Amer. J. Bot. 75(9): 1275-1285. 1988.

EFFECTS OF POLLEN AND RESOURCES ON SEED NUMBER AND OTHER FITNESS COMPONENTS IN ARBOREA (: MALOIDEAE) 1

DAVID L. GORCHOV2 Department of Biology, University of Michigan, Ann Arbor, Michigan 48109

ABSTRACT Most Amelanchier arborea flowers have 10 ovules, but the number of filled (embryo-con­ taining) seeds per is usually less than 10 and is highly variable within each individual . Because fruit developmental time correlates with seed number, this variation in seed number results in asynchronous fruit ripening. Field experiments tested whether seed number was pollen- or resource-limited. Manipulation of resources in the shoot at the time of fruit initiation by defoliation, girdling, fruit thinning, or foliar feeding had no significant effect on seed number per fruit, although fruit set and seed weight were affected. Supplemental cross­ also had no demonstrable effect on seed number. Most ovules that do not become filled seeds are visible as small "undeveloped seeds"; these are not necessarily aborted seeds as this was also the fate of ovules in unpollinated carpels. Alternative hypotheses for the deter­ mination of seed number are proposed and discussed.

THE NUMBER OF SEEDS that mature per fruit is few developing seeds are more likely to abort variable in many plant species and has several than those with many seeds (see reviews by important ecological, evolutionary, and hor­ Stephenson, 1981; Stephenson and Bertin, ticultural implications. Seed number has been 1983). By aborting with low seed num­ studied as a component ofyield (or fitness) for bers, may improve the average quality its importance to both the adjustment of re­ of their offspring, either by eliminating fruits productive output under varying environmen­ resulting from self-fertilization (Darwin, 1876, tal conditions (Wilbur, 1977; Primack, 1978; pp. 398-400) or in which there has been little Wyatt, 1981; Schemske and Pautler, 1984; Ste­ pollen competition for ovules (Lee, 1984). phenson, 1984; Garwood and Horvitz, 1985; Seed number can affect seed dispersal in sev­ Marshall, Levin, and Fowler, 1985, 1986; Ga­ eral ways. Seed number is correlated with ma­ len and Weger, 1986) and the improvement of ture fruit size in many wild (Willson and agricultural crop yield (Olsson, 1960; Adams, Schemske, 1980; Bookman, 1983; Garwood 1967; Harper, 1977). In addition to its direct and Horvitz, 1985; Gorchov, 1985; Bronstein, contribution to total plant reproductive out­ 1986) as well as cultivated (Crane, 1964, p. put, seed number per fruit often influences the 303; Wareing and Phillips, 1978, p. 123) proportion of flowers that develop into fruits species; dispersers sometimes discriminate (fruit set). For several plant species, fruits with among fruits on the basis ofsize (Wheelwright, 1985). I Received for publication 22 June 1987; revision ac­ The ratio ofpulp weight to seed weight usu­ cepted 8 December 1987. I am grateful to K. Childs, J. Larsen, L. Whittingham, ally decreases with increasing seed number(e.g., B. Crespi, and K. Olsson for helping with field work, W. Willson and Schemske, 1980; Janzen, 1982). Frasch and J. Palmer for use of equipment, and D. Bay Because pulp is reward and seed is ballast to for photography. C. Campbell examined ovules for signs fruit-eating animals that disperse seeds, pulp: of apomictic seed development. M. Cruzan, L. Nooden, seed ratio should influence dispersers' prefer­ J. Hancock, and E. Hanson advised me on experimental methods. J. Bronstein, G. Estabrook, B. Rathcke, D. Gold­ ences (Howe and Vande Kerckhove, 1980; berg, B. Barnes, W. A. Robinson, and an anonymous re­ Herrera, 1981a, b). Similarly, seed number in­ viewer provided useful comments on earlier drafts ofthis fluences dispersal distance of some wind-dis­ manuscript. R. Kushler, G. Estabrook, and R. Bush aided persed species via its effects on mass/volume in data analysis. The Museum of Zoology, University of Michigan, provided access to the E. S. George Reserve ratio (Casperand Wiens, 1981; Augspurgerand and two ESGR scholarships. Additional support was pro­ Hogan, 1983; Augspurger, 1986). vided by Rackham Block grants from the University of Seed number may also affect the number of Michigan. This work represents part ofa dissertation sub­ seedlings germinating in one place, and there­ mitted in partial fulfillment ofthe requirements for a Ph.D. at the University of Michigan. fore sibling competition (Casper and Wiens, , Current address: Dept. of Biology, Princeton Univer­ 1981). Predispersal seed predators may prefer sity, Princeton, NJ 08544. fruits with more seeds (Herrera, 1984); where

