M05_DAVI4493_08_SE_C05.qxd 8/9/10 8:00 PM Page 140 5 Principles and Practices of Seed Selection INTRODUCTION learning objectives Many annual and biennial crop, forage, vegetable, and ornamental selec- • Define breeding systems. tions are produced by plant breeding to be propagated by seed (7, 8, 29, • Categorize seed-propagated 34). Breeding involves selection of parents, specific breeding procedures, cultivars 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 plants 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, fruit, 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 agriculture, horticulture, 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 principles and practices of seed selection chapter five 141 self-pollination paired alleles at many flowers open to permit cross-pollination between flow- A breeding system in chromosome loci and ers and produce offspring (seeds) 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 garden 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 cultivar 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 nature, 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 142 142 part two seed propagation 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-pollinations. 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 principles and practices of seed selection chapter five 143 does not ensure cross-pollination but reduces the ratio of self- to cross-pollinated flowers (40).