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5 Principles and Practices of Selection

INTRODUCTION learning objectives Many annual and biennial , forage, vegetable, and ornamental selec- • Define breeding systems. tions are produced by breeding to be propagated by seed (7, 8, 29, • Categorize seed-propagated 34). Breeding involves selection of parents, specific breeding procedures, and species. and genotype stabilization (1, 3, 47). The last process is sometimes • Define procedures to control referred to as “fixing the genotype.” “fixing” The process genetic variability. Seed is used to reproduce most woody of stabilizing the • Describe systems of seed perennial in forestry as well as in genotype of a seedling selection and production. the landscape. Propagation of many population to make it • Define legal controls on ornamental, , and nut trees utilizes homozygous so that it genetic purity. seedlings for rootstocks that are then will “breed true.” grafted (49, 58). However, characteristics important in , , and forestry may not be consis- tently perpetuated into the next seedling generation unless appropriate principles and procedures are followed. This chapter deals with seed selection and the management of genetic variability in seedling popula- tions in both herbaceous and perennial plant species for the purposes of propagation.

BREEDING SYSTEMS The main objective of a breeding program is to use the observed variability available within a particular genus or species to create new, stable popula- tions with improved plant characteristics. Variability in seed-propagated plants can be described both at the pheno- homogenous type (appearance) and genotype (genetic) A population of seedlings levels. Seedlings that are phenotypi- that are phenotypically cally very similar in appearance to each similar. other are termed homogeneous, while those that are dissimilar are described heterogeneous as heterogeneous. When more A population of seedlings specific information is known that are phenotypically about the seedling popula- dissimilar. tion’s genetic makeup, they can homozygous be described as homozygous or A population of seedlings heterozygous. Homozygous whose genotypes are populations share many com- very similar. mon paired alleles (genes) at heterozygous each chromosome loci and A population of seedlings breed true-to-type offspring. Hete- whose genotypes are rozygous populations have dissimilar dissimilar. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 141

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self- paired alleles at many flowers open to permit cross-pollination between flow- A breeding system in chromosome loci and ers and produce offspring () with generous which the plant flower is generally lead to genetic diversity. These same plants also produce pollinated by itself diverse genetic off- underground cleistogamous flowers in the autumn because of flower spring. These charac- that never open and self-pollinate. Although this structure or isolation. teristics are determined restricts genetic diversity, it does not require the cross-pollination by the breeding sys- same level of plant resources for seed production and A breeding system in which tem, characteristics of provides insurance against poor seed production from the plant is pollinated by the crop species, and earlier out-crossing flowers. pollen from a separate management conditions The degree to which self-pollination occurs can genotype either because under which seed vary among species. Some are highly self-pollinated of flower structure or populations are grown (i.e., less than 4 percent cross-pollinated) such as artificial control during (1, 3, 22). Three impor- cereal grains [barley (Hordeum), oats (Avena), wheat pollination. tant considerations for (Triticum), rice (Oryza)], legumes [field pea (Pisum), determining a plant and bean (Phaseolus)], flax (Linum), and some apomixis A breeding system in which the breeding system are grasses. There are also those that are self-fertile but can embryo is apomictic whether the plants cross-pollinate at more than 4 percent, including cot- (i.e., produced from a reproduce primarily ton (Gossypium), pepper (Capsicum), and tomato vegetative cell and not from self-pollination, (Solanum). Self-pollination is not typically found in as a result of reduction cross-pollination, and most woody plant species, but some exceptions occur, division and fertilization). apomixis (22). such as peach (Prunus) (58). Homozygosity in a self-pollinated herbaceous Self-Pollination is “fixed” by consecutive generations of self- fertilizations (Table 5–1) (1, 22, 47). To produce a Self-pollination occurs when pollen germinates on “true-breeding” homogeneous and homozygous culti- the stigma and the pollen tube grows down the style to var, plant breeders will start with a single plant and fertilize the same flower or a flower of the same plant then eliminate the off-type plants each generation for a or clone. Self-pollination is a natural condition in period of six to ten generations. If one assumes a more some species because of flower structure. The extreme or less homogeneous population with individuals pos- case is when pollination occurs before the flower opens sessing homozygous traits, self-pollination will result in (Fig. 5–1). This type of behavior is called cleistogamy a population of individuals that will remain homoge- and occurs in some crop plants like peanuts (Arachis). neous and homozygous. If a mutation occurs in one of A wonderful example of this reproductive strategy is the alleles and is recessive, the genotype for that trait found in several types of violets (Viola). Violets can becomes heterozygous. Then the next generation will produce two types of flowers. Chasmogamous (open) produce homozygous plants that are similar in appear- flowers are produced in the spring or summer when ance but genetically heterozygous for the mutant allele. pollinators are plentiful and active. Chasmogamous The proportion of homozygous individuals with the two traits will increase in consecutive generations, (a) (b) (c) while the proportion with heterozygous genotypes will decrease by a factor of one-half each generation. The group of descendants of the original parent will segre- gate into a heterogeneous mixture of more or less true- breeding lines.

Flower Fruit Seeds Cross-Pollination In , many, if not most, species are naturally cross- pollinated, a trait that seems to be desirable both for Figure 5–1 the individual and its population. Not only does the A cleistogomous flower in violet. The flower remains underground and never opens, forcing self-pollination. increased heterozygosity provide the opportunity (a) Unopened cleistogomous flower. (b) Fruit with developing for evolutionary adaptation within the population seeds. (c) Fruit with ovary wall removed to show the seeds. confronted with environmental change, but plant vigor M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 142

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Table 5–1 EFFECT OF SELF-POLLINATION AND ROGUING FOLLOWING CROSSING OF A TALL (DD) PEA AND DWARF (dd) PEA (SEE FIG. 2-14). “Fixing” of the two parental phenotypes can be observed in succeeding generations in the proportion of tall and dwarf plants. Continuous roguing for the recessive trait never quite eliminates its segregation from residual heterozygous individuals.

A. Continuing self-pollination proportions B. Roguing of all dwarfed plants DD Dd Dd Percent homozygous Tall Dwarf %dd

P1 1 1 100 F1 1 0 all F2 1 2 1 50 3 1 25 F3 3 2 3 75 14 1 7.1 F4 7 2 7 87.5 35 1 2.8 F5 15 2 15 93.75 143 1 0.7 F6 31 2 31 96.88 535 1 0.2 F7 126 2 126 98.44 2143 1 0.05

also tends to be enhanced. Enforced self-pollination of androecious. This type of flower arrangement usually naturally cross-pollinated plants through consecutive forces cross-pollination. generations may result in homozygous plants and a • Monoecy. Monoecious plants have pistillate (female) homogeneous popula- and staminate (male) flowers in separate flowers inbred line tion (inbred line), but on the same plant. This A population of seedlings vigor, size, and produc- system occurs in cucurbits monoecious Plant that produced a tivity may be reduced, (Cucurbita), corn (Zea), trait in which the consecutive series of a condition described walnut (Juglans) (Fig. 5–3), male and female self-. as inbreeding depres- oak (Fagus), and many parts are in different sion. If, however, two inbred lines are crossed, the conifers. Although this flowers but on the vigor of the plants of the resulting population may not facilitates cross-pollination, same plant. only be restored but may show more size and vigor than either parent, a Vigor hybrid vigor phenomenon known expressed by a seedling as heterosis or hybrid population that vigor. In this case, the exceeds that of either individual plants will of the parents. be heterozygous, but the population is likely to be homogeneous and have uniform characteristics. Many species have also developed morphological or genetic mechanisms to prevent self-pollination and promote cross-pollination. Here are four illustrations of morphological adap- tations to facilitate cross-pollination (6): • Dioecy. Dioecious plants have pistillate (female) and staminate (male) flowers present in separate plants, (a) (b) such as asparagus (Asparagus), pistachio (Pistacia) and holly (Ilex) (Fig. 5–2). Figure 5–2 Holly (Ilex) plants are dioecious, producing female (a) and dioecious Plant trait in Plants with only female male (b) flowers on separate plants, forcing cross-pollination. which male and female flowers are called gynoe- Many flowers in dioecious plants produce remnant female flowers are produced cious, and those with and male parts that are usually non-functional. Note the on different plants. only male flowers are non-functional male stamens present in the female flowers. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 143

