Unmanipulated Plants Research Articles

Reproduction in an Orchid Can Be Resource‐Limited over its Lifetime1

ABSTRACTMost orchids studied thus far show long‐term resource adjustments to increases in fruit production within a flowering season, but none of these offers rewards to their potential pollinators. If nectar production is energetically expensive, then resources utilized to produce fruits and seeds may be even more limited in pollinator‐rewarding orchids than in non‐rewarding ones. Thus, resource adjustments may be more dramatic or entirely different in nectar producing plants. In this study, we performed artificial hand‐pollinations for two consecutive flowering seasons in die nectar producing orchid Comparettia falcata, and tested whether or not fruit set, seed set, and seed viability were limited by the quantity of pollinations or by resources. In addition, we compared mechanisms of short‐term (fruit abortion within seasons) and long‐term consequences (percent change in leaf length and change in flower number per plant between seasons, probability of shoot and inflorescence production, and mortality) between hand‐ pollinated and unmanipulated plants. The relationships among plant traits related to vegetative size and reproduction also were examined. Hand‐pollinations showed some negative effects. Fruit set was higher in hand‐pollinated plants in the first season but was similar to the controls in the second. Seed set was significantly lower and abortions were higher than in unmanipulated plants. On the other hand, some of our measurements were unaffected by die hand‐pollination treatment. Unexpectedly, there were no significant differences between groups in percent change in leaf length, change in flower number per plant between seasons, or die probability of shoot and inflorescence production. Although there was a strong correlation between leaf size and die number of flowers produced within a season, associations between leaf size and traits related to current or future reproduction were not consistent. Like other epiphytic orchids, pollination limitation occurred within a single season in C. falcata., but increases in fruit production also resulted in reduced lifetime fitness as estimated by a compounded fitness index. Contrary to all other epiphytic orchids studied, long‐term adjustments to increased fruit production in C. falcata through reduction in future growth or flower and inflorescence production were either minor or lacking. Our results suggest that the nature of plant strategies associated with resource constraints during sexual reproduction may be dependent on whether or not plants have evolved traits that are costly.

Reproduction in an Orchid Can Be Resource-Limited over its Lifetime1

Most orchids studied thus far show long-term resource adjustments to increases in fruit production within a flowering season, but none of these offers rewards to their potential pollinators. If nectar production is energetically expensive, then resources utilized to produce fruits and seeds may be even more limited in pollinator-rewarding orchids than in non-rewarding ones. Thus, resource adjustments may be more dramatic or entirely different in nectar producing plants. In this study, we performed artificial hand-pollinations for two consecutive flowering seasons in the nectar producing orchid Comparettia falcata, and tested whether or not fruit set, seed set, and seed viability were limited by the quantity of pollinations or by resources. In addition, we compared mechanisms of short-term (fruit abortion within seasons) and long-term consequences (percent change in leaf length and change in flower number per plant between seasons, probability of shoot and inflorescence production, and mortality) between hand-pollinated and unmanipulated plants. The relationships among plant traits related to vegetative size and reproduction also were examined. Hand-pollinations showed some negative effects. Fruit set was higher in hand-pollinated plants in the first season but was similar to the controls in the second. Seed set was significantly lower and abortions were higher than in unmanipulated plants. On the other hand, some of our measurements were unaffected by the hand-pollination treatment. Unexpectedly, there were no significant differences between groups in percent change in leaf length, change in flower number per plant between seasons, or the probability of shoot and inflorescence production. Although there was a strong correlation between leaf size and the number of flowers produced within a season, associations between leaf size and traits related to current or future reproduction were not consistent. Like other epiphytic orchids, pollination limitation occurred within a single season in C. falcata, but increases in fruit production also resulted in reduced lifetime fitness as estimated by a compounded fitness index. Contrary to all other epiphytic orchids studied, long-term adjustments to increased fruit production in C. falcata through reduction in future growth or flower and inflorescence production were either minor or lacking. Our results suggest that the nature of plant strategies associated with resource constraints during sexual reproduction may be dependent on whether or not plants have evolved traits that are costly.

