This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. CHAPTER 2 Generalized Life Cycle

Differences between life cycles, particularly phe- nological shifts, constitute an important basis for taxo­ nomic distinctions in Arceuthobium. Here we discuss the salient features of the dwarf mistletoe life cycle, excluding the more detailed aspects of sexual repro­ duction, which are treated separately in chapter 3. Shoots, flowers, and fruits of Arceuthobium are illus­ trated in figures 2.1-2.4. The critical features of the life cycle of a representative species CA. americanum) are shown in figure 2.5.

Figure 2.2 -Flowers and fruits of Arceuthobium americanum. A: terminal portion of staminate shoot showing mature flower buds; B: terminal portion of staminate shoot with open 3-merous flowers; C: terminal portion of pistillate shoot showing flowers shortly after pollination (upper portion of right branch) and developing fmits (left branch) that are about 1 year old and still require approximately 3 months to complete maturation and initial dispersal. (B. Velick).

The life cycles of the following species have been studied in some detail: Arceuthobium abietinum-Scharpf and Figure 2.1 -Shoots of Arceuthobium. A: young shoots showing the Parmeter 1967, 1976, 1982. decussate arrangement of the internodes; B: older shoots showing elongated internodes and branching; C: typical flabellate (fan-like) Arceuthobium americanum-Gilbert and branching pattern of most New World species; D: verticillate Punter 1990, 1991; Hawksworth 1965b; (whorled) branching pattern of Old World and a few New World Hawksworth and Johnson 1989a. species. All species show the primary branching type (A and B), but most species also develop secondary branching, which may be Arceuthobium campylopodum-Roth 1959, either flabellate (C) or verticillate (D). Wagener 1962. Arceuthobium chinense-Tong and Ren 1980. Arceuthobium douglasii-Wicker 1965, 1967a. Arceuthobium laricis-Smith 1966a; Wicker 1965, 1967a.

Generalized Life Cycle 7 Chapter2

(fig. 2.4); the dispersed seed sticking to a needle by means of its viscin coating (fig.2.1OA); the dispersed seed sliding down the needle in the hygroscopically endosperm pedicel expanded viscin mass (fig. 2.10B); the seed and viscin at the base of a needle (fig. 2.10C)j the germinated seed and elongated hypocotyl that has developed a I~ holdfast from which infection can occur (fig. 2.11); the microscopic ~penetrat ion wedge entering the host tis­ sue (fig. 2.12); and young shoots emerging from the swollen tissue of a new infection (fig. 2.13). viscin cells pericarp \ \ ./" Seed Dispersal and Intercepltion Our discussion of the dwarf mistletoe life cycle begins with the seed. lne fruit normally contains a Figure 2.3 -Mature froit and seed of Arceulhobfllm. Diagrammatic single seed with one embl)'o, but fruits may rarely cross~section through a mature fruit (left) and fruit disdurging its contain two seeds or a single seed with two embl)'os seed (right). (8. Velick) ~eir1914,Hawkswonh 1961b). Dwarfmistletoes possess one of the most effective hydrostatically con­ trolled, explosive mechanisms of seed dispe.rsal known in flowering plants (fig. 2.4) (Hinds and others 1963, Hinds and others 1965; van der PijI1972). The only exception to this mode of dispersal among the dwarf mistletoes is exhibited by Arceuthobium vertidl­ lijIorum, which has the largest seeds in the g'enus (11 x 6 rom). In fact, these seeds are greater than l'wice the size of those of any other dwarf mistletoes. Fruits of A. verticillijlorum exhibit a weak explosive me,:::hanism that accomplishes little more than removing the peri­ carp and exposing the seed. Seeds of A. verticillijIo­ rum are undoubtedly dispersed primarily by birds. Among temperate species of dwarf mistletoes, seeds are explosively discharged during late summer Figure 2.4 -8I:plosive seed discharge inArcel4lhobillm. Seed is in flight immediately after db;charge. (photograph taken at 5-milliomhs at velocities of about 27 m per second (Hinds and oth­ ofasecond, Hinds and others 1963) ers 1963) (fig. 2.4). Maximum dispersal distance is about 16 m, but dispersal distances of 10 m or less are Arceuthobium oxycedri-Heinricher 1915a, more typical. Most seeds are intercepted within 2 to 1915b, 1924; Rios Insua 1987_ 4 m by host needles. Seeds have a viscous coating Arceuthobium pusillum-Baker 1981; Baker (viscin) that readily adheres to any object rh(!y strike, and French 1979, 1986; Baker and others especially conifer needles (fig. 2.10A) (Roth 1959, Hawkswonh 1965b). Intercepted seeds usually 1981, 1985. remain on the needles until the first fall rain wets the Arceulhobium tsugense-Carpenrer and others hygroscopic viscin (fig. 2. 10B). Gravity then pulls the 1979; Shaw 1982b; Shaw and Loopstra 1991; well-lubricated seed to the base of an upright needle Smith 1966., 1966b, 1971. (fig. 2.1 OC). As the viscin dries, the seed is wmented to Arceuthobium vaginatum subsp. cryptopodum­ the shoot surface. Seeds intercepted by downward­ Hawkswonh 1961a, 1965b. pointing needles generally fall to lower branches or to Various features of the life cycle are illustrated the ground (Shaw and Loopstra 1991). To achieve photographically: pistillate plant with mature fruit and infection, seeds must lodge on shoot segments usually characteristic swelling at the point of infection (fig. less than 5 years old; only this portion of the host can 2.6); mature staminate flowers at anthesis with moist­ be considered a "safe-site," i.e., a place where a seed ened nectaries (fig. 2.7); pistillate flowers at anthesis can germinate and establish an infection. with pollination droplets (fig. 2.8); mature fruit ready for dispersal (fig. 2.9); explosive dispersal'of the seed