1275 1276 AMERICAN JOURNAL OF BOTANY [Vol. 75 they don't have such a preference, spreading Cantrall (1943), and Cooper (1958). All Ame­ seeds few to a fruit reduces seed loss (Delph, lanchier arborea trees chosen for this study 1986). were from a dense population in oak-hickory Seed numbercan also affect dispersal through forest within 15 m ofa 270 m stretch ofa dirt its effects on ripening phenology. Within in­ road that runs along the top ofan esker (ridge). dividual plants ofsome species, the number of seeds is strongly negatively correlated with the duration of fruit development and with rip­ Study species-i-Amelanchier arborea (Michx. ening date (Aalders and Hall, 1961; Dempsey f.) Fern. (Rosaceae: Maloideae), Downy Ser­ and Boynton, 1965; Brewerand Dobson, 1969; viceberry, is a large shrub to small tree com­ Varga and Bruinsma, 1976; Lee and Bazzaz, mon in dry and mesic sites in eastern North 1982; Gorchov, 1985). For these plants, vari­ America (Fernald, 1950; Barnes and Wagner, ance in seed number results in ripening that is 1981). Species identifications were made using much more asynchronous than flowering (Gor­ Voss (1985); voucher specimens were depos­ chov, 1985). Thompson and Willson (1979) ited at the University ofMichigan Herbarium. proposed that the asynchronous ripening of Amelanchierarborea is one ofthe first shrubs temperate, summer-fruiting, -dispersed to bloom in the spring (late April--early May) plants is an adaptation to avoid satiation of at the study site; the white flowers open syn­ seed dispersal agents. chronously (90% offlowers on each plant open In the course oftesting Thompson and Will­ in two to five days, Gorchov, 1987) and are son's (1979) hypothesis, I became interested visited primarily by small (mostly An­ in the proximate (mechanistic) causes ofasyn­ drenidae and Halictidae) but also by other Hy­ chronous ripening. In the case ofAmelanchier menoptera, Diptera, and Coleoptera. arborea, a summer-ripening small tree with Flower morphology is similar to that de­ bird-dispersed seeds, flower-fruit interval is scribed for A. laevis (Campbell et al., 1985) and highly negatively correlated with the number A. alnifolia (Olson and Steeves, 1982): the gy­ offilled seeds (Gorchov, 1985). Hence, a prox­ noecium consists of five carpels each contain­ imate explanation for asynchronous ripening ing two ovules (hence a potential of 10 seeds in this species requires an understanding of per fruit), as in other Maloideae. The five styles why seed number varies among fruits. The im­ appear fused but are distinct ("pseudo-syn­ portance of resources, pollination, and seed carpous" cf. Carr and Carr, 1961) and diverge predation in the determination ofseed number at the stigmas, enabling separate pollination of raises the larger question of which of these individual carpels (Fig. 1). The inflorescence factors usually limits total plant reproductive is an elongate panicle and the terminal flower output (Bierzychudek, 1981; Stephenson, 1981; usually opens first. Inflorescences contain four Rathcke, 1983; Bawa and Webb, 1984). Snow to eleven flowers, with variability in flower (1986) showed that seed number is related to number relatively low within individual plants pollination intensity primarily in species with (SO = 0.7-2.2). very many seeds per fruit. Other studies have The fruits (pomes) are juicy, purple, and demonstrated a relationship between seed about 1 em in diameter when fully ripe. The number and resources (reviewed in Marshall number offully developed seeds per fruit ranges et al., 1986). from oneto 10, and the balance ofthe 10 ovules One objective ofthis study was to determine develop into small or empty "seeds" that have whether manipulation of resources in a shoot seed coats but no embryo. Amelanchier ar­ at the time offruit initiation affected fruit set, borea is the first species to ripen fleshy fruits seed number, or seed weight in A. arborea. I each summer in southeastern Michigan; fruits also tested whether addition of xenogamous ripen throughout June and early July. Fruit pollen to unbagged flowers increased fruit set ripening is relatively asynchronous within trees; or seed number compared to open-pollinated 90% of fruits ripen within 15-27 days (Gor­ controls. In addition I attempted to determine chov, 1987). Fruits are eaten by a variety of whether "undeveloped seeds" were aborted or and mammals; at the study site the prin­ unfertilized ovules. cipal seed disperser is the Cedar Waxwing, Bombycilla cedrorum. Chipmunks and other MATERIALSANDMETHODS-Studysite-Field rodents are seed predators (Robinson, 1986), work was done at the E. S. George Wildlife as are larvae oftortricid and geometrid moths. Reserve, Livingston County, Michigan, a re­ search area maintained by the University of Shoot-resource manipulations-Ten plants Michigan. Climate and vegetation for the Re­ with many inflorescences accessible from the serve have been described in Rogers (1942), ground were chosen during flowering (18-26 September 1988] GORCHOV- FITNESS COMPONENTS IN AMELANCHIER 1277

