Effects of defoliation and shading on the physiological cost of reproduction in silky locoweed Oxytropis sericea

Background

The production of flowers, fruits and seeds demands considerable energy and nutrients, which can limit the allocation of these resources to other plant functions and, thereby, influence survival and future reproduction. The magnitude of the physiological costs of reproduction depends on both the factors limiting seed production (pollen, ovules or resources) and the capacity of plants to compensate for high resource demand.

Methods

To assess the magnitude and consequences of reproductive costs, we used shading and defoliation to reduce photosynthate production by fully pollinated plants of a perennial legume, Oxytropis sericea (Fabaceae), and examined the resulting impact on photosynthate allocation, and nectar, fruit and seed production.

Key Results

Although these leaf manipulations reduced photosynthesis and nectar production, they did not alter photosynthate allocation, as revealed by ¹³C tracing, or fruit or seed production. That photosynthate allocation to reproductive organs increased >190 % and taproot mass declined by 29 % between flowering and fruiting indicates that reproduction was physiologically costly.

Conclusions

The insensitivity of fruit and seed production to leaf manipulation is consistent with either compensatory mobilization of stored resources or ovule limitation. Seed production differed considerably between the two years of the study in association with contrasting precipitation prior to flowering, perhaps reflecting contrasting limits on reproductive performance.     

Patterns of wing size variation in seeds of the lily Cardiocrinum cordatum (Liliaceae)

We examined the patterns of variation in wing-loading and its related characteristics in Cardiocrinum cordatum to clarify the factors that determine the variation in seed dispersal ability in this species. The square root of wing-loading of a seed of a plant was not significantly correlated with basal stem diameter of a plant, indicating that large plants did not necessarily produce seeds with high dispersal ability. This result was inconsistent with the hypothesis that large plants produce seeds with high dispersal ability to avoid high mortality of seeds and seedlings in the vicinity of the parents. On the other hand, the square root of wing-loading of a seed of a fruit was negatively dependent on seed number of a fruit. Thus, many-seeded fruits produced seeds with high dispersal ability. This was because the projected surface area per seed was large in large fruits and large fruits contained large numbers of seeds. The cost per seed of producing fruit structures was small for many-seeded fruits. Thus, high dispersal ability of seeds in many-seeded fruits may be a result of an effective resource allocation pattern in which a high proportion of resources are allocated to those many-seeded fruits, enabling seeds to develop large wings and thus reducing the structural cost of fruits per seed.     

Maternal regulation of fecundity: non-random ovule abortion inCassia fasciculata Michx.

Summary We examined the extent of ovule abortion and the within-fruit pattern of abortion inCassia fasciculata, an annual legume, and tested the hypothesis that abortion can result from competition for limited maternal resources among developing fruits and seeds. In a natural population at Mayview, IL, 53.4% of ovules in mature fruits matured as seeds; 43.4% showed some development but aborted, and 3.1% showed no development over virgin ovules. In a greenhouse experiment in which treatments were applied after most fruits were initiated, nutrient addition and partial root removel had no effect on abortion, but drought reduced the proportion of ovules maturing to 75% of the control mean. A fruit thinning experiment was conducted in which the number of fruits initiated on certain plants was limited. Control plants had more ovule abortion than fruit-thinned plants, suggesting that abortion resulted from competition for limited maternal resources. A position effect was observed in both field and greenhouse populations; ovules toward the fruit base (pedicellar end) had higher frequencies of abortion than those at the distal end. Thus, ovule abortion, like fruit abortion in this species, is non-random. Indivisuals regulate fecundity at both the whole fruit and individual seed levels.Scientific contribution no. 1357 from the New Hampshire Agricultural Experiment Station     

Factors regulating fruit and seed production in the desert annual Lesquerella gordonii

Summary This study examines the pattern of reproduction and factors that limit the maternal reproductive output in the self-incompatible annual, Lesquerella gordonii. Greenhouse experiments showed that fruits do not abscise and they mature if they contain at least one fertilized (outcrossed) ovule. Field studies showed that flowering increased to a peak and then fell off rapidly, while seed production/fruit decreased through the season. Pollen addition did not increase the number of fruit or seeds per fruit, suggesting that fruit and seed production are not pollinator limited. Addition of water increased fruit and seed production by extending the life of the plant, by increasing the number of stalks which flowered, and by increasing the number of seeds per fruit. This indicates that fruit and seed production are resource limited. Fruits are relatively inexpensive as compared to seeds as over 93% of the water in fruits is contained in the seeds. Fifty-three percent of the plants were lost to herbivory as were 64.6% of the fruits on the remaining plants, indicating that herbivory also limited fruit and seed production. It is hypothesized that high fruit set coupled with variable seed set in L. gordonii is an adaptive response to unpredictable, variable resource levels, and high herbivory risk.     