8 Generalized Life Cycle Chapter 2

developing ! - fruit seeds washed seed dispersal and to twigs by interception by rain \ ~ pine needles ~ .~ Y1J fertilization megasporogenesis1

pollination

penetration

m ic rosporogenes~ is '-

first shoots

Figure 2.5 -Generalized life cycle of dwarf mistletoe as exemplified by Arceuthobium americanum on Pinus contorta.

Studies of three species of dwarf mistletoes indi­ remaining seeds will be discharged inward. Only 40% cate that about 40% of dispersed seeds are intercepted of the outwardly dispersed seeds will escape from the by trees (Hawksworth 1965b, Smith 1985). Seed inter­ host crown. Dwarf mistletoe shoots located closer to ception rates, however, are highly variable and depend the interior of the crown will disperse only 20 to 30% of on stand structure and composition, position of the their seeds out of the crown (Smith 1985). Because dwarf mistletoe on the host, and needle characteristics few seeds escape the host crown, secondary infection of the host tree. For example, an adjoining tree within is common, and int~nsification proceeds rapidly once 2 to 3 m of an infected host will intercept about 90% of a host tree becomes infected. the seeds dispersed in its direction. From an infection Many intercepted seeds are not deposited at safe­ site on the outer edge of a host crown, about 70% of sites, and a high proportion of those that do arrive at discharged seeds will be dispersed outward, and the safe-sites are lost to disease, predation, or removal by

Generalized Life Cycle 9 Chapter 2

Figure 2.6 -Arceulhobium lsugense subsp. I$ugense on PimtS con­ Figure 2.8-Pistillat,e plant of Arceulhobfum cyanoca1pum at anthe­ lorta var. coII/orla. Shoots of pistillate plant with m:lture fruit; note sis; note the pollination droplets at the tips of the flowers. characteristic branch swelling at the point of infection. (D. L. Nickrent)

Figure 2. 7 -Staminate flowers of Arcelllhobium ameriCflIl1lm; note Figure 2.9-Mature fruit of Arceuthoblum bfcarlnatum ready for the moist, glistening surfaces of the tripartite central nectary. explosive seed discharge; note old sepals and stigma at the distal end of fruit.

10 Generalized Life Cycle Chapter2

\ . \. ... ~ . . ~ '. " ~' e- '" '-.. . , -. \/ ''''', '''-._.,-. ") ' . "'. . '" -.....,' ...... ".,'. ... .' .. ~ ~ ', ...... • "'~~. '. ... . / . ' '" Q ",',.." . .. ..,' ' ."_;;-;.j,.~''''-. .. ?.' ••: ., '. . .. . ;.'.~-: ",-,~;'r:~,- ...... , .. '.~. . _ .'.' "', .C"'· . •_ ".~ .. _ ... .. - . "-' - , .,;..: ,~ ..~;:~. )., "4.,1:.' ...... _...... ' ..... '...... ~-l..~ ~ _','.". , , .. ~...... " ,,, .- ~..... -.. . - .~''''' \ ~- ...... "' . .' .• e'!t.;...... •: ....-'['"" . ••..•-.... Figure ZII--Germinated seed of Arceuthobium abietinum with extended radicle and temlinal disc-like holdfast. (R F. Scharpf)