Nitrogen effects on an interaction chain in a salt marsh community.

Nutrients can structure communities by influencing both plant interactions and plant herbivore interactions, though rarely do studies integrate these processes. In this study we examined how nitrogen fertilization influenced (1) the positive interaction between the marsh elder, Iva frutescens, and the black rush, Juncusgerardi, and (2) the quality of Iva as a host plant for the aphid, Uroleuconambrosiae. Previous studies have shown that by mitigating soil salt accumulation and hypoxia, Juncus is essential to the survival of Iva and its aphid herbivore at mid-marsh elevations. To address the effects of nitrogen on this interaction, we compared fertilized and unfertilized Iva plants subject to Juncus removal and control treatments in the field. Additionally, we measured the monthly population growth rates of aphids transplanted onto these Iva plants. Iva leaf biomass and flower number results indicated that fertilizing Iva eliminated its dependence upon Juncus, such that fertilized plants grown without Juncus were not different from unmanipulated plants. Aphid monthly population growth rates through mid-summer revealed that fertilization also eliminated the indirect dependency of aphids on Juncus, so that aphid growth rates on fertilized Iva without Juncus neighbors were similar to rates on unmanipulated Iva. Results also indicated that fertilizing Iva grown with Juncus increased Iva size, potentially enabling these plants to support larger aphid populations. Our results suggest that only under conditions of nitrogen limitation are the positive effects of Juncus essential to the mid-marsh persistence of Iva and its aphid herbivore. Furthermore, we found that nitrogen effects on aphid populations may arise not only from a direct effect of nutrients on Iva size but also through the indirect effects of nitrogen on the interaction between Juncus and Iva. We argue that studies integrating processes occurring both within and between trophic levels, are important to fully understanding the community-wide effects of nutrients.

Patterns and Consequences of Benzyl Acetone Floral Emissions from Nicotiana attenuata Plants

The emission of a single compound, benzyl acetone (BA, 4-phenyl-2-butanone), is barely detectable during the day in the headspace of flowers of the self-compatible disturbance species Nicotiana attenuata, but it increases dramatically (50×) in the evening, becoming the dominant component in the floral headspace. This striking temporal pattern of emission may be sculpted by its potential ecological roles (e.g., synomonal and kairomonal), which we examine here. We measured the nightly BA emissions from individual flowers at six different branch positions on plants receiving either self-pollen or pollen from another genotype and calculated the nightly whole-plant emission. The first flowers produced on a branch have a lower rate of emission than flowers produced later on the same branch; however, cross pollination did not influence the quantity of BA emitted from subsequently produced flowers. Informed by these measures of whole-plant emission, we constructed a device that released BA at a constant rate equivalent to that of a plant with 240 open flowers (an approximate 10× increase in emissions). This device and a control device were attached to 50 matched pairs of plants growing in a native population in Utah to estimate the fitness consequences of enhanced, constant BA emission. Plants with elevated BA emissions in the field were browsed more frequently than control plants and produced fewer capsules, so that lifetime seed production was reduced by 3.1%. However, both treatment and control plants were heavily attacked by negro bugs (Cormelina spp.) and produced light seeds with low viabilities, representing 47% and 23% of the mass per seed and viability, respectively, of unmanipulated plants, which flowered two weeks later in the same population. From glasshouse experiments, we estimated the consequences of out-crossing and attack by negro bugs on seed production. Out-crossing did not significantly affect seed production, seed mass or viability. In contrast, negro bug infestation dramatically decreased seed mass and viability. We conclude that while the phenological variation in attack rates might have obscured our ability to estimate the fitness consequences of enhanced BA emission, the effects are likely to be dominated by kairomonal rather than synomonal interactions for this self-compatible species.