Effect of ovule position within the pod on the probability of seed production in Bauhinia ungulata (Fabaceae)

BACKGROUND AND AIMS: It has been claimed that ovules linearly ordered within a fruit differ in their probabilities of reaching maturity. This was investigated by studying the effect the position of an ovule within the pod has on seed abortion and seed production in Bauhinia ungulata. METHODS: Fruits collected during the dry seasons of 1999, 2000 and 2001 were opened, and the number, position and status of each ovule within the fruit were recorded. A GLM model was used to assess the effects of population, tree identity and ovule position within the pod on ovule fertilization, seed abortion, seed damage and seed maturation in two populations of B. ungulata. KEY RESULTS: Nearly 30% of the ovules were not fertilized in 1999; this percentage dropped to 5% the following two years. Seed abortion (50%) and seed damage (15%) were the same every year during the study period. Only 15% of the initial ovules developed into mature seeds in 1999; this value increased to 35% in 2000 and 2001. However, seed survivorship was dependent on the position of the ovule within the pod; non-fertilized and early aborted ovules were found more often near the basal end of the ovary. The frequency of seed damage was not affected by position. Mature seeds were found mainly in the stylar half of fruits, where ovules are likely to be fertilized by fast pollen tubes. CONCLUSIONS: The pattern of seed production in B. ungulata is non-random but is dependent upon the position of the ovule within the pod. The results suggest that the seeds produced within a fruit might differ in their vigour.     

Selective seed abortion affects the performance of the offspring in Bauhinia ungulata

BACKGROUND AND AIMS: Under the microgametophytic competition hypothesis, a non-random pattern of seed abortion is expected, in which only the most vigorous seeds reach maturity. In a previous study, it was found that Bauhinia ungulata (Fabaceae) exhibits a pattern of seed abortion dependent on the position of the ovule within the ovary; ovules located in the stylar half of the fruit, close to the point of entry of pollen tubes to the ovary, have a low probability of seed abortion, whereas ovules in the basal half of the fruit are aborted with a high probability. METHODS: A series of experimental fruits was generated, in which ovules from either the stylar (treatments 1 and 2) or the basal (treatments 3 and 4) half of fruits were destroyed, to evaluate whether these patterns of selective seed abortion have an effect on the vigour of the offspring in B. ungulata. KEY RESULTS: Only 53 % of the seed from control fruits germinated. Seed set in fruits from treatments 1 and 2 showed a significantly lower (33-43 %) percentage of germination; the germination of seeds from fruits in treatments 3 and 4 (49-51 %) did not differ from control seeds. In addition, it was found that the differences in vigour of the offspring are not random with respect to the position of the ovule in the pod. CONCLUSIONS: The overall performance of the seeds correlated with their likelihood of maturation. Seeds located at the basal half of the treatment fruits showed lower values of vigour than seeds located on the stylar half. The differences were more marked for early measures of fitness.     

Patterns of seed mass variation and their effects on seedling traits inAlliaria petiolata (Brassicaceae)

Seed mass is considered to be the least plastic component of reproductive yield. Yet, in invasive populations of garlic mustard, Alliaria petiolata, seed mass was highly variable (eightfold among populations, 2.5-7.5 fold within populations, two-threefold within individuals, and 1.4-1.8 fold within fruits). Variation in seed mass among populations explained nearly half of the total variance. Variation among seeds within fruits accounted for a further 25% of variance. Individual seed mass within a plant decreased with increased distance from the main stem, suggesting that access to parental resources limits seed size in a predictable manner. MANOVAs and Roy-Bargmann stepdown analyses revealed significant effects of seed mass, but not seed position (within a fruit or within an infructescence), on an array of subsequent seedling traits. Smaller seeds germinated significantly earlier, and seedlings from small seeds produced their first primary leaves significantly later and grew significantly taller. After accounting for seed mass as a covariate, only one seedling trait, date of first leaf emergence, was affected by seed position in a fruit. Differences in seed mass may therefore affect seedling recruitment via effects on early seedling growth in this weedy species.     