Figure 2.10-<:aplUre and settlement of a seed of Arceuthobium abfeltnum on a shoot of Abies sp. A:. Seed adhering to a needle by means of itS dried viscin coating. B: seed sliding down needle in the Figure Z12 -Microphotograph of a penetf3tion wedge of hygroscopically swollen viscin mass. C seed and hygroscopically Arceulhobillm abietinum entering host tissue from the holdfast to enlarged viscin mass at the base of a needle where penetration of the initiate infection. (R F, Scharpf) host can occur. (R F. Scharpf)

Generalized Life Cycle 11 Chapter2

Germination consists of little more than the initia­ tion of meristematic activiry at the radicular apex. The role of moisture in germination varies among species. Germination is vi:rrually independent of humidity in Arceuthobium abietinum (Scharpf 1970), but free water greatly enhances embryo growth inA. pusillum (Bonga 1972). In temperate zones, seeds typically ger­ minate with the onset of higher temperatures in the spring. Optimal E!mbryo elongation occurs from 15 to 20 0 C (Gill and Hawkswollh 1961, Scharpf 1970). light significantly enha nces germination of mistletoe seeds in general (Lamont 1983a) and that of several dwarf mistletoe species in pallicular (Scharpf 1970, Wicker 1974,). Field germinabiliry in excess of 90% is reported for Arceuthobium americanum, A. vaginatum subsp. Figure 2.13 -Young shoots of Arcellihobillm ablells-religlO$oe cryptopodum (Hawksworth 1965a). and A. abietinum emerging from a swollen branch ofAbles religlosa. (D. L Nlckrem) (Scharpf and Parmeter 1982). For other dwarf mistle­ toes, however, field germinabiliry is apparently much rain. Less than 10% of seeds reach safe-sites (Hawks· lower: A. pusil/um-7% (Baker and others 1979) to 25% worth 1965b), and less than 5% of these establish new (French 1968); A. tsugense-45% (Carpenter and oth­ infections (Hawksworth 1961b, Wicker 1967a). ers 1979), 3% (Smith 1965), 23% (Smith 1985), to 38% (Shaw and Loopstra 1991). Environmental factors Within infested stands in Colorado, Arceuthobium undoubtedly playa strong role in germination. D. L americanum and A: vaginatum subsp. cryptopodum Nickrent (personal communication) indicates that produce 0.9 to 1.3 million dwarf mistletoe seed" per mature seeds rypiically exhibit high percentages of ger­ hectare (Hawksworth 1965b). Smith (1973) estimated mination under laboratory conditions. that a single Tsuga heterophyJla tree infected by A. tsugense produced 73,000 dwarf mistletoe seeds annu­ The time of germination of many species is poorly ally, and Wicker (1967a) calculated that A. campy/opo­ known, but most temperate species germinate in the dum on Pinus ponderosa produced an average of spring following fall dispersal. However, a few 32,000 (range 800 to 2.2 million) seeds per tree. species (notably Arceuthobium vaginaturn subsp. cryptopodum and A. guaternalense) germinate imme­ The large number of seeds produced compen­ diately after seed dispersal in the autumn. Seeds of sates fOf the high proportional loss of seeds before some tropical Mexican and Central American species infection. Consequently, explosive dispersal is a suffi­ may also genninme soon after dispersal at the end of ciently effective mechanism for short distances so that the wet season (late August to early September). dwarf mistletoes can spread rapidly within infested stands (Hawkswollh 1965a). Beyond the explosive Dormancy in the traditional sense does not exist in range of the dwarf mistletoe fruits, however, animal dwarf mistletoes. Seeds stored under optimal condi­ vectors are required for dispersal (chapter 8), tions retained 58/% germinabiliry for 15 months (Knutson 1969, 1984); under laboratory conditions, some seeds remained viable up to 4 years (Beckman Germination and Roth 1968). In the field, however, there is no evi­ dence that seeds survive longer than the season fol­ The seeds of most dwarf mistletoes have few char­ lowing dispersal. Likewise, traditional "after-ripening" acteristics that are typical for seeds of flowering plants. is not characteristic of dwarf mistletoe seeds (Scharpf Because there are no true ovules in either the 1970, lamont 1983a), although varying periods of Viscaceae or Loranthaceae, there are no testa and con­ stratification did (mhance germination for some sequently no true "seeds." The "seed" is an embryo species (Wicker :I 965, Holmes and others 1968). embedded in a chlorophyllous endosperm surround­ Substrate appears to play no role in germination. ed by a layer of viscin. The embryo is green, rod­ shaped, and only several millimeters long; and it pos­ Perhaps the most unusual feature of all viscaceous sesses a meristematic radicular apex without a root seeds is the chlofophyllous endosperm. Although the cap. The coryledons are vestigial. embryo is also chlorophyllous, this condition is com­ mon for plant elnbryos. The growing hypocoryl pos­ sesses stomata, a:nd the seed is photosynthetic (Tocher 12 Generalized Life Cycle Chapter 2