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does not ensure cross-pollination but reduces the ratio of self- to cross-pollinated flowers (40). • Polymorphism. Floral polymorphisms refer to differ- ent arrangements of flower parts in flowers from the same or different plants within the same species. Many of these adaptations are designed to alter the ratio of self- to cross-pollination. A range of flower structures is illustrated in asparagus (Asparagus) (Fig. 5–5, page 144). These types of polymorphisms were of particular inter- est to Darwin (14) as he described the different flower forms in primrose (Primula) referred to as heterostyly. (a) (b) Plants exhibiting heterostyly have two or three different Figure 5–3 flower morphologies where the style of the female and Some nut-producing tree species have pollination systems that the filaments of the male are produced at differernt ensure cross-pollination. Walnuts (Juglans) are monoecious lengths (Fig. 5–6, page 144). In addition to the differ- with female (a) and male (b) flowers produced separately on ent heterostylous morphologies, each style and filament the same plant. length combination may be linked to a sexual incom- patibility system to limit which flowers can cross with self-pollination is usually possible in monoecious each other (23). plants unless another barrier to self-pollination is present. Sexual incompatibility (10, 15) is a general • Dichogamy. Dichogamy is the separation of female term that describes the inability of plants that are not and male flower function in time (50). There are genetically related to cross and produce offspring. two types of dichogamy Self-incompatibility sexual incompatibility dichogamy Genetic depending on whether is a form of sexual Genetic trait in which trait in which male and the female becomes incompatibility that the pollen either fails to female flowers on the receptive before the has evolved to prevent grow down the style or same plant bloom at male sheds pollen (pro- self-pollination within does not germinate on different times. togyny) or the male closely related species the stigma of a plant sheds pollen before the female is receptive (protandry). and has been found in with the same There are numerous examples of this type of flowering over 250 plant genera incompatibility alleles. including carnation (Dianthus) (Fig. 5–4). Dichogamy from at least 70 fami- lies. Some horticulturally important plants showing self-incompatibility include lily (Lilium), cabbage (Brassica), Petunia, almond (Prunus dulcis), apple (Malus), cherry, and plum (Prunus). Self-incompatible crosses are characterized by a lack of pollen germination or arrested pollen-tube growth an (53). Self-incompatibility is a genetic mechanism con- trolled by a single gene locus (in diploids) with several dif- ferent S alleles. It is controlled by protein-to-protein s s recognition determined by the type of S allele in the male an and female partners. The two most common forms of self-incompatibility are gametophytic and sporophytic (Fig. 5–7, page 145). Gametophytic self-incompatibility is the most common form of self-incompatibility, and the interaction between the male and female partners is Figure 5–4 determined by a single S-allele derived from the haploid Sweet William carnation (Dianthus) flowers show dichogamy. within the pollen grain. Recognition only occurs Note how the flower on the left has anthers (an) shedding pollen before the style (s) has fully developed and the flower after pollen germination and tube growth. When the on the right that has fully receptive female parts after the male and female share a common S-allele genotype, there anthers have withered. is a protein-to-protein interaction that stops pollen-tube M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 144

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Figure 5–5 Range of flower structure types expressed in different asparagus flowers of individual plants. Type 1. Completely female. Dioecious. Flowers contain only the pistil; stamens (male) are reduced and nonfunctioning. Type 2. Completely male. Dioecious. Flowers only contain stamens. The pistil is reduced and nonfunctioning. Type 3. Both male and female structures are functioning. Perfect. Type 4. Both male and female structures are nonfunctioning. Sterile. Commercial seed production of asparagus results from growing Type 1 and Type 2 plants together to enforce cross-pollination and produce the desirable hybrid plants. Courtesy Bryan Benson.

Pin Thrum

an

s

(a) (b)

Pin Thrum

s Figure 5–6 a an Examples of heterosyly in primrose (Primula). s On the left are “pin” flowers where the stigma (s) is elevated above the corolla and the anthers (an) held on a short filament. On the right are “thrum” flowers with elongated filaments exposing the anthers above the corolla, and a shortened style, keeping the (c) stigma within the corolla tube. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 145

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bypass incompatibility in lily (Lilium) is to remove the upper two-thirds of the style (including the stigma) before applying pollen. This allows time for the pollen tube to reach the embryo sac before being arrested by the incompatibility reaction. Sporophytic self-incompatibility differs from gametophytic self-incompatibility because it is the diploid S-allele pair from the male and female parents that determines compatibility (32). Protein types determined by different S alleles are deposited on the surface of the pollen grain where they interact with proteins on the stigmatic surface to determine whether the pollen grain will germinate and initiate tube growth. Examples of plants with this type of incom- patibility are found in the Brassicaceae, Asteraceae, and Convolvulaceae families. Because multiple S alle- les are involved in this recognition system, pollen/ stigma interactions can be complex (Fig. 5–7). Cross-pollination is mostly carried out by the movement of pollen by wind or insects. Also, pollina- tion is sometimes by bats, birds, and water (48). Insect pollination is the rule for plants with white or brightly colored, fragrant, and otherwise conspicuous flowers Figure 5–7 that attract insects. The honeybee is one of the most Incompatibility mechanisms prevent self-pollination in some species. Top (cabbage): Sporophytic incompatibility. Each important pollinating insects, although wild bees, but- pollen contains genes of both S1 and S2 alleles, and the pollen terflies, moths, and flies also obtain pollen and nectar tube will only grow down a style with a different genotype. from the flower (Fig. 5–8). Generally, pollen is heavy, Bottom (clover): Gametophytic incompatibility. Each pollen sticky, and adheres to the body of the insect. Some grain has a single S allele. A pollen tube will not grow down a important seed that require cross-pollination style where that allele is represented. Redrawn with permission from are alfalfa (Trifolium), birdsfoot trefoil (Lotus), red Stoskopf, et al. Theory and Practice. Westview Press: Boulder, CO. clover (Trifolium pratense), white clover (Trifolium repens), onion (Allium), watermelon (Citrullis) and growth. In some families (i.e., Papaveraceae), the pollen sunflower (Helianthus) (Fig. 5–8c). In addition, tube stops growing soon after initial germination, while many flower and vegetable crops are insect pollinated in others (i.e., Solanaceae, Rosaceae) the pollen tube will as are many fruit plants, ornamental plants, and growth a considerable distance down the style before its deciduous and broad-leaved evergreens used in the growth is arrested (21). A unique breeding technique to landscape.

(a) (b) (c) Figure 5–8 Important insect pollinators include (a) bees and (b) butterflies. (c) Bee hives are included in production fields to help pollination. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 146