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

Most Amelanchier arborea flowers have 10 ovules, but the number of filled (embryo-containing) seeds per fruit is usually less than 10 and is highly variable within each individual plant. 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-pollination 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 determination of seed number are proposed and discussed.     

Long-term variation in seed mass and seed production in populations of Paris quadrifolia

Seed production of the perennial herb Paris quadrifolia L. (Liliaceae) was investigated in five populations in northern Poland. The long-term seed production per square metre differed significantly among populations and years. Moreover, throughout the 7-year study period, 30% of both whole ripe fruits and seeds alone were predated. Variation in seed mass per fruit in both space and time was significant. Throughout the 7-year study, nearly all the marked individuals produced fruits once every 2 years. The most frequent break between fruiting was 2 years and the longest was 5 years. Only the mean seed mass in fruits of the same individual varied significantly over subsequent years. In the five populations, the number of ovules, number of seeds in the fruit and seed mass varied significantly between populations. However seed/ovule ratio did not differ in fruits in the five populations. The seed mass/number trade-off in fruits was strongly partially correlated when the effect of total seed mass was considered. Breeding experiments suggest that P. quadrifolia has a substantial capacity for both inbreeding and outbreeding. There were no significant differences in the seed/ovule ratio, seed number or seed mass in fruits produced from bagged or control flowers. However, both seed/ovule ratio and number of seeds were significantly lower in fruits from emasculated flowers.     

Self pollination and resource availability affect ovule abortion inCassia fasciculata (Caesalpiniaceae)

We examined the effects of pollen source and resource availability on ovule abortion in the annual legumeCassia fasciculata. Pollen source was controlled by hand-pollinating flowers with cross- or self-pollen. Resource availability to developing fruits was controlled by adjusting fruit loads (heavy versus light) on each plant and exposing plants to different photoperiod cycles (16 vs 12 h of light; short days favor fruit growth at the expense of vegetative growth). In mature fruits the proportion of ovules expanding (showing some development over virgin ovules) ranged from 89–95% and did not increase with resource availability, suggesting that unexpanded ovules were either unfertilized or obligately aborted shortly after fertilization. The proportion of expanded ovules maturing in mature fruits was near 97% for both self- and crosspollinations in the treatment with highest resource availability (light load, short days) and lower in the remaining treatments, where self-pollination resulted in up to 9% lower seed maturation than cross-pollination. In the latter three treatments a pollen source effect was dependent upon the maternal plant; in some plants selfing increased abortion and in others it did not. Collectively, the results suggest that (1) both pollen source and resource availability affect ovule abortion, (2) at least some abortion is facultative, and (3) when resources are limited, self-pollination increases abortion in some but not all maternal plants.     

Patterns of fruit and seed production inBauhinia ungulata (Leguminosae)

Patterns of seed and fruit production ofBauhinia ungulata, a small tree legume indigenous in tropical America, were studied in Costa Rica. Only about 8% of flowers produced fruits. The average pod had 19 ovules and about two thirds of these began seed development, with mature pods containing an average of 9.7 mature undamaged seeds. About half of the mature pods were damaged by herbivores and within these, 27% of ovules or seeds had been eaten. Among trees there was no significant variation in pod production, but the number of ovules per pod and seed production per pod varied significantly. Within infructescences most pods were retained at middle positions. Within pods, the probability of an ovule developing into a seed increased toward the distal end. The pattern of seed and fruit production in this species agrees well in general with that reported for other neotropical legumes. The abortion of seeds and fruits can be regarded as a way of controlling maternal investment, and as a response to herbivory.     

Effects of self-pollination and maternal resources on reproduction and offspring performance in the wild lupine, Lupinus perennis (Fabaceae)

We examined the effects of self-pollination and resource addition to maternal plants of Wild Lupine on seed production in the field, and on offspring performance in the greenhouse. Although 24% of flowers set fruits when open-pollinated, only 11% of flowers set fruits when self-pollinated. Self-pollination significantly reduced fruit and seed production per inflorescence and increased aborted seeds per fruit. Resource addition to maternal plants significantly increased fruit and seed number in the field. Moreover, selfed plants exhibited greater variability in seed production in the absence of resource addition to the maternal plant. We planted a total of 1,306 of the seeds from this experiment in the greenhouse. While self-pollination did not affect the proportion of seeds emerging, it slowed seedling emergence by 5–10%, and reduced offspring biomass by 25–35%. Interestingly, resource addition to the maternal plants significantly decreased proportion of seedlings surviving after 5 months. Moreover, offspring from maternal plants with resource addition expressed more inbreeding depression in the seedling stage compared to offspring from maternal plants without resource addition, for which more inbreeding depression occurred during seed maturation and emergence. These results indicate that conservation efforts using benign environments to increase number of seeds or offspring may face compensating reductions in survivorship at other life stages.     