and others 1984). Lamont (l983a) suggests that simple (negative phototropism), irrespective of the gravita­ sugars produced photosynthetically are a more effi­ tional considerations (neutral geotropism). When the cient source of energy for radicular growth than the radicular apex encounters an obstruction such as a complex carbohydrates typical of storage endosperm. needle base, it responds (positive thigmotropism) by The autotrophic capability of germinating seeds of developing a rounded structure termed a "holdfast" dwarf mistletoes undoubtedly increases their longe­ (Bonga 1969b) (fig. 2.11). vity beyond the availability of stored nutrients and, The center of the holdfast then develops a region therefore, increases the likelihood of infectio,n. Of of intense meristematic activity known as a "penetra­ course, growth of the hypocotyl is ultimately limited, tion wedge" (fig. 2.12). The penetration wedge grows and only those seeds that germinate within 5 mm of into the host cortex by exerting mechanical pressure susceptible young shoots are likely to cause infection. (Scharpf 1963, Scharpf and Parmeter 1967). After the penetration wedge has entered the cortex, a rootlike endophytic system ramifies throughout the bark. Infection and Initial Shoot Those portions of the endophytic system that subse­ Development quently become embedded in successive layers of 'xylem are described as "sinkers" (chapter 11). Infection is the equivalent of seedling establish­ ment among terrestrial flowering plants. Successful Once infection is established, an incubation peri­ infection by a dwarf mistletoe requires penetration of od of 2 to 5 years elapses before young shoots appear the host cortex by a growing embryo and develop­ (fig. 2.13; see also 16.112 and table 2.1). A swelling at ment of an endophytic system. For most combinations the point of infection usually precedes shoot produc­ of the host and dwarf mistletoe, infection can take tion by a year or more. The incubation period or laten­ place only through young stem tissues, usually a seg­ cy between infection and appearance of shoots (or ment less than 5 years old. Arceuthobium ameri­ swellings) varies by species of dwarf mistletoe, species canum, however, can penetrate through the thin, of host, and various environmental factors. For exam­ chlorophyllous bark of Pinus contorta branches as old ple, in British Columbia, about half of the infections of as 60 years (Hawksworth 1954). Arceuthobium tsugense produced shoots in the sec­ ond year after infection and an additional third pro­ The growing radicular apex has a unique combi­ duced shoots the following year (Smith 1971), where­ nation of tropistic responses that promote infection. as the incubation period extends 3 to 6 years in Alaska The radicular apex typically grows toward the low (Shaw and Loopstra 1991). The incubation period in light intensities that characterize the surface of the host other species lasts for 4 years-A. pusillum (Baker and

TABLE 2.1- Incubation times for 6 taxa of Arceuthobium frQm infection to production of the first shoot

Percent of inoculations producing shoots Arceuthobium 1 yr 2 yrs 3yrs 4yrs 5yrs 6 yrs 7yrs 8yrs 9 yrs 10 yrs 11 yrs 12 yrs

A. abietinum f. sp. concoloris 0 0 19 31 12 9 11 9 3 3 0 3 A. abietinum f. sp. magnificae 0 0 39 25 11 9 4 6 3 3 0 0 A. americanum 0 2 32 34 19 8 4 1 0 0 0 0 A. campylopodum 0 0 23 62 11 4 0 0 0 0 0 0

A.~ugensesubsp.~ugense 0 51 33 6 6 6 0 0 0 0 0 0 A. varginatum subsp. cryptopodum 0 1 30 32 30 4 2 1 0 0 0 0

Note: Data for A. abietinum f. sp. concoloris and f. sp. magnificae from CA (Scharpf and Parmeter 1982), A, americanum from CO (Hawksworth and Johnson 1989), A. campylopodum from CA (Wagener 1962), A. tsugense subsp. tsugense from BC (Smith 1971), andA. vaginatum subsp. crytopodum fromAZ (Hawksworth 1961a).