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Wind pollination is the rule for many plants that from imported seed of Eucalyptus from Australia and have inconspicuous flowers, or those with monoecious, pear (Pyrus) species from China and Japan (30) could be dioecious, or dichogamous flowers. Examples are traced to hybridization with other species nearby. grasses, corn, olive, and catkin-bearing trees such as the walnut (Juglans), oak (Quercus), alder (Alnus), cottonwood Apomixis (Populus), and conifers (Fig. 5–9). The pollen produced Apomixis occurs when an embryo is asexually produced from such plants is generally light and dry and, in some from a single cell of the sporophyte and does not develop cases, carried long distances in wind currents. from fertilization of two gametes (28). This new “vegeta- Most trees and shrub species are both heterozy- tive” embryo may arise by mechanisms that were gous and cross-pollinated such that considerable poten- described in Chapter 4. In each case, the effect is that seed tial for genetic variability exists among the seedling production becomes asexual and seed reproduction results progeny. Selection of seed source plants must take into in a clone. In some species, both apomictic and sexual account not only the characteristics of the plant itself seeds are produced, sometimes within the same ovule but also the potential for cross-pollination with other (facultative); bluegrass facultative apomictic species in the surrounding population. For example, the (Poa pratensis) falls into A plant in which both presence of off-type individuals in seedlings propagated this category. Other sexual and asexual species are essentially embryos are produced 100 percent apomictic by the same plant. (obligate); for example, Bahia grass (Paspalum obligate apomictic A plant in which all the notatum) and buffelgrass embryos are apomictic. (Pennisetum ciliare). Breeding of apomictic cultivars requires that a genetic source for apomictic reproduction be found within that species. This trait is not identifiable by visual inspection of the parent plant but by its genetic performance (i.e., unexpected uniformity of its prog- eny from among normally variable populations). Apomixis has been most important in the breeding of grasses, forage crops, and sorghum. Introduced culti- vars have included ‘King Ranch’ bluestem, ‘Argentine’ (a) (b) Bahia grass (Paspalum), and ‘Tucson’ side oats grama, ‘Bonnyblue’ and ‘Adelphi’ Kentucky bluegrass (Poa) (24), and buffelgrass (Pennisetum) (28). Relatively few Female flower genes apparently control apomixes, and breeding sys- tems have been described to incorporate this trait into cultivars and particular species. Apomictic reproduction in woody plant species and cultivars is found in many Citrus (9), mango (Mangifera), and some apple (Malus) species (52). Although apomixis produces genetically uniform Male flower seedlings, it is not necessarily useful for growing spe- cific fruit cultivars because of undesirable juvenile ten- dencies, such as thorniness, excess vigor, and delayed fruiting. On the other hand, these characteristics make (c) apomictic seedlings useful as rootstocks, characteristics Figure 5–9 exploited extensively in Citrus. Conifers are usually wind pollinated. Male strobili (a) release In apomixes, the seedling population is immedi- pollen that is deposited on the female cone (b). True ately stabilized as a “true-breeding” line without flowering plants (Angiosperms) developed along with insect pollinators. Wind pollination is a derived character that is seedling variation. Such plants exhibit the apomictic usually associated with reduced flower parts (i.e., no petals) cycle and express typical juvenile traits of the seedling and unisexual flowers (c) as illustrated for chestnut (Castanea). population. Apomixis is particularly appropriate for M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 147

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plants whose value lies in their vegetative characteris- Sometimes, new cultivars lack adaptation to local envi- tics—as occurs in forages and grasses—rather than in ronments. Although the trend has been to increase the plants whose value depends on fruiting characteristics. world supply of essential food crops, concerns have been raised that a parallel loss of genetic diversity and germplasm has occurred. Exploration and conservation CATEGORIES OF SEED- efforts have expanded to maintain these important raw PROPAGATED CULTIVARS materials for future use (19). AND SPECIES Cultivars A cultivar is a uniform and stable plant population that possesses recognizably distinct char- Herbaceous Annual, Biennial, acteristics. Stated another way, a cultivar is a plant and Perennial Plants population that shows a minimum of variation, that Landraces Historically, farmers throughout the can be propagated true-to-type for at least one charac- world have maintained seed-propagated plants by sav- teristic, and is unique compared to the wild species or ing selected portions of the crop to be used to produce other cultivars. The term variety is often used inter- the next cycle. These populations, called landraces, changeably with cultivar especially when describing evolved along with flower and vegetable populations. Care should be landrace Primitive human societies and taken not to confuse varieties developed and are still found in some variety with the con- botanical variety maintained before the A population of plants parts of the world (56). cept of a true botani- modern era of genetics. originating in nature These populations are cal variety (varietas or that are within one variable but identifiable and have local names. This var.) that describes a species but are practice results in genetic populations adapted to a type of naturally occur- phenotypically distinct. localized environment. Their inherent variability pro- ring population. vides a buffer against environmental catastrophe and Categories of seed-propagated cultivars include preserves a great deal of genetic diversity (Fig. 5–10). open-pollinated, lines, hybrids, synthetic, F2, and Changes in cropping patterns have occurred dur- clonal cultivars. ing the 20th Century, particularly since about 1960. Open-pollinated cultivars can be maintained in Many of the older populations around the world are cross-pollinated species that produce a relatively homo- being replaced by modern cultivars, which tend to be geneous population for specific traits important for uniform and high yielding, particularly when grown in production of that crop. Open-pollinated seed is often conjunction with high irrigation and fertility inputs. cheaper to produce compared to because

Figure 5–10 A landrace of soybeans (Glycine) in Africa showing the diversity inherent in seeds saved over many generations. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 148

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they do not require hand pollination to maintain because in the next generation, variability in size, vigor, the cultivar. However, because open-pollinated culti- and other characteristics may appear. vars are a genetically heterogenic population, they can Synthetic cultivars are derived from the first be more variable than hybrids (41). generation of the open cross-fertilization of several Historically, many open-pollinated vegetable lines or clones. For and flower varieties were maintained by families in example, ‘Ranger’ alfalfa synthetic line Acultivar seedling their “kitchen .” Many of these varieties have seed is made from inter- population that is since been maintained by generations of cropping five seed- produced by combining and local farmers and are being offered as heirloom propagated lines that a number of separately varieties. The preservation and distribution of infor- results in genetically dis- developing lines to mation concerning these varieties has been an objec- tinct but phenotypically produce a heterozygous tive of certain groups including Seed Savers Exchange, similar seedlings in the but homozygous Inc., in Decorah, Iowa (4, 54, 59). There are also seeded crop. Other Cultivar. numerous commercial flower and vegetable crops pro- crops in this category duced as open-pollinated cultivars including Begonia, include pearl millet (Pennisetum glaucus), bromegrass marigold (Tagetes), cucumber (Cucumis), and squash (Bromus) and grass (Dactylis). (Cucurbita). F2 cultivars are derived for open-pollination of line A population of Lines result in an . Some flower crops, (Petunia, pansy seedling plants whose seedling populations (Viola), and Cyclamen) and vegetables (tomato and genotype is maintained whose genotype is main- melon) can be maintained as F2 populations. to a specific standard in tained relatively intact Clonal seed cultivars are maintained through consecutive generations. during consecutive gen- apomictic seed production (25, 51). Apomixis occurs erations. These may be when an embryo is asexually produced and does not maintained as self- or cross-pollinated lines. An impor- develop from fertilization of two gametes (28). The tant type of seed population in this category is the result is a clonal copy of the parent plant. Apomixis inbred line, which are mainly used as parents for later is discussed in detail in Chapter 4. The degree of production of F1 hybrids (55). clonal seed production depends on whether the species has a facultative or obligate form of apomixis. In Hybrid Cultivars include groups of individuals species with facultative apomixis, both apomictic reconstituted each generation from specific parents. F 1 and sexual seeds are produced, sometimes within the hybrids are the first generation of a planned cross. For same seed. Bluegrass (Poa pratensis) falls into this seed production, they result from the cross between category. Other species show essentially 100 percent seedling populations of two or more inbred lines. obligate apomictic seed production. Examples include When crossed with another inbred line, the result is a Bahia grass (Paspalum notatum) and buffelgrass population of uniform, but heterozygous, plants. Often (Pennisetum ciliare). these populations exhibit greater vigor than the parents due to hybrid vigor (heterosis), depending on the com- bining ability of the parents. Hybridization is a means of “fixing” the genotype of the population similar to Woody Perennial Plants that described for self-pollinated lines. Hybrid lines Wild Populations In nature, most species can be were first produced in recognized as a more or less phenotypically (and geno- hybrid line A seedling corn (Zea mays) (55) typically) uniform seedling population that has population that is but have since been evolved over time through consecutive generations to produced by cross- applied to many agro- be adapted to the environment at a particular site. If pollinating two or more nomic, vegetable, and the species covers a wide area, local variation in envi- parental lines. flower crops (1, 26). ronment can result in different populations becoming Hybrids may be produced between two inbred adapted to different areas even though the plants may lines (single-cross), two single-crosses (double-cross), appear phenotypically similar. Plants within the an inbred line and an open pollinated cultivar (top- species that show morphological differences cross), or between a single-cross and an inbred line compared to the species, but that are reproduced by (three-way cross) (55). Seeds saved from the hybrid seed, may be designated as botanical varieties or population normally are not used for propagation varietas or var. The term form indicates a particular M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 149