Selective fruit and seed maturation in Asphodelus albus Miller (Liliaceae)

The fruiting patterns of the rhizomatous perennial Asphodelus albus Miller (Liliaceae) were studied in five populations during 1989 and in one population over 3 years (1988–1990). Fruit/flower (Fr/Fl) ratio and seed/ovule (S/O) ratio varied markedly between populations. Although there were differences between years within the population studied over 3 years, these variations, although statistically signifcant, were less important than those between populations. Neither flowering phenology nor plant size influenced Fr/Fl or S/O ratios. Field experiments tested whether fruit and seed set were pollenor resource-limited. Hand-pollination had no effect on Fr/Fl or S/O ratios, but the reduction of resources by defoliation at the time of flowering decreased both relative components of fecundity. Manipulation of resources by hand-thinning flowers and tiny fruits had no demonstrable effect on these ratios, although brood size of individual fruits was significantly affected. It may be concluded that fruit maturation is resource-limited rather than pollen-limited. Most of the fruits aborted early in the fruiting season, and fruits with higher numbers of developing seeds had a lower probability of abortion than fruits with fewer seeds. Analyses of position effects revealed that the fruits in lower positions in the inflorescence matured preferentially. Furthermore, the two ovules in the same carpel tended either both to fail or both to develop into seeds. The plant's ability to selectively mature only high quality embryos may be limited.     

The role of pollination level on the reproduction of females and hermaphrodites in the gynodioecious plant Gypsophila repens (Caryophyllaceae)

In gynodioecious plant species, females are expected to have more resources available for maturing seeds because pistillate flowers are smaller, do not produce pollen, and are thus less costly that perfect flowers. The potential female advantage arising from more abundant resources is, however, likely to vary depending on whether seed production is limited by resource or pollen availability. Here we experimentally investigated the influence of pollen and resource limitation on female advantage in a gynodioecious species using two levels of pollination. Total seed production of females was always greater than that of hermaphrodites: females produced more flowers and more fruits that contained similar numbers of seeds of similar mass. Under low pollination, female and hermaphrodite plants allocated resources to increased flower production rather than to increased seed size or quality. We did not detect any influence of pollen or resource limitation on female advantage, which remained similar under low (= abundant resources) and full pollination. Outcrossed fruits performed better than selfed fruits when the same plant received both selfed and outcrossed pollen on different flowers. These differences were not greater under high pollination, possibly because resources available for each fruit did not differ between our pollen intensity treatments.     

DEVELOPMENTAL AND GENETIC SOURCES OF SEED WEIGHT VARIATION IN RAPHANUS RAPHANISTRUM L. (BRASSICACEAE)

Seed weight is known to have a marked impact on emergence and post-emergence productivity in wild radish (Raphanus raphanistrum). In this paper, I describe several levels of seed weight variation in plants taken from a natural population in Hamden, Connecticut. Six maternal plants from the 1981 season were analyzed in detail: the weights and positions of all seeds within a fruit were recorded, and some of these seeds were used the following summer for competition studies and progeny analysis. Within a plant, average seed weight decreased as the number of seeds within a fruit increased, suggesting that developing embryos compete for maternal resources. Seed weight also varied significantly among the six maternal plants used in the study. Comparison of the average weights of seeds produced by offspring of those six plants with the average weights of seeds borne by the maternal plant revealed a significant genetic component to seed weight variation. Seed weight varied up to six-fold within single fruits of R. raphanistrum; large seeds tend to occur near the pedicel or in the middle positions. Seed size variation seen within single fruits is of sufficient magnitude to result in differential reproductive output among closely related seeds under competitive field conditions.     