most flowers. However, one shoot on each of 5 plants had no initiated fruits. These 5 shoots were not treated; instead the treatment they were designated to receive was applied to the shoot designated to receive treatment 10. (Be­ cause of this, these five plants lacked a treat­ ment 10 shoot.) Treatment 10 was done 29 April. Foliarfeedings were done twice, 30 April and 12 May, days that were chosen because they were cloudy but rain was not forecast (minimizing the risks of fertilizer bum and wash-off, respectively). Shoots were checked twice weekly until fruit Fig. I. Gynoecia from two Amelanchier arborea flow­ ripening began 28 May; checks were then in­ ers stained with aniline blue and photographed under ul­ creased to every second day until all fruits had traviolet light (Martin, 1959). Fluorescing strands are pol ­ ripened or aborted (15 July). To minimize len tubes. Each ofthe five styles ofeach flower has a distinct set ofpollen tubes. Because ovaries were squashed, locules damage to developing seeds and fruits by moth of the ovaries do not appear distinct. larvae moving from fruit to fruit, small drops of'Tanglefoot'" were applied to larval entrance holes and pedicels of fruits that appeared to April 1986). On each plant 10 shoots with in­ contain such larvae and branches were ringed florescences that were near the outside of the with Tanglefoot e proximal to each shoot on plant, were on different branches, and had the 17 May. To prevent removal offruits by birds same number of flowers (or differed only by and mammals, and to catch fruits that aborted one flower) were chosen. On each plant, shoots or were knocked off, on 20 May mesh bags were randomly assigned to the 10 treatments (made from green Orbit nylon net with 4 mm described in Table 1. Three of the treatments diameter openings) were placed around shoots reduced the number of sinks competing with containing one or more undamaged fruits. the target fruits for resources (Infr. Thin, Branch Fruits were collected when they were first ob­ Thin, Infr. + Branch Thin). Two others were served nearly ripe (pink or red), when they were intended to increase the availability ofmineral found loose in the mesh bag, or when it was nutrients to the target fruits by foliar feeding clear that the tissue was dead. (NPK, Trace). Four treatments were designed Fresh weights of ripe fruits were obtained to decrease photosynthesis in, and/or trans­ within two hours ofcollection using a Mettler location ofphotosynthate into, the shoot (De­ Pl60 balance. Both ripe and aborted fruits were foliation, Girdling, Defol. + Girdl., Defol. + kept in individual glazed paper coin envelopes 112 Girdl.). and refrigerated until dissected for seed counts Most treatments were done 28 April, the first (at most 15 days). Seeds were scored as filled day that styles had senesced on most flowers (embryo present), empty(no embryo butlonger on all 10 plants. By this date, ovary swelling than 2 mm), eaten (by insect larvae), or un­ indicated that fruit initiation had occurred in developed (shorter than 2 mm). Presence ofan

TABLE I. Summary ofshoot resource manipulation treatments . In each case, "shoot" refers to flowers, , and stem that grewf rom a single that opened April 1986