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ecotype A genetically phenotypic difference, Burmese race of teak, Douglas-fir (Pseudotxuga men- distinct group of plants as a blue or white color. ziesii) from the Palmer area in Oregon, ponderosa pine within a species that is Subgroups of a particu- (Pinus ponderosa) from the Lolo Mountains in adapted to a specific lar species that are Montana, and white spruce (Picea alba) from the ecological location. morphologically simi- Pembroke, Ontario (Canada) area (16, 20, 31). lar but specifically Douglas-fir (Pseudotsuga menziesii) has at least three cline Continuous adapted to a particular recognized races—viridis, caesia, and glauca—with var- genetic variation from environmental niche ious geographical strains within them that show differ- one area to another in are known as ecotypes. ent adaptations. For instance, progeny tests showed ecological adaptation. Variations that occur that a viridis strain from the United States West Coast continuously between locations are known as clines was not winter-hardy in New York but was well suited (35, 43). to Western Europe. Those from Montana and Wyoming were very slow growing. Trees of the glauca Provenance The climatic and geographical locality (blue) strain from the Rocky Mountain region were where seed is produced is referred to as its seed origin winter-hardy but varied in growth rate and appearance. or provenance (2, 3, 38, 54). Variation can occur Strains collected farther inland were winter-hardy and among plants associated with latitude, longitude, and vigorous; similar differences occurred in Scotch pine elevation. Differences may be shown by morphology, (Pinus sylvestris), mugho pine (P. mugho), Norway , adaptation A forestry spruce (Picea abies), and others. provenance to climate and to soil, term used to indicate and in resistance to the climatic and Improved Seed Sources Nursery propagation by diseases and insects. geographical locality woody plant seed can be upgraded significantly by the Natural plant popula- where the seed selection and development of improved seed sources. tions growing within originated. This practice applies to the production of rootstocks a given geographical for fruit and nut trees (49, 58), shade trees (20, 43), area over a long period of time evolve so that they and trees in the landscape. Likewise, foresters have become adapted to the environmental conditions at been engaged in recent years in “domesticating” and that site. upgrading forest tree production over that of “local” Consequently, seeds of a given species collected seed (38, 40, 60). in one locality may produce plants that are completely inappropriate to another locality. For example, seeds Elite Trees. Foresters refer to single seed-source trees collected from trees in warm climates or at low alti- with a superior genotype, as demonstrated by a progeny tudes are likely to produce seedlings that will not stop test, as elite trees. Nursery progeny tests can identify growing sufficiently early in the fall to escape freezing and characterize specific elite tree An when grown in colder regions. The reverse situation— seed sources (e.g., for individual tree with collecting seed from colder areas for growth in landscape or Christmas outstanding phenotypic warmer regions—might be more satisfactory, but it tree uses). characteristics to be also could result in a net reduction in growth resulting used as a seed source. from the inability of the trees to fully utilize the grow- Clonal Seed Sources. ing season because of differences in the response to Superior (elite) seed- clonal seed source photoperiod (61). source trees can be Cultivar maintained as a Distinct ecotypes have been identified by means maintained as clones in clone selected for of seedling progeny tests in various native forest tree seed to pre- producing outstanding species, including Douglas-fir (Pseudotsuga menziesii), serve the genotype of seedlings. ponderosa pine (Pinus ponderosa), lodgepole pine the parent. Seeds from (Pinus murrayana), eastern white pine (Pinus strobus), this clonal source are then used to produce seedling slash pine (Pinus caribaea), loblolly pine (Pinus sp.), trees in the nursery. This procedure is used to pro- shortleaf pine (Pinus echinata), and white spruce (Picea duce rootstock seed for fruit and nut cultivars. For alba) (66). Other examples include the Baltic race of instance, ‘Nemaguard’ is a peach hybrid whose nema- Scotch pine (Pinus sylvestris), the Hartz Mountain tode-resistant seedling progeny are used for almond, source of Norway spruce (Picea abies), the Sudeten peach, plum, and apricot trees in central California (Germany) strain of European larch (Larix), the (49). Named cultivars of ornamental trees have been M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 150

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identified as producing uniform, superior seedling CONTROL OF GENETIC progeny (16). VARIABILITY DURING SEED selected families Selected Families. PRODUCTION Consecutive groups of Genetic improvement Herbaceous Annual, Biennial, progeny trees related of forest-tree species has by origin and showing brought about family and Perennial Plants superior characteristics. selection by growing Isolation Isolation is used to prevent mechanical progeny trees either mixing of the seed during harvest and to prevent from controlled crossing or selection from single open- contamination by unwanted cross-pollination with a pollinated superior (elite) trees. Seed orchards then may different but related cultivar. Isolation is achieved pri- be established either from seedlings of these trees or marily through distance, but it can also be attained by established by grafting the parent trees. A minimum enclosing plants or groups of plants in cages, enclos- number of individual genotypes are selected—usually ing individual flowers, or removing male flower parts around twenty-five—to avoid the dangers of inbreed- (i.e., de-tasseling corn) and then manually applying ing and limits to the genetic range. Progeny trees are pollen of a known source by hand or various other planted in test sites and evaluated for various forestry devices (Fig. 5–11). On a large scale, this goal can be characteristics. Over time, superior sources are identi- achieved by using male-sterile parents (36). In a fied and preserved as parents to produce the next gener- number of crop species [e.g., tobacco (Nicotiana) ation of new families of improved seed genotypes. and onion (Allium)], specific genes have been identi- Inferior seed sources are identified and eliminated. fied that prevent normal formation of the male (pollen) reproductive structures (Fig. 5–12). This Hybrid Seed Sources. F1 hybrids of two species usually means that no viable produce uniform populations of plants in the same man- pollen is produced. The pollen sterility ner as hybrid seeds of corn and other inbred lines (see most common form of Genetic phenomenon p. 157). For example, hybridization has been valuable in pollen sterility is cyto- in which the pollen is producing vigorous almond × peach hybrids for almond plasmic male sterility, nonviable. and peach rootstocks (33, 34), Paradox hybrids (Juglans which is a complex inter- hybrid seed Seed hindsii × J. regia) for walnuts and fast-growing poplars action between nuclear produced by the from crosses with North American and European poplars and mitochondrial plant crossing of two (like Populus trichocarpa. × P. deltoides). Forest tree genes (12). Such traits can dissimilar parents, hybrids, such as Pinus rigida × P. taeda in Korea and Larix be bred into parental lines usually produced decidua × L. leptolepsis in Europe are not necessarily uni- of specific cultivars for when made between form, however, but have been a focus of improved forests. the production of hybrid species. Because of expense and uncertainty of production, seeds seed. Using molecular of F1 hybrids of the forest trees have been used to produce to induce male sterility has also become a F2 seedling populations. The more vigorous hybrid plants potential strategy to limit gene flow from transgenic dominate and the weaker trees are crowded out. plants into the environment (11).

