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