Treatment Description I . Control No manipulation 2. Infr. Thin Most proximal developing fruit identified, all other developing fruits and past flowers on infructescence cut off at end of pedicel 3. Branch Th in All developing fruits and past flowers on the four shoots closest to the target shoot (as measured along branches) cut at end of pedicel 4. Infr. + Branch Thin Fruits thinned on target shoot (as in 2) and on neighboring shoots (as in 3) 5. NPK Shoots sprayed twice with 2,000 ppm solution of Peters Professional Soluble Plant Food (20-20-20 N-P-K) 6. Trace Shoots sprayed twice with 2,500 ppm solution of Peters Soluble Trace Element Mix 7. Defoliation All leaves on shoot cut off at base of blade 8. Girdling Bark scraped off on a 2 mm wide band of wood j ust proximal (within I em ) ofshoot base 9. Defol. + Gi rdl. Shoot defoliated and girdled as abo ve 10. Defol. + 1/2 Girdl. Same as 9 except shoot girdled only halfway around stem 1278 AMERICAN JOURNAL OF BOTANY [Vol. 75 embryo could usually be determined by the set of bagged flowers is extremely low unless plumpness of the seed; seeds of intermediate hand-crossed (Gorchov, 1985) indicated that plumpness were cut open for inspection. Fresh through-the-bag cross pollination is infrequent weights of seeds of ripe fruits were obtained at best. Inflorescences were bagged shortly be­ by drying seeds with a paper towel and then fore the first flowers opened: 17 April for one weighing on a Mettler HK60 balance. A dry plant, 12 April for the other four plants. B3C weight was obtained for the pulp plus skin of inflorescences were checkeddaily duringflower each ripe fruit ("pulp dry weight") by carefully opening. On the first day a flower was open, placingeach in a papercoin envelopeanddrying two of its five styles were pinched off with a at 40 C for over one month, before reweighing fine forceps and the remaining three cross pol­ on the Mettler HK60. linated. Bags were removed afterthe last flower On one of the 10 plants no fruits matured had been hand pollinated, allowing insects to on any ofthe treated or control shoots, so data visit the flowers. Inflorescences in the Bagged analysis was limited to the other nine plants. treatment were kept bagged until all flowers showed signs ofsenescence (browning) on their Pollination experiments-In order to deter­ styles (26-30 April). mine whether augmentation ofpollination re­ In order to keep track of individual flowers sults in increased seed number, two treatments and the fruits derived from each, inflorescences were carried out: 1) open pollination (Open)­ were checked daily from flower opening to 1 flowers notmanipulated, and 2) open plus cross May, every second day 3-9 May, then twice pollination (0 + C)-flowers cross-pollinated weekly until fruit ripening began 28 May. Fruits by hand. To determine whether fruits ever set were then checked every second day until all from apomixis or selfing, other inflorescences fruits had ether ripened or aborted (23 July). were bagged to exclude insects and theirflowers Moth larvae and fruits believed to contain lar­ not pollinated (Bagged).To determine whether vae (entrance hole with frass extruding) were unpollinated ovules become "undeveloped removed in mid-May, and Tanglefootw was seeds" or are resorbed when they are in a fruit applied 17 Mayas described above for the that matures, a fourth treatment was carried shoot resource manipulations. Fruits were pro­ out in which inflorescences were bagged, two tected with mesh bags (25 May), collected when styles per flower were pinchedoff, and the other ripe or aborted (but not weighed), stored, dis­ three cross-pollinated (B3C). sected, and seeds counted (but not weighed) as Five trees with a large number of inflores­ described for the shoot resource manipula­ cences accessible from the ground were chosen tions. for pollen manipulations. A few days before Fruit set (number offruits ripened -;- number flowering began, the modal number of flower of flowers sampled) was calculated for each per inflorescence on each tree was esti­ plant for each treatment. Flowers that were mated, and 20 widely spaced inflorescences killed by a hard frost on 22 April or collected that had the modal number(± 1)offlower buds for pollen-tube staining were not included in were selected. On each plant five inflorescences these calculations, norwere flowers whose styles were randomly assigned to each of the four senesced before I was able to cross-pollinate treatments. them in the 0 + C (N = 11) and B3C (N = Cross pollen for the 0 + C and B3C treat­ 29) treatments. Data from two plants were not ments was a mixture obtained from anthers of included in further analyses. One plant lost four other trees immediately prior to polli­ most of its flowers in the 22 April frost, and nation. These trees (pollen donors) were lo­ another plant had very low fruit set for un­ cated in the same general vicinity as the pollen known reasons. Data for the other three plants receiving trees, but each pollen donor was at were pooled for further analyses, as one-way least 3.4 m from the nearest pollen recipient. analysis ofvariance (ANOVA) revealed no sig­ Equal numbers of mature anthers from each nificant plant effects on seed number. pollen donor were placed in an Eppendorfcen­ trifuge tube and mashed with a wooden tooth­ Data analysis -Comparison of fruit set pick. Pollen was applied to stigmas using a among the shoot resource treatments, between toothpick. Nearly all hand were each ofthese treatments and the Control, and done 22-25 April. between the Open and 0 + C pollination treat­ Inflorescences in the Bagged and B3C treat­ ments was done by G test. To determine the ments were bagged with white nylon cloth effects ofthe resource manipulation treatments (RosebarGlitterOrgandy) with a mesh ofabout and individual plants on 1) filled seed number 0.2 mm. These bags may not have excluded and 2) mean weight of filled seeds per fruit, wind-borne pollen, butearlier findings that fruit two-way ANOVA was used, modelling treat- September 1988] GORCHOV- FITNESS COMPONENTS IN AMELANCHIER 1279 ment as a fixed effect and plant and plant­ TABLE 2. Number ofA. arborea flowers and ripe fruits. treatment interaction as random effects. Clas­ andfruit set (fruits ripened -0-flowers) in shoot resource sical ANOVA was not appropriate for this manipulations. Treatments are described in Table 1. Fruit set in each treatment was comparedto the control mixed model design because the data were un­ by G test balanced. Instead, maximum likelihood esti­ mation ofthe ANOVA model parameters were Signifi- cance calculated with the BMDP:3V program (Dix­ com- on, 1983). This program calculates likelihood % pared Aow- Fruits Fruit to ratio statistics for each null hypothesis (no Treatment en; ripened set control treatment effect, no plant effect, and no inter­ Control 73 20 27 2 action) which are tested for significance by X Infr. + Branch Thin 9 8 89 *** test and not by the F test ofclassical ANOVA. Infr. Thin 9 4 44 ns Only fruits without larval insect damage were Branch Thin 72 29 40 ns used in these analyses. Fruits from the Defol. NPK 71 21 30 ns + Girdl. and Defol. + 1/2 Girdl. treatments Trace 75 17 23 ns were pooled because the treatments and the Defoliation 66 18 27 ns resultant seed numbers and weights were sim­ Girdling 70 16 23 ns ilar. In order to test the homogeneity of vari­ DefoI. + 'h GirdI. 46 5 II * ances assumption of two-way ANOVA, the DefoI. + GirdI. 69 3 4 *** Box test was used to determine whether the ns P > 0.05. variances in a) filled seed number, b) mean * P < 0.05. weight of filled seeds, and c) pulp dry weight *** P < 0.00I. were homogenous 1)across the nine plants and 2) across the 10 treatments. Variances of all but fruit set was significantly lower than the three variables were homogenous across treat­ Control on shoots that were both defoliated ments, but only variance in seed weight was and girdled (4%) or defoliated and half-girdled homogenous across plants. For seed number, (11 %). the assumption of homogeneity of variances The total number of seeds (including eaten, was satisfied by omitting the plant with the empty, and undeveloped seeds) perfruit ranged lowest variance. For pulp dry weight, this as­ from eight to 11, but equalled 10 for 92% of sumptionwas met by omittinga different plant, fruits. that with the highest variance in pulp dry Filled seed number per fruit did not differ weight. Therefore the two-way ANOVAs of among the treatments (Table 3). The two-way seed number and pulp dry weight on plant and ANOVA of seed number on plant and treat­ treatment were done only on fruits from eight ment showed a significant plant effect (X2 = plants. Seed number and pulp dry weight of 4.0, df = 1, P < 0.05) but no significant treat­ Control fruit were compared to those offruits ment or interaction effects. Seed number in in each of the other resource treatments (all each of the treatments was not significantly nine plants pooled) by Student t test. Com­ different from the Control. parison of seed weight between Control and Fruits that aborted had fewer filled seeds other treatments was done by Mann-Whitney than fruits that ripened. Fruits that aborted 24­ U test, since variances were often unequal. 31 May had a mean of 0.5 ± 1.5 filled seeds Comparisons between Open and 0 + C pol­ and those that aborted during June had 1.8 ± lination treatments in seed number were done 2.4 filled seeds, compared to 6.7 ± 2.6 filled by U test for the same reason. Means are ex­ seeds for fruits that ripened (all treatments pressed ± 1 SD. pooled). Mean filled seed weight per fruit did differ RESULTS-Shoot resource manipulations­ among the shoot resource treatments (Table The overall effect ofshoot resource treatments 3). The two-way ANOVA revealeda significant on fruit set was highly significant (G = 55.1, treatment effect (x2 = 36.7, df= 8, P < 0.001) df = 9, P < 0.005; Table 2). Fruit set was as well as significant plant (x2 = 27.1, df = 1, higher than the Control (27%) for treatments P < 0.001) and interaction (X2 = 4.6, df = 1, that reduced the number of competing sinks, P < 0.05) effects. Seeds were heaviest in the although this was significant only for the Infr. Infr. + Branch Thin and Infr. Thin treatments. and Branch Thin treatment (89%). Fruit set Sample size in each of these two treatments did not differ significantly from the Control in was too small for Mann-Whitney U test, but treatments where mineral nutrients were added when pooled, these seed weights were signifi­ by foliar feeding. There was no significant effect cantly heavier than those of Control fruits (U on fruit set from girdling or defoliation alone, = 29, N, = 10, N 2 = 18, P < 0.01). There was 1280 AMERICAN JOURNAL OF BOTANY [Vol. 75