Figure 5–11 Isolation is used to prevent unwanted cross-pollination during seed production. (a) An onion hybrid cross being isolated with an individual sac. (b) A small cage where flies will be introduced to pollinate onions for hybrid production. Female plants in the cage are made sterile to (a) (b) prevent self-pollination. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 151

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In seed production areas, such as regions of Oregon, Washington, and California, seed companies cooperate to locate seed production fields at appropri- ate isolation distances from each other. The fields are located on “pinning” maps (each colored pin indicating a field and crop type), and the maps are located in County Extension offices within the production areas. Recently, these have also become available as virtual maps on the Internet, as is the case in California. Roguing The removal of off-type plants, plants of other cultivars, and weeds in the seed production field Figure 5–12 is known as roguing (37). roguing The act of Male sterility in tobacco (Nicotiana). Note how the flower on During the development of removing off-type the left lacks developed anthers compared to the perfect a seed-propagated cultivar, flower on the right with elongated and fertile anthers (arrow). plants, weeds, and positive selection is practiced plants of other to retain a small portion of cultivars in seed Self-pollinated cultivars of herbaceous plant desirable plants and to maxi- production fields. species need only to be separated to prevent mechanical mize the frequency of desir- mixing of seed of different cultivars during harvest. The able alleles in the population. During seed production, minimum distance usually specified between plots is roguing following visual inspection exerts selection by 3 m (10 ft), but may be up to 50 to 65 m (150 to 200 ft) eliminating the relatively small population that is not “true depending on the degree of cross-pollination capacity in to type,” thus keeping the cultivar “genetically pure.” the crop. For example, bell pepper is a self-pollinating Off-type characteristics (i.e., those that do not crop but, given the opportunity, will cross-pollinate to a conform to the cultivar description) may arise because high degree from bee pollinators. Careful cleaning of the recessive genes may be present in a heterozygous condi- harvesting equipment is required when a change is made tion even in homozygous cultivars. Recessive genes aris- from one cultivar to another. Sacks and other containers ing by mutation would not be immediately observed used to hold the seed must be cleaned carefully to in the plant in which they occur. Instead, the plant remove any seed that has remained from previous lots. becomes heterozygous for that gene, and, in a later gen- More isolation is needed to separate cultivars eration, the gene segregates and the character appears cross-pollinated by wind or insects. The minimum dis- in the offspring. Some cultivars have mutable genes tance depends on a number of factors: that continuously produce specific off-type individuals • the degree of natural cross-pollination (45). Off-type individual plants should be rogued out • the relative number of pollen-shedding plants of the seed production fields before pollination occurs. • the number of insects present Systematic inspection of the seed-producing fields by • the direction of prevailing winds trained personnel is required. Other sources of off-type plants include contami- The minimum distance recommended for insect- 1 nation by unwanted pollen due to inadequate isolation pollinated herbaceous plant species is 0.4 km ( ⁄4 mi) to or volunteer plants arising from accidentally planted 1.6 km (1 mi). The distance for wind-pollinated plants 1 seed or from seed produced by earlier crops. Seed pro- is 0.2 km ( ⁄8 mi) to 3.2 km (2 mi), depending on duction fields of a particular cultivar should not have species. grown a potentially contaminating cultivar for a num- Effective cross-pollination usually can take place ber of preceding years. between cultivars of the same species; it may also occur Weeds are plant species that have been associated between cultivars of a different species but in the same with agriculture as a consequence of their ability to genus; rarely will it occur between cultivars belonging to exploit disturbed land areas when cultivation occurs another genus. Since the horticultural classification may (29). Some weed species have evolved seed types that not indicate taxonomic relationships, seed producers closely resemble crop seeds and are difficult to screen should be familiar with the botanical relationships out during seed production. among the cultivars they grow. It is also important to iso- late GMO (genetically modified organisms) crops from Seedling Progeny Tests Planting representative seeds non-GMO seed crops of the same cultivar or species. in a test plot or garden may be desirable to test for M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 152

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trueness-to-type. This procedure is used in the devel- concept could apply pure stand An opment of a cultivar to test its adaptability to various both to plants growing interbreeding group of environments. The same method may be necessary to in a natural environment phenotypically similar test whether changes have occurred in the frequency of or in a planting such as a plants of the same kind particular genes or new gene combinations may have wood lot. These popula- growing in a given site. developed during seed increase generations. These tions are useful in seed changes can result from selection pressure exerted by collection because cross-pollination would likely occur management practices or environmental interaction. from among this group of plants and one can judge For example, intensive roguing may result in a genetic both the female and the male parents. Although the drift due to changes in individuals are likely to be heterozygous, they should genetic drift Change in the frequency of the frequency of partic- produce good seeds and vigorous seedlings. The popu- specific genes as a ular genes or gene com- lation should be homogeneous and reproduce the result of environmental binations (24). Shifts parental characteristics. or other types of may also occur due to selection. environmental expo- Phenotypic Tree Selection Versus Genotypic Tree sure in a growing area Selection. Phenotypic selection refers to evaluation which is different from the initial selection area. of a seed source through Seedlings of particular genotypes may survive better visual inspection of the phenotypic selection than others and contribute more to the next generation. source plant(s). The basis Selection of a seed If sufficiently extensive, genetic drift could produce of this procedure is that source based on the populations of progeny plants that differ somewhat many important traits in phenotypic from those of the same cultivar grown by other produc- forestry—such as stem appearance of the ers. Or the cultivar may have changed from the original form, branching habit, source tree. breeder’s seed. growth rate, resistance to diseases and insects, presence Problems can result if seed crops of particular of surface defects, and other qualities—are inherited perennial cultivars are grown in one environment (such quantitatively. Geneticists refer to this relationship as as a mild winter area) to produce seed to be used in a high phenotypic correlation between parents and off- different and more severe environment (such as an area spring. In practical terms, this means that the parental requiring cold-). This situation has occurred, performance can be a good indicator of the perform- for example, with alfalfa (24) where rules for produc- ance of the offspring (57). tion of forage crop seed in a mild winter area can spec- When individual trees in native stands show a ify only one seedling generation of increase. superior phenotype, foresters call them “plus” or elite trees and sometimes leave them for natural reseeding or Woody Perennial Plants as seed sources. Such dominant seed trees may con- tribute the bulk of natural reseeding in a given area. Use of Local Seed “Local seed” means seed from a Genotypic selection refers to evaluation of a seed natural area subjected to a restricted range of climatic source based on the and soil influences. As applied to forest tree seed, this genotypic selection performance of their usually means that the collection site should be Selection of a seed seedling progeny test within 160 km (100 mi) of the planting site and source based on the (39). Seeds may be pro- within 305 m (1,000 ft) of its elevation. In the phenotypic appearance duced by open pollina- absence of these requirements, seed could be used of the seedling progeny. tion (OP) where only from a region having as nearly as possible the same one parent is known. Or the test may be made from a climatic characteristics. The reason for the historical controlled cross, where both parents are known and the emphasis on local seed is the phenomenon previously contributions of each can be evaluated. A progeny test defined as seed origin or provenance (27). The use of establishes the breeding value of a particular seed local seed for herbaceous and woody plants is partic- source (5) because genetic potential is based on actual ularly important in the effort to restore any native performance of the progeny. ecosystem where the use of exotic species would be A representative sample of seeds is collected, inappropriate (43). planted under test conditions, and the progeny Pure Stands. Pure stand refers to a group of pheno- observed over a period of years. A high correlation typically similar seedling plants of the same kind. This between the average phenotypic traits of the parent(s) M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 153

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high additive and the average pheno- heritability High typic response of the correlation between offspring is referred to phenotypic traits of as high additive heri- offspring with the tability and justifies phenotypic traits of the using the “best” trees parents. for seed sources of the low additive next generation (38). A heritability Low low statistical correla- correlation between tion between parent phenotypic traits of and progeny character- offspring with the istics is referred to as phenotypic traits of the low additive heritabil- parents. ity in that the desired traits of the progeny cannot be predicted from inspection of the parents. Progeny testing is useful to verify the suitability of individual seed sources for future seed collecting. The procedure is an important component to the improve- Figure 5–13 ment of woody plants whether in forestry or horticulture. Pedigree system for seed production. See text for discussion.