Generalized Life Cycle 13 Chapter2 others 1981); for 6 years-A. campy/opodum (Wagener Flower and Fruit Production 1962) andA. tsugense (Shaw and Loopstra 1991); for 8 Meiosis may either occur immediately before years-A. americanum (Hawksworth and Johnson flower production (direct flowering) or approximately 1989a) and A. vaginatumsubsp. cryptopodum 5 to 8 months before anthesis (indirect flowering) (Hawksworth 1961a); for 10 years-A. abielinum f. sp. (Wiens 1968). Most species exhibit definite annual magnificae (Scharpf and Parmeter 1982); or as long as flowering periods, but a few tropical species (e.g., 12 years-A. abietinum f. sp. concoloris (Scharpf and Arceulhobium aureurn subsp. aureum) appear to Parmeter 1982). Dwarf mistletoe plant.. begin to flower continuously throughout the year. Arceuthcr tlower 1 or 2 years after the initial shoot.. appear. blUm abieUs-religlosae and A. nigntrn (both species Dwarf mistletoes are typically parasites on branch­ found in Mexico) display two distinct flowering peri­ es or main stems of conifer trees, but they rarely occur ods, andA. hawksworlhii (found in Belize) may pro­ on roots. Known instances of root parasitism include duce three flower crops annually (Wiens and Shaw Arceuthobium occidentale on digger pine in California 1994). Arceuthobium juniperiprocerae (found in East (reported by Scharpf in Kuijt 1969a), A. globosum Africa) appears to produce several discrete flower subsp.grandicaule on pines in Guatemala (Steyer­ crops annually. Flowering may occur as early as mark MICH 36940 and our observations in central February or March (e.g., by A. globosum) or as late as Mexico), A. vaginatum subsp. vagina tum on Pinus November-January (e.g., by A. occidentale). For a hartwegii in Mexico (Vasquez 1991), andA. vagina­ given species and locality, flowering usually lasts 4 to tum subsp. cryptopodum on ponderosa pine in 6 weeks, but most of the pollen is dispersed within a Arizona (our observations) (fig. 2. 14). These cases are shorter, 2- to 3-week period. abnormal, however, and result from vegetative growth The staminate flowers and terminal portions of the into the roots from infections that originated on a main shoots are usually shed a few weeks after anthesis. stem near the root collar. This phenomenon is not However, the entire staminate flowering spikes of comparable to root parasitism in typical terrestrial Arceuthobiurn verticillijJorum dehisce following mistletoes of the Loranthaceae (Gaiadendron, anthesis. Individual shoots o f most species produce Nuytsia, Alkinsonia), where infection takes place ini­ crops of flowers over several successive years; A. pusil­ tially through the roots (Kuijt 1969a). lurn produces a Single crop. There were early reports that dwarf mistletoe shoot.. die after fruits mature (Peirce 1905, Korstian and Long 1922), but this is not typical of species other than the diminutive A. pusil/um and perhaps A. minutissimum. Most shoots produce at least two successive crops of flowers. In Colorado, individual pislillate shoots of both A. americanurn and A. vaginatum subsp. cryptopodum have produced successive fruit crops for 5 years (making these shoots at least 7 years old). Kuijt (1970) also reports that sev­ eral species have relatively long-lived shoots. The time required from pollination to fruit maturity varies considerably. Arceuthobium plisillum and per­ haps some tropical species (e.g., A.juniperi-procerae) require about 5 months fo r fruit to develop. Fruit mat­ uration may occur in about 4 months in A. hawkswor­ Figure 2.14 -ArU!/I/hobillm vagina/mil suOOp. crylopodllm infect­ thii from Belize. Most temperate species need one or ing the roots of PimlS /xmtierosa. more years for fruit to mature; A. gil/Ii requires 19 months. The minimum time from infection to initial seed production averages 6 years for A. americanum (Hawksworth and Johnson 1989a) and 7 to 8 years for A. vagina/urn subsp. cryptopodurn (Hawksworth 1961a).

14 Generalized Life Cycle