TABLE 3. Filled seed number and meanfilled seed weight per undamagedA. arboreafruit in shoot resource manipulations

Undamaged Mean filled seed weight Treatment ripe fruits Filled seeds per fruit per fruit (mg) Pulp dry weight (mg) Control 18 7.4 (2.3)' 4.8 (0.8) 37.9 (12.0) Infr. + Branch Thin 6 6.2 (3.0) 7.0 (1.0) 41.0 (14.8) Infr. Thin 4 9.0 (0.8) 5.9 (1.4) 39.0 (10.7) Branch Thin 25 6.6 (2.3) 5.0 (1.4) 36.4 (13.8) NPK 16 5.8 (2.6) 4.9 (1.5) 35.9 (13.0) Trace 16 6.2 (2.9) 5.4 (1.4) 41.5 (14.4) Defoliation 12 7.0 (2.1) 4.1 (1.0) 38.3 (14.0) Girdling 14 6.0 (3.2) 5.5 (1.3) 43.2 (13.4) Defo!. + I;' Gird!. 5 8.2 (2.5) 4.8 (0.8) 34.0 (4.4) Defo!. + Gird!. 3 6.7 (3.5) 4.7 (0.8) 39.9 (18.9)

a Mean (SD). a marginally significant tendency for seeds to in the Open + Cross (0 + C) treatment com­ be lighter in the Defoliation treatment than in pared to Open pollinated flowers (Table 4). the Control (U = 63, N, = 12, N 2 = 18, P = The number of filled seeds per fruit did not 0.06). The othertreatments that interfered with differ between the two treatments butwas more photosynthesis and/or translocation had no ef­ variable in the 0 + C treatment. The number fect. Nutrient addition also did not affect seed ofempty seeds was greater and more variable weight. in the Open treatment. The number of unde­ Plants that averaged more filled seeds per veloped seeds was not different between treat­ fruit tended to have lighter mean filled seed ments, but was more variable in the 0 + C weights(r= -0.65, N= 9, P= 0.06). Similarly, treatment. within individual plants, fruits with more seeds Fruit set was zero in the Bagged treatment, tended to have lighter seeds: the correlation confirming earlier findings ofno (or very low) between filled seed number and mean weight self-compatibility or apomixis (Robinson, was negative for eight of the nine plants, al­ 1982; Gorchov, 1985). Other evidence that A. though significantly so for only two. Overall, arborea is not apomictic is the low frequency mean weight of filled seeds declined slightly of multiple megagametophytes in the ovules with increasing filled seed number (slope = (Campbell, Greene, and Bergquist, 1987; C. -0.22 mgseed", N= 119, F= 24, P < 0.001, Campbell, unpublished data). r 2 = 0.17). Fruit set was low in the B3C treatment. These There was a significant plant effect on pulp fruits had one to six filled seeds and most of dry weight (x2 = 15.1, df= 1, P < 0.001) but the balance ofthe 10 ovules in each fruit were there was no significant treatment effect or in­ accounted for by undeveloped seeds. As would teraction. There were no significant differences be expected if these undeveloped seeds rep­ in pulp dry weight between Control fruits and resent unfertilized ovules, all but one of these any treatment(Table 3). Among the nine plants fruits contained two adjacent carpels contain­ mean pulp dry weight was not correlated with ing two undeveloped seeds each. mean filled seed weight (r = 0.44, P > 0.2) or number (r = 0.15). Among all fruits, pulp dry DISCUSSION - Shoot resources - Shoot re­ weight increased with filled seed number (slope source levels were affected by at least some of = 1.8 mg! seed, N = 119, F = 17.3, P < 0.001, the manipulations, as evidenced by the changes r2 = 0.13). However, on a per filled seed basis, in fruit set and seed weight. Reducing the num­ pulp dry weight declined (slope = -1.3 mg­ ber ofcompeting sinks by thinning fruits tend­ seed:", N= 119, F= 89, P < 0.001, r2 = 0.43), ed to increase fruit set and seed weight. These indicating a decrease in pulp:seed ratio with effects were greatest in the Infr. and Branch increasing seed number. Thin treatment, where there was only one ini­ tiated fruit per five infructescences. Thinning Pollination experiments-The total number an entire smallA. arboreashrub to one initiated ofseeds (atl categories) ranged from nine to 11 fruit per infructescence also resulted in high (91% with 10) in the Open treatment and from mean weight of filled seeds (7.0 ± 1.3 mg, N eight to 10 (96% with 10) in the 0 + C treat­ = 9 fruits, Gorchov, 1987). Treatments that ment. both reduced local photosynthesis (by defol­ Fruit set was higher, but not significantly so, iation) and interfered with transport of re- September 1988] GORCHOV- FITNESS COMPONENTS IN AMELANCHIER 1281