with the production of a small quantity of seeds SEED PRODUCTION SYSTEMS (breeder’s seed) that is maintained by the originating Herbaceous Annual, Biennial, institution as the primary reference for the cultivar. Phase and Perennial Plants 2 is a maintenance phase in which a quantity of seed called foundation seed (for certified seed classes; see Box Commercial Plantings Traditional seed selection of 5.1, page 154) or stock seed (in commercial enterprises) herbaceous plants utilized a portion of the seed from one is maintained under high standards of isolation, inspec- year’s crop to plant a crop for the next year. This system tion, and roguing. Phase 3 is the distribution phase, which would be satisfactory for self-pollinated cultivars that may include two steps: a second-generation increase block are easy to maintain genetically. For cross-pollinated and a third-generation block to produce commercial seed cultivars, knowledge of the production requirements of for distribution to the public. A foundation planting orig- individual crops is needed and specific conditions are inates only from breeder’s seed or another foundation practiced depending upon the plant (3). Note, how- planting. An increase block originates only from a foun- ever, that inadequacies of this method led to its replace- dation seed or another increase planting. A seed produc- ment by the pedigree system. tion planting originates from foundation seed or Pedigreed Stock System (4, 42) Commercial seed pro- increase block seed. This entire production process is duction of most self-pollinated and cross-pollinated lines carried out either by large commercial firms or groups is carried out in three steps (Fig. 5–13). The purpose of a of independent growers joined within a Crop pedigreed stock system is to maintain genetic purity Improvement Association to produce certified seed. through consecutive pedigreed stock seed generations fol- Seed Certification (4, 13, 17, 42) Seed certification is system A controlled lowing appropriate a legalized program that applies the previously men- seed-production system standards of isolation, tioned principles to the of consecutive inspection, and rogu- production of specific seed certification generations with ing (with high costs) at seed-propagated plant cul- A system of seed standards to maintain the initial release with tivars to ensure the main- production utilizing genetic purity leading decreased standards tenance of seed purity. pedigreed stock prin- to commercial (and lower costs) in the The system was estab- ciples, which provides distribution. distribution of com- lished in the United States for legally enforceable mercial seed. The overall program includes three phases. and Canada during the standards of quality Phase 1 includes the development phase, which ends early 1920s to regulate the and genetic purity. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 154

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BOX 5.1 GETTING MORE IN DEPTH ON THE SUBJECT CLASSES OF CERTIFIED SEEDS

Breeder’s seed: that which originates with the sponsoring approved and certified by the certifying agency and plant breeder or institution and provides the initial source designed to maintain satisfactory genetic identity and of all the certified classes. Foundation seed: progeny of purity. Bags of registered seed are labeled with a purple breeder’s seed that is handled to maintain the highest tag or with a blue tag marked with the word “registered.” standard of genetic identity and purity. It is the source of Certified seed: progeny of registered seed (or sometimes all other certified seed classes but can also be used to pro- of breeder’s, foundation, or other certified seed) that is duce additional foundation plants. (Select seed is a com- produced in the largest volume and sold to crop produc- parable seed class used in Canada.) Foundation seed is ers. It is produced under specified standards designed to labeled with a white tag or a certified seed tag with the maintain a satisfactory level of genetic identity and purity word “foundation.” Registered seed: progeny of founda- and is approved and certified by the certifying agency. tion seed (or sometimes of breeder’s seed or other Bags of certified seed have a blue tag distributed by the registered seed) produced under specified standards seed-certifying agency.

commercial production of new cultivars of agricultural plants are maintained either as inbred lines (corn, onion) crops then being introduced in large numbers by state or as vegetatively propagated clones (asparagus). The and federal plant breeders. The principles (as described same standards of isolation as for nonhybrid seed pro- for the pedigreed stock system) and accompanying regu- duction may be required. To mass-produce hybrids, lations of seed maintenance were established through the some system must be used to prevent self-pollination cooperative efforts of public research, extension, regula- and to enforce cross-pollination. Hand pollination is tory agencies, and seed-certifying agencies known as sometimes practiced to produce seed in crops or situa- Crop Improvement Associations, whose membership tions in which the production of seed per flower is very included commercial producers. These organizations high and/or the high value of the seed justifies the were designated by law through the Federal Seed Act expense (Fig. 5–15, page 156). Hand pollination is used (1939) to conduct research, establish production stan- to produce some hybrid flower seeds and in breeding dards, and certify seeds that are produced under these new cultivars (Fig. 5–16, page 157). standards. Individual state organizations are coordinated through the Association of Official Seed Certifying Perennial Sources Agencies (AOSCA) (4) in the United States and Canada. Commercial Sources Seeds for fruit and nut crops Similar programs exist at the international level where historically have been collected more or less successfully certification is regulated through the Organization for from commercial orchards particularly where the spe- Economic Cooperation and Development (OECD). cific cultivar or origin is known. Fruit tree seeds such as The principal objective of seed certification is to apple (Malus), pear (Pyrus), and peach (Prunus) have provide standards to preserve the genetic qualities of a been collected from canneries and dry yards where spe- cultivar. Other requirements of seed quality also may cific commercial cultivars, such as ‘Lovell’ peach, are be enforced as well as the eligibility of individual culti- used. Pure stands of local seedling landscape trees might vars. The seed-certifying agency may determine pro- be used. In several forest-tree species, seed has been col- duction standards for isolation, maximum percentage lected from phenotypically above-average trees in com- of off-type plants, and quality of harvested seed; make mercial plantations. In New Zealand, seed from such regular inspections of the production fields to see that trees is designated “CS” (“climbing selects”) and rated the standards are being maintained; and monitor seed higher in value than the seed from the surrounding processing. trees, but below that of seed orchards or from well- The international OECD scheme includes simi- tested families. lar classes but uses different terms. These include basic Seed-Collection Zones seed-collection zone (equivalent to either foundation or registered seed), A seed-collection zone Naturally occurring zone certified first-generation (blue tag) seed, and second- for forest trees is an area (forest plants) designated generation (red tag) seed. with defined boundaries by elevation, latitude, and Hybrid Seed Production (1) Hybrid cultivars (Fig. 5–14) and altitudinal limits in longitude that identifies a are the F1 progeny of two or more parental lines. Parent which soil and climate specific seed source. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:01 PM Page 155

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Figure 5–14 Hybrid seed corn production. Four inbred lines are produced to be used as parents for cross-pollination utilizing either detasseling (removal of male flower) or a pollen-sterile parent. The resulting F1 plants are used as parents of the next (F2) generation which are then sold for commercial crop production. The individual progeny plants are highly heterozygous, but the population is highly homogeneous, showing high vigor and production. Redrawn from USDA Yearbook of Agriculture 1937, Washington, D.C.: U.S. Government Printing Office.

are sufficiently uniform to indicate a high probability lished in Washington and Oregon and in the central of reproducing a single ecotype. Seed-collection region of the United States. zones, designating particular climatic and geographi- cal areas, have been established in most of the forest- Seed-Production Areas (43) seed-production tree–growing areas in the world (61). California, for A seed-production area area An area of example, is divided into six physiographic and cli- contains a group of plants source plants matic regions, 32 subregions, and 85 seed-collection that can be identified and set specifically utilized zones (Fig. 5–17, page 158). Similar zones are estab- aside specifically as a seed for seed collection. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:01 PM Page 156

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(a) (b)

(c) (d)

(e) (f) Figure 5–15 (a and b) Hybrid pollination in petunia. Hand pollination involves removal of the male anthers (emasculation) before the flower opens; (c and d) followed by transfer of the pollen to the receptive stigma. Pollen is collected and stored dry in small vials at cold temperature. Pollen is transferred to the stigma using a small transfer stick or brush. (e) A successful pollination/fertilization is evident by continued development (red arrow). (f) Seed yield per capsule is high and the seed is very valuable justifying the use of hand labor.

source. Seed plants within the area are selected for their species that would interfere with the operations. desirable characteristics. The value of the area might be Competing trees can be eliminated to provide adequate improved by removing off-type plants, those that do space for tree development and seed production. In not meet desired standards, and other trees or shrub forestry, an isolation zone at least 120 m (400 ft) wide M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:01 PM Page 157

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(a)

(b)

Figure 5–16 F1 hybrid cultivars of many vegetable and flowering annuals are created following large-scale hand pollination in a con- trolled environment. (a) Removing male parts in snapdragon prior to hand pollination. (b) Hand pollination. (c) Pepper fruit (c) prior to harvest for seed extraction.

seed orchard from which off-type origin or source. For example, fruit tree nurseries A planting used in forestry plants are removed maintain seed orchards to produce seeds of specific or in fruit tree nurseries to should be established rootstock cultivars under conditions that will prevent maintain seed sources as around the area. cross-pollination and the spread of pollen-borne seedling populations of viruses. A clonal cultivar such as ‘Nemaguard’ peach selected seed families or Seed orchards Seed is budded to a rootstock, planted in isolation to avoid of a clone (fruit and nut orchards are estab- chance cross-pollination by virus-infected commer- trees) or collections of lished to produce tree cial cultivars, and grown specifically for rootstock clones (forestry). seeds of a particular seed production as part of the nursery operations.