From flowering to dispersal: reproductive ecology of an endemic plant,Astragalus australis var. olympicus (Fabaceae)

Astragalus australis var. olympicus is an endemic plant of the Olympic Mountains, Washington. It is considered a Species of Concern by the U.S. Fish and Wildlife Service. This study focused on the reproductive biology of the plant from flower production through seed germination to identify possible weak points that might contribute to its rarity and impede its conservation. Most plants produced a large mean number of flowers and ovules (314.8 and 4106, respectively), but relatively few of these formed fruits and seeds (25.8 and 3.8%, respectively). In decreasing importance, ovules in fruits were lost to predation, seed abortion, and lack of fertilization. The percentages of these fates differed among sites and years. Excluding pollinators by bagging flowers reduced fruit set by ∼50%, but seed set per fruit and seed mass were unaffected. Germination was affected by scarification, temperature, and moisture availability. About 11% of seeds damaged by predispersal seed predators (weevil larvae) remained viable and were released from dormancy. I hypothesize that predispersal seed predation (over 80% at one site) has a negative effect on population growth. Conservation of this species could benefit from improved fruit set and decreased seed predation.     

荒漠植物准噶尔无叶豆结实、结籽格局及其生态适应意义

连续2a采用野外记录的方式对荒漠植物准噶尔无叶豆的结实、结籽格局进行了比较研究。结果表明:植株内花序生成格局表现为不同大小的花序在总花序中的比重与其结实率存在显著正相关,其中含有中等花数目的花序所占的比重及其结实率均最高,此种分布格局能够最大程度的保证繁殖成功;果序内果实生成格局表现为单花着生位置(从近柄端算起)与其结实量占总结实量的比例成线性负相关,花序基部的结实比例最高,顶部结实比例近为0;荚果内种子生成格局表现为:中间位置的胚珠败育率最低。另外,年际间荚果内仅有(1.08±0.03)粒和(1.07±0.03)粒种子能够完全成熟,此种结籽格局是准噶尔无叶豆保证后代质量的最佳策略。     

Pollen and Resource Limitation in Veratrum nigrum L. （Liliaceae）, an Andromonoecious Herb

Pollen limitation and resource limitation were invoked to account for the pattern that flowering plants produce more flowers and ovules than fruits and seeds. This study aimed to determine their relative importance in Veratrum nigrum, a self-compatible, perennial, andromonoecious herb. In order to determine whether female production was limited by pollen grains on stigmas or by available resources, we performed supplemental hand pollination in three populations, male-flower-bud removal in three other populations, and emasculation of hermaphroditic flowers in still another population, resulting in a total of seven populations experimentally manipulated. Across the three populations, supplemental hand pollination did not significantly increase fruit set, seed number per fruit, and total seed production per individual, nor did emasculation of hermaphroditic flowers. Taken together, our results suggest that pollen grains deposited on stigmas were abundant enough to fertilize all the ovules. Male-flower-bud removal significantly increased the mean size of hermaphroditic flowers in all three populations. Female reproductive success was increased in one population, but not in the other two populations possibly due to heavy flower/seed predation. We concluded that the female reproductive success of V. nigrum was not limited by pollen grains but by available resources, which is consistent with Bateman＇s principle. Furthermore, the female reproduction increase of male-flower-bud removal individuals might suggest a trade-off between male and female sexual functions.     

Influence of plant growth regulators on flowering,fruiting, seed oil content,and oil quality of Jatropha curcas

Field experiments were conducted to determine the effects that plant growth regulators (PGRs) have on seed production of Jatropha curcas when they are used for chemical pruning. In the subsequent year, following a single foliar application of PGRs, flowering, fruit set, fruit characteristics, seed total oil content and oil free fatty acid (FFA) content were evaluated. The number of flowers per plant, number of fruits per bunch, fruit- and seed characteristics and seed oil content were significantly affected by the different treatments. However, there were no variations in the degree of fruit set or oil FFA content. A single foliar application of N⁶-benzyladenine produced more flowers per plant, more fruits per bunch, heavier and bigger fruits and seeds with more oil compared to manual pruning. Treatment with 2,3,5-triiodobenzoic acid yielded more flowers per plant and heavier fruits with a higher oil content than the control and manually pruned plants. Treatment with 2,3:4,6-di-O-isopropylidene-2-keto-l-gulonic acid yielded similar results. More fruits per bunch and more seeds per fruit were also produced. Maleic hydrazide treatment yielded more flowers per plant, heavier and bigger fruits with more, heavier, oil rich seeds compared to the control and manual pruning. This study indicates that foliar application of PGRs as chemical pruners in J. curcas may have a sequential effect in boosting seed production, seed oil content and improves fruit quality.