TABLE 4. Fruit set and seed numbers for four pollination treatments on three A. arborea plants

Seeds per fruit: mean (SD) Treatment Flowers Fruits ripened- % Fruit set Filled Empty Undeveloped Open 118 45 (41) 38 7.5 (1.8) 1.1 (1.3) 1.4(1.5) ns" ns ** * *** ns *** O+C' 101 48 (41) 46 7.2 (2.6) 0.5 (0.8) 2.1 (2.5) B3Cd 93 9(6<) 10 3.4 (2.1) 1.0 (1.0) 5.6 (2.5) Bagged 117 0

a Total (undamaged). Only undamaged fruits were included in the analysis of seeds per fruit. b Statistical significance of differences between Open and 0 + C treatments are given in this row. Fruit set was compared by G test, seed number by Mann-Whitney U test, and variance in seed number by F test. * P < 0.05, ** P < 0.01, ***P < 0.005, ns P > 0.05. c Open + Cross-pollinated. d Bagged + three styles cross-pollinated; two styles pinched off. e One fruit not inspected so sample size equals 5 for seed numbers. sources to the shoot (halfor complete girdling) dependent on resources comes from small A. dramatically reduced fruit set. However, de­ arborea plants thinned throughout to one ini­ foliation alone had no significant effect on fruit tiated fruit per infructescence. On one plant set and only marginally reduced seed weight, filled seed numbers were low (4.9 ± 2.5, N = and girdling alone did not affect either. 30) but on another they were high (8.1 ± 1.6, Despite the effects of some of these treat­ N = 19, Gorchov, 1987). This difference is con­ ments on fruit set and/or seed weight, none sistent with the significant differences among resulted in fruits with filled seed numbers dif­ plants in filled seed number detected by the fering significantly from the control. These re­ two-way ANOVA. Plants that averaged more sults suggest that A. arborea does not adjust seeds per fruit tended to have lighter seeds, seed number to local resource levels. Alter­ suggesting intrinsic differences among plants natively, resources may influence the number in basic allocation patterns. of seeds that begin to develop, but due to the The results of the shoot resource manipu­ preferential abortion of few-seeded fruits this lations suggest that photosynthate, rather than effect is manifested in differences in fruit set. any mineral nutrient, was limiting to fruit In this scenario mean seed number per ripened number and seed weight. Neither macronutri­ fruit should still reflect shoot resources, but ents (NPK) nor trace element fertilization had differences among treatments would be less­ a demonstrable effect on any of the measured ened by differential abortion. Thus this hy­ components of fitness, whereas reducing the pothesis predicts a correlation between fruit number of physiological sinks competing for set and seed number among the treatments, a resources, by fruit-thinning, increased fruit prediction not supported by these data (r = number and/or seed weight. Either girdling or -0.12, N = 10). Another prediction is that defoliation alone had no effect on fruit set. differences in seed number among treatments However, the combination of defoliation and would be more apparent when aborted fruits partial or complete girdling drastically reduced were included in the analysis. When fruits fruit set. This supports theconclusion that pho­ aborting after 20 May (when mesh bags were tosynthate was the limiting resource: due to put in place) were included, the finding that defoliation, fruits on these shoots did not have treatments did not differ in seed number was local sources of photosynthate (except from not qualitatively changed (one-way ANOVA storage in the small segment ofwood distal to F = 1.2, df = 8, 120, P > 0.5). Most fruit the girdle) and as a result ofgirdling they were abortions occurred before 20 May but seed partially or completely cut offfrom photosyn­ numbers were not obtained for these. Thus, thate translocated from the rest of the plant. while there is no direct support for the hy­ These fruits could still be suppliedwith mineral pothesis that resource effects on fruit set are nutrients through xylem, although defoliation mediated by seed number, criticial data are could reduce this supply by slowing transpi­ lacking. Even if preabortion seed number is ration. determined by resources, but seed-number-de­ An alternative explanation ofthe lack ofre­ pendent fruit abortion somehow compensates sponse to foliar feeding ofmineral nutrients is for it, seed number in maturefruits would still that these minerals were not absorbed by the end up independent of resources. tissue. This is unlikely, since foliar ab­ Further evidence that seed number is not sorption ofminerals is generally good in another 1282 AMERICAN JOURNAL OF BOTANY [Vol. 75 member of the Maloideae, the , and is it is determined mostly by seedabortion, which better in young leaves than old (Swietlik and may be mediated by resources. Faust, 1984). Another caveat is the possibility In order to determine whether unfertilized that the other manipulations affected fruit set ovules became undeveloped seeds, I examined and seed weight through changes in endoge­ mature fruit in which some ofthe ovules were nous hormone levels, rather than resources. known to be unfertilized. These fruits were Although each ofthe classes ofresource ma­ obtained by taking advantage of A. arborea's nipulation described here has been shown to five-carpellate morphology (Fig. 1). The two affect seed number in other species, seed num­ ovules in each carpel can only be fertilized by ber is less frequently affected by resource levels pollen germinating on that carpel's stigma. than are other fitness components such as fruit Pinching offtwo adjacent styles and cross-pol­ set and seed weight. (See Marshall et al., 1986, linating the other three on flowers from which for a review.) Most ofthe studied to date have pollinators were excluded (treatment B3C), re­ been done on herbaceous plants, particularly sulted in fruits that could mature but would crops and wild annuals. Woody plants have contain at least four unfertilized ovules. These been less often considered in these studies. The fruits each had a total seed count (filled + data available for woody species generally show empty + undeveloped) of 10 (as did nearly all an effect of resource manipulation on fruit fruits in the other treatments) indicating that number (Jaumien, 1968; Aalders, Hall, and unfertilized ovules are not resorbed. Further Forsyth, 1969; Stephenson, 1980; Wyatt, 1981; evidence that the ovules in the unpollinated Bertin, 1982; Swietlik and Faust, 1984; Den­ carpels become "undeveloped seeds" is the nis, 1986). Changes in seed number or weight finding that most of these fruits had two ad­ have been reported less frequently, but both jacentcarpels with two undeveloped seeds each. declined following defoliation in two tree Therefore, the undeveloped seeds found in un­ species (Janzen, 1976; Stephenson, 1980) and manipulated fruits may also derive from un­ seed number was increased by flower thinning fertilized ovules, although some may result in apple (Weinbaum and Simons, 1976). from ovules that are fertilized but later abort.