BOX 5.2 GETTING MORE IN DEPTH ON THE SUBJECT TREE CERTIFICATION CLASSES

Certification of forest-tree seeds is available in some seeds carry a yellow tag. Selected: tree seed collected states and European countries similar to that for crop seed from trees that have been selected for promising pheno- (17, 18, 61). Recommended minimum standards are given typic characteristics but have not yet been adequately by the Association of Official Seed Certifying Agencies (4). progeny-tested. The source and elevation must be stated Forest-tree seed have different standards than agricultural (44). These seeds are given a green label. Certified: two seeds. types of seed are recognized. Seeds are from trees grow- Tree certification classes are defined as source- ing in a seed orchard whose genetic potential is based on identified: tree seed collected from natural stands where phenotypic superiority. These are identified by a pink tag. the geographic origin (source and elevation) is known and When seedlings or seeds have been proven to be geneti- specified or from seed orchards or plantations of known cally superior in a progeny test, they are classified as provenance, specified by seed-certifying agencies. These tested and identified by a blue tag. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:01 PM Page 158

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Fruit-tree rootstock clones that are self-pollinated are include a range of genotypes in a suitable arrangement planted in solid blocks. An isolation zone 120 m to ensure cross-pollination and to decrease the effects of (400 ft) wide should be established around the inbreeding. Seven to thirty unrelated genotypes have orchard to reduce pollen contamination from other been recommended to avoid this problem in a purely cultivar sources. The size of this zone can be reduced clonal orchard. if a buffer area of the same kind of tree is present around the orchard. Hybrid seed production involves Nursery Row Selection Sometimes phenotypically planting both the parental clones in adjoining rows unique individuals appear in nursery seed populations under the same condition. planted in the nursery row and can be identified visu- Three general types of seed orchards are used for ally. This is referred to as nursery row selection. forest trees (42, 61): (a) seedling trees produced from Identification requires nursery row selection selected parents through natural or controlled pollina- that the character be A system of selection tion; (b) clonal seed orchards in which selected distinctive in vigor, where specific progeny clones are propagated by grafting, budding, or rooting appearance, or both trees can be identified in cuttings; and (c) seedling-clonal seed orchards in (Fig. 5–11). For exam- the nursery row due to which certain clones are grafted onto branches of ple, Paradox hybrid wal- phenotype. some of the trees. The choice depends on the nut seedlings (Juglans particular strategy used in the seed improvement regia ( J. hindsii) used as a rootstock in California are program. produced by planting seeds of specific seed tree sources A site should be selected for good seed produc- of black walnut (Juglans hindsii). Once germination tion. Forest trees and most other native species should takes place, leaf characteristics, bark color, and greater

Physiographic and climatic region boundaries

Physiographic and climatic subregion boundaries

Zones

Figure 5–17 Seed collection zones in California. The 85 zones are identified by a three-digit number. The first gives 1 of the 6 major physiographic and climatic regions, the second gives 1 of the subregions, of which there are 32, and the third gives the individual zone. 0 = Coast; 1 = South coast; 3 = North coast; 5 = Mountains of Sierra Nevada and Cascade ranges; 7 = Northeast interior; 9 = Valley and desert areas, further divided into the central valley (6), southern California (9), and the desert areas (7). Redrawn from G. H. Schubert and R. S. Adams. 1971. Reforestation Practices for Conifers in California. Sacramento: Division of Forestry. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:01 PM Page 159

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BOX 5.3 GETTING MORE IN DEPTH ON THE SUBJECT PLANT VARIETY PROTECTION ACT

Breeders of a new seed-reproduced plant variety (cultivar) must be stable (i.e., essential characteristics must remain in the United States may retain exclusive propagation unchanged during seed propagation). A certificate is rights through the Plant Variety Protection Act, estab- good for 20 years. The applicant may designate that the lished in 1970 (7), and revised in 1994 (46). The breeder cultivar be certified and that reproduction continue only applies to the U.S. Department of Agriculture for a Plant for a given number of seed generations from the breeder Variety Protection Certificate. For one to be granted, the or foundation stock. If designated that the cultivar be cultivar must be “novel”: it must differ from all known certified, it becomes unlawful under the Federal Seed cultivars by one or more morphological, physiological, or Act to market seed by cultivar name unless it is certified. other characteristics. It must be uniform; any variation The passage of this law has greatly stimulated commer- must be describable, predictable, and acceptable. It cial cultivar development.

vigor of the hybrid seedlings allow identification of the Sometimes the phenotype of the propagated desired hybrid seedlings, whereas the black walnut plant is sufficiently striking as to allow for selection in seedlings are rogued out of the nursery row or separated the nursery. For example, blue seedlings of the at a later date. Genotypic progeny tests based on previ- Colorado spruce (Picea pungens) tend to appear among ous commercial experience indicate which walnut tree seedling populations having the usual green form. sources produce the highest percentages of hybrids, pre- Differences in fall coloring among seedlings of sumably from natural crossing with surrounding Persian Liquidambar and Pistacia chinensis necessitate fall walnut (J. regia) orchards. selection of individual trees for landscaping.

DISCUSSION ITEMS

Propagators of many plant species and cultivars may not 2. What are some major reasons why seed producers be involved directly in the selection and handling of the like to produce hybrid seed lines? seeds used but depend on the skill and knowledge of the 3. What are the differences and similarities among specialized seed industry. Nothing is more important, apomictic, inbred, and hybrid seed lines? however, than using seeds that are true-to-type and true- 4. What is the function of seedling progeny tests in to-name. Consequently, knowledge of the basic princi- seed production? ples and practices that are required to produce genetically 5. Why is the seed origin (provenance) important to pure seeds is important to propagators whether or not users of tree crops? they are directly involved in seed selection. 6. Would it be better to collect seeds from a single woody plant or from multiple plants? 1. From the propagator’s standpoint, why do you think crop plants such as wheat, rye, and barley played such an important role in human history?

REFERENCES 1. Acquaah, G. 2008. Principles of plant genetics 5. Barker, S. C. 1964. Progeny testing forest trees and breeding. Wiley Press. for seed certification programs. Ann. Rpt. Inter. Crop 2. Ahuja, M. R., and W. L. Libby. 1993. Clonal Imp. Assn. 46:83–7. Forestry: I. Genetics and . II. Conservation and 6. Barrett, S. C. H. 2002. The evolution of plant application. New York: Springer-Verlag. sexual diversity. Nature Reviews Genetics 3:274–84. 3. Allard, R. W. 1999. Principles of plant breeding. 7. Barton, J. H. 1982. The international breeder’s 2nd ed. New York: John Wiley & Sons. rights system and crop plant innovation. Science 4. Assoc. Off. Seed Cert. Agencies. Seed certifica- 216:1071–5. tion. http://www.aosca.org M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:01 PM Page 160

160 part two seed propagation

8. Callaway, B., and Callaway. 2000. Breeding 24. Garrison, C. S., and R. J. Bula. 1961. ornamental plants. Portland, OR: Timber Press. Growing seeds of forages outside their regions of use. In 9. Cameron, J. W., R. K. Soost, and H. B. Frost. Seed yearbook of agriculture. Washington, DC: U.S. 1957. The horticultural significance of nucellar embry- Govt. Printing Office, pp. 401–6. ony. In J. M. Wallace, ed. Citrus virus diseases. Berkeley: 25. Geneve, R. L. 2006. Alternative strategies for Univ. Calif. Div. Agr. Sci. pp. 191–96. clonal plant reproduction. Comb. Proc. Intl. Plant Prop. 10. Charlesworth, D., X. Vekemans, V. Castric, Soc. 56:269–73. and S. Glemin. 2005. Plant self-incompatibility sys- 26. Goldsmith, G. A. 1976. The creative search tems: A molecular evolutionary perspective. New for new F1 hybrid flowers. Proc. Intl. Plant Prop. Soc. Phytologist 168:61–9. 26:100–3. 11. Chase, C. D. 2006a. Cytoplasmic male 27. Haddock, P.G. 1968. The importance of prove- sterility: A window to the world of plant mitochondrial– nance in forestry. Proc. Intl. Plant Prop. Soc. 17:91–8. nuclear interactions. Trends Genetics 23:81–90. 28. Hanna, W. W., and E. C. Bashaw. 1987. 12. Chase, C. D. 2006b. Genetically engineered Apomixis: Its identification and use in plant breeding. male sterility. Trends Plant Sci. 11:7–9. Crop Science 27:1136–9. 13. Cowan, J. R. 1972. Seed certification. In 29. Harlan, J. R. 1992. Crops and man, 2nd ed. T. T. Kozlowski, ed. Seed biology, Vol. 3. New York: Madison, Wis.: Amer. Soc. Agron. Crop Sci. Academic Press. pp. 371–97. 30. Hartmann, H. 1961. Historical facts pertain- 14. Darwin, C. 1862. On the two forms, or ing to root and trunkstocks for pear trees. Oreg. State dimorphic condition, in the species of Primula, and on Univ. Agr. Exp. Sta. Misc. Paper 109:1–38. their remarkable sexual relations. J. Proc. Linnaean Soc. 31. Heit, C. E. 1964. The importance of quality, () 6:77–96. germinative characteristics and source for successful 15. De Nettancourt, D. 1993. Self- and cross- seed propagation and plant production. Proc. Intl. incompatibility systems. In M. D. Hayward, Plant Prop. Soc. 14:74–85. N. O. Bosemark, and I. Romagosa, eds. Plant breeding: 32. Hiscock, S. J., and D. A. Tabah. 2003. The dif- Principles and prospects. London: Chapman and Hall. ferent mechanisms of sporophytic self-incompatibility. 16. Dirr, M. A., and C. W. Heuser, Jr. 1987. The Philos. Trans. R. Soc. Lond. B. Biol. Sci. 358:1037–45. reference manual of woody . Athens, 33. Jones, R. W. 1969. Selection of intercompati- Ga.: Varsity Press. ble almond and root knot nematode resistant peach 17. Edwards, D. G. W., and F. T. Portlock. 1986. rootstocks as parents for production of hybrid Expansion of Canadian tree seed certification. For. rootstock seed. J. Amer. Soc. Hort. Sci. 94:89–91. Chron. 62:461–66. 34. Kester, D. E., and C. Grasselly. 1987. 18. Ehrenberg, C., A. Gustafsson, G. P. Forshell, Almond. In R. C. Rom, and R. Carlson, eds. Rootstocks and M. Simak. 1955. Seed quality and the principles of for fruit trees. New York: John Wiley & Sons. forest genetics. Heredity 41:291–366. 35. Langlet, O. 1962. Ecological variability and 19. Esquinas-Akacazar, J. T. Plant genetic of forest trees. In T. T. Kozlowski, ed. Tree resources. In M. D. Hayward, N. O. Bosemark, and growth. New York: Ronald Press pp. 357–69. I. Romagosa, eds. Plant breeding: Principles and 36. Lasa, L. M., and N. O. Bosemark. 1993. prospects. London: Chapman and Hall. Male Sterility. In M. D. Hayward, N. O. Bosemark, 20. Flint, H. 1970. Importance of seed source to and I. Romagosa, eds. Plant breeding. Principles and propagation. Proc. Intl. Plant Prop. Soc. 20:171–78. prospects. London: Chapman and Hall. 21. Franklin-Tong, V. E., and F. C. H. Franklin. 37. Laverack, G. K., and M. R. Turner. 1995. 2003. The different mechanisms of gametophytic self- Roguing seed crops for genetic purity: A review. Plant incompatibility. Philos. Trans. R. Soc. Lond. B. Biol. Sci. Varieties and Seeds 8:29–45. 358:1025–32. 38. Libby, W. J., and R. M. Rauter. 1984. 22. Frey, K. J. 1983. Plant population manage- Advantages of clonal forestry. For. Chron. pp. 145–49. ment and breeding. In D. R. Wood, K. M. Rawal, and 39. Lindgren, D., J. Cui, S. G. Son, and M. N. Wood, eds. Crop breeding. Madison, Wis.: J. Sonesson. 2004. Balancing seed yield and breeding Amer. Soc. Agron. and Crop Sci. Soc. Amer. value in clonal seed orchards. New Forests 28:11–22. pp. 55–88. 40. Macdonald, B. 1986. Practical woody plant 23. Ganders, F. R. 1979. The biology of propagation for nursery growers, Vol. 1. Portland, OR: heterostyly. New Zealand J. Bot. 17:607–35. Timber Press. M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:01 PM Page 161

principles and practices of seed selection chapter five 161

41. Maynard, D. N., and G. J. Hochmuth. 1997. 51. Savidan, Y. 2000. Apomixis: Genetics and Knott’s handbook for vegetable growers. New York John breeding. Plant Breeding Rev. 18:13–86. Wiley & Sons. 52. Sax, K. 1949. The use of Malus species for 42. McDonald, M. B., Jr., and W. D. Pardee, eds. apple rootstocks. Proc. Amer. Soc. Hort. Sci. 1985. The role of seed certification in the seed industry. 53:219–20. CSSA Spec. pub. 10. Madison, WI: Crop Sci. Soc. 53. Schopfer, C. R., M. E. Nasrallah, and Amer., ASA. J. B. Nasrallah. 1999. The male determinant of self- 43. Millar, C. I., and W. J. Libby. 1991. Strategies incompatibility in Brassica. Science 266:1697–700. for conserving clinal, ecotypic, and disjunct population 54. Seed Savers Exchange. 2001. Web site http:// diversity in widespread species. In D. A. Falk and K. E. www.seedsavers.com. Holsinger, eds. Genetics and conservation of rare plants. 55. Sprague, G. F. 1950. Production of hybrid Oxford: Oxford University Press. corn. Iowa Agr. Exp. Sta. Bul. P48. pp. 556–82. 44. National Tree Seed Laboratory. 2001. Seed 56. Teshome, A., A. H. D. Brown, and T. Hodgkin. certification. http://www.ntsl.net/Ntsl_dcert.htm. 2001. Diversity in landraces of cereal and legume 45. Pearson, O. H. 1968. Unstable gene systems crops. Plant Breeding Rev. 21:221–61. in vegetable crops and implications for selection. 57. Weng, Y. H., K. Tosh, G. Adam, M. S. HortScience 3(4):271–74. Fullarton, C. Norfolk, and Y. S. Park. 2008. Realized 46. Plant Variety Protection Office. 2001. genetic gains observed in a first generation seedling Mission and general information. Web site http://www.as seed orchard for jack pine in New Brunswick, Canada. .usda.gov./science/pvp.htm. New Forests 36:285–98. 47. Poehlman, J. M. 1995. Breeding field crops, 58. Westwood, M. N. 1994. Temperate zone 4th ed. Westport, CT: AVI. pomology, 3rd ed. Portland, OR: Timber Press. 48. Raven, P. H., R. F. Evert, and S. E. Eichhorn. 59. Whealy, K. 1992. Garden seed inventory, 2005. Biology of plants, 7th ed. New York: Freeman/Worth. 3rd ed. Decorah, IA: Seed Saver Publications. RR 3, 49. Rom, R. C., and R. F. Carlson, eds. 1987. Box 239. Rootstocks for fruit crops. New York: John Wiley & Sons. 60. Young, J. A., and C. G. Young. 1992. Seeds of 50. Routley, M. B., R., I. Bertin, and B. C. woody plants in North America. Portland, OR: Husband. 2004. Correlated Evolution of Dichogamy Dioscorides Press. and Self-Incompatibility: A phylogenetic perspective. 61. Young, J. A., and C. G. Young. 1986. Seeds of Intl. J. Plant Sci. 165:983–93. wild land plants. Portland, OR: Timber Press.