Pollination-Theabsence ofasignificantdif­ ference in filled seed number between open­ Alternative hypotheses-Neither quantity of pollinated flowers and those receiving supple­ compatible pollen nor resources in the shoot mental cross pollination indicates that filled during fruit initiation appear to influence the seed numberis not pollen limited in A. arborea. number of filled seeds per fruit in A. arborea, In her review, Snow (1986) reported that seed yet the numberoffilled seeds in unmanipulated number is not determined by quantity ofcom­ fruits varies from 1 to 10, and is variable within patible pollen deposited on the stigma in most each plant. I propose several alternative hy­ species, but this result varied with average seed potheses and evaluate each in light ofthe avail­ number. Only one of 14 species averaging 2­ able data. 15 seeds per fruit has been shown to have seed Seed predation: Although many fruits par­ number pollen limited, compared to five of 11 asitized by lepidopteran larvae failed to ripen, species with 21-500 seeds (Snow, 1986, table some ripened buthad some oftheirseeds eaten. 4). Presumably, for those species with a small This seed predation may explain some of the to moderate number of ovules, if compatible variation in filled seed number, but most rip­ pollen reaches a flower there is usually enough ening fruits were undamaged and filled seed to fertilize a sufficient number of ovules to number was highly variable among them. In­ cause a fruit to develop; the number of seeds cluding parasitized fruits in the analyses did in the fruit may vary but is not correlated with not qualitatively change the findings that nei­ the amount ofpollen applied. Amelanchier ar­ ther shoot resources nor pollination treatments borea falls into this category, as it has only 10 significantly affected seed number. Fungi may ovules per flower. be responsible for the death of some or all of the empty seeds, but the latter were uncommon "Undeveloped seeds"-Most ovules that did compared to undeveloped seeds. Seed preda­ not mature into filled seeds became small "un­ tion or fungal infection could explain the re­ developed seeds." It is crucial to determine maining variability only if these undeveloped whether the latter represent unfertilized ovules seeds were the result of attack early in seed or ovules that were fertilized but later aborted. development. If the former, then seed number is primarily Pollen blockage: Self-pollen deposited by in­ determined by fertilization; if the latter, then sects or autogamously might cover enough of September 1988] GORCHOV- FITNESS COMPONENTS IN AMELANCHIER 1283 the surface of a stigma to prevent the germi­ eration due to inhibition from the terminal nation of compatible pollen arriving subse­ flower. quently. Similarly, the upper part of a style Although earlier initiating fruits might be might clog with the pollen tubes from self­ expected to have a resource advantage over pollen (incompatibility in the Maloideae is ga­ later fruits, there was no correlation between metophytic [de Nettancourt, 1977] which is seed number and fruit initiation date (Gor­ associated with stylar inhibition). Growth of chov, 1987). pollen tubes through a style may be blocked Carpel not adequately vascularized: Some by improperly developed stylar transmitting ovules may abort due to resource shortage, but tissue or wounds in the style. In any of these the shortage may be due to inadequate vascular situations, seed number would be limited by connections, rather than the nutritional status fertilization, but supplemental cross pollina­ of the shoot or developing fruit. tion would have no effect, as reported here. Ovule infertility: Some ovules may not de­ Testing the alternatives-Itis possible to dis­ velop into seeds because they are incapable of tinguish among the above alternative hypoth­ being fertilized. One proximate cause ofovule eses by examining the spatial pattern of filled infertility is ovule degeneration, which refers seeds. Certain ofthe hypotheses (pollen block­ to the cessation of development of an ovule age, carpel vascularization) predictthat the two before fertilization, as has been reported in ovules in a carpel should either both fail or Quercus (Mogensen, 1975), soybean (Aber­ both develop into seeds. Others (ovule degen­ nethy et al., 1977), and pear (Jaumien, 1968). eration, postzygotic incompatibility) predict An alternative proximate cause ofovule in­ that the fate ofan ovule should be independent fertility is ovule senescence. In apple (Dennis, ofthat ofits neighbor. An examination ofthe 1986), pear (Herrero, 1983), and sweet cherry distribution of ovules within carpels in a set (Stosser and Anvari, 1983) ovules mature a ofopenpollinatedfruit, andcomparisonto that few days after anthesis but begin to senesce generated by a simulation, revealed that ovule several days later if not fertilized. Ovule se­ failure is partially, but not completely, depen­ nescence appears to be the reason fruit set (and dent on carpel membership (Gorchov and Es­ for pear, seeds per fruit) declines with increas­ tabrook, 1987). Thus neither a hypothesis that ing time between anthesis and hand-pollina­ predicts both ovules should develop or fail to­ tion in these species. However, seeds per fruit gether, nor one that predicts that ovule fate is was not correlated with number of days be­ independent of its neighbor, can be the sole tween anthesis and fruit initiation in an inten­ determinant ofseed number in A. arborea, al­ sively studied A. arborea tree (Gorchov, 1987). though either may be among the causes. Early abortion offertilized ovules: Guth and Weller (1986) found that a large proportion of Ecological consequences and evolutionary Oxalis magnifica ovules aborted during the first significance-Aborted A. arborea fruits have few days after fertilization. A fertilized ovule fewer filled seeds than matured fruits, sug­ might fail to develop due to 1) genetic incom­ gesting selective abortion offew-seeded fruits, patibility between the male and female ge­ as has been reported in several other species nomes manifested postzygotically, or 2) ma­ (Stephenson, 1981; Stephenson and Bertin, ternal resources limitation. 1983). Whether this results in higher average The shoot resource manipulations reported offspring quality (Darwin, 1876, pp. 398-400; here had no effect on seed development, but Lee, 1984) was not determined. Multiseeded they were done at or shortly after the time of fruits are more "efficient" for the parent plant, fruit initiation. It is possible that resource levels as the mass ofpulp and skin per seed declined at some stage before fruit initiation do influ­ with increasing seed number. This decrease in ence whether a fertilized ovule develops into pulp to seed ratio would make many-seeded a filled seed. This hypothesis is supported by fruits less profitable to frugivores (Howe and the fact that fruits from terminal flowers, which Vande Kerckhove, 1980; Herrera, 1981a, b), may have better vascular connections, average but it is not known whether frugivores dis­ significantly more filled seeds than those from criminate among Amelanchier fruits based on lateral flowers (Gorchov, 1987). However, their seediness. Fruits ripening early in the sea­ among the lateral fruits, there was no trend of son, before fruits ofotherspecies become avail­ seed number with flower position. An alter­ able, tend to be eaten more promptly. Since native explanation for lower seed number in ripening in A. arborea occurs earlier for fruits lateral fruit, advanced for (Weinbaum with more filled seeds (Gorchov, 1985), dis­ and Simons, 1976), is premature ovule degen- persal may be more successful for seeds in 1284 AMERICAN JOURNAL OF BOTANY [Vol. 75

many-seeded fruits, although subsequent seed­ the evolution of paternal and maternal reproductive ling competition may be greater. patterns. Amer. J. Bot. 71: 736-751. BERTIN, R. I. 1982. The ecology ofsex expression in red Filled seed number is variable within each buckeye. Ecology 63: 445-456. A. arborea individual and this variability per­ BIERZYCHUDEK, P. 1981. Pollinator limitation of plant sists despite changes in available resources. As reproductive effort. Amer. Naturalist 117: 838-840. a consequence, great variability exists in the BOOKMAN, S. S. 1983. Costs and benefits of flower ab­ flower-fruit interval and ripening date of the scission and fruit abortion in Asclepias speciosa. Ecol­ fruits on individual plants (Gorchov, 1985). ogy 64: 264-273. BREWER, J. W., AND R. C. DOBSON. 1969. Seed count Such asynchronous ripening in a summer­ and berry size in relation to pollinator level and har­ fruiting, animal-dispersed plant has been pro­ vest date for the highbush blueberry, Vaccinium cor­ posed to be an adaptation to avoid satiation ymbosum. J. Econ. Entomol. 62: 1353-1356. of resident seed-dispersal agents (Thompson BRONSTEIN, J. L. 1986. Coevolution and constraints in and Willson, 1979), although field manipula­ a Neotropical fig-pollinator wasp mutualism. Ph.D. tions of fruit displays do not support this hy­ dissertation. University ofMichigan, Ann Arbor, Ml. pothesis (Gorchov, 1988). Although it is dif­ CAMPBELL, C. S., C. W. GREENE, B. F. NEUBAUER, AND J. M. HIGGINS. 1985. Apomixis in Amelanchier laevis, ficult to evaluate an evolutionary hypothesis Shadbush (Rosaceae, Maloideae). Amer. J. Bot. 72: such as this, it is made much less plausible if 1397-1403. the purported adaptation can be shown to be ---,--, AND S. E. BERGQUIST. 1987. 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