Within‐patch foraging behaviour of the aphid parasitoid Aphidius funebris: plant architecture, host behaviour, and individual variation

AbstractThe influence of plant architecture, host colony size, and host colony structure on the foraging behaviour of the aphid parasitoid Aphidius funebris Mackauer (Hymenoptera: Aphidiidae) was investigated using a factorial experimental design. The factorial design involved releasing individual parasitoid females in aphid colonies consisting of either 10 or 20 individuals of Uroleucon jaceae L. (Homoptera: Aphididae) of either only larval instar L3 or a mixture of host instars, both on unmanipulated plants and on plants that had all leaves adjacent to the colony removed. Interactions between the parasitoid and its host were recorded until the parasitoid had left the plant. The time females spent on the host plant and the number of eggs laid varied greatly among females. Host colony size significantly affected patch residence time and the number of contacts between parasitoids and aphids. Plant architecture influenced the time‐budget of the parasitoids which used leaves adjacent to the aphid colony for attacking aphids. Female oviposition rate was higher on unmanipulated plants than on manipulated plants. No further significant treatment effects on patch residence time, the number of contacts, attacks or ovipositions were found. Oviposition success of A. funebris was influenced by instar‐specific host behaviour. Several rules‐of‐thumb proposed by foraging theory did not account for parasitoid patch‐leaving behaviour.

Resource Allocation in Relation to Leaf Retention Time of the Wintergreen Rhododendron Lapponicum

Extended leaf longevity has been explained both as an adaptation to conserve nutrients and to increase seasonal or life—time carbon gain. The relative importance of these possible adaptations was investigated in the wintergreen dwarf shrub Rhododendron lapponicum by analyses of responses of growth and nutrient allocation to early and late spring defoliation and to natural reduction of leaf biomass following flowering. Removal of old leaves at the time of budbreak had no effect on biomass accumulation and nutrient pools of the new shoots 1 mo later, although the defoliation prevented the transport of resorbed nutrients from old leaves to the new shoots. This transport accounted for by far the largest part of the increase in shoot N and P in underfoliated controls. Extra nutrients were not transported from stems and roots to compensate for the lost import from old leaves. Hence, defoliated plants compensated for the losses solely by increasing nutrient uptake. In contrast, defoliation 3 wk earlier caused a 29% reduction of annual shoot growth and a proportional decline of nutrient pools in the shoots, while the nutrient concentrations, reflecting the shoot nutrient status, did not decline. Almost identical responses were observed in unmanipulated plants the year after heavy flowering. Reproductive branches do not form leaves, resulting in a decreasing photosynthetic capacity and a reduction of resource storage and allocation to next year's leaf cohort. Although early and late spring defoliation caused equal losses of mobile nutrients, only the early defoliation decreased growth, suggesting that it was the loss of carbon rather than nutrients that affected growth. This support earlier observations that the extended retention time of leaves in wintergreen and evergreen species plays a greater role in improving the carbon balance than in supplying the new leaves with nutrients.

Mirror image flowers and their effect on outcrossing rate in Chamaecrista fasciculata (Leguminosae)

The influence of enantiostyly (reciprocal segregation of anthers and stigmas to different sides of the flower) on outcrossing rate was examined in Chamaecrista fasciculata (Leguminosae). I hypothesized that enantiostyly has not evolved to increase the female component of outcrossing and actually acts to increase the selling rate through geitonogamy. To quantify the role of enantiostyly to outcrossing, plants of known isozyme genotype were manipulated to be either completely left‐ or right‐styled (nonenantiostylous) or to have equal numbers of left‐ and right‐styled flowers (enantiostylous). Flower number was varied to quantify any interaction between floral display size and enantiostyly on outcrossing rate. These “target” plants were surrounded by unmanipulated plants homozygous for the alternative allele. Outcrossing rates of the target plants were determined by scoring the presence or absence of heterozygotes. The contribution of enantiostyly to geitonogamy may be reduced if pollinators discriminate among the floral types. Thus, observations of pollinator movement between flowers on the same plant were made to determine if pollinators discriminate between the floral types. Although pollinators moved randomly between flower types, outcrossing rate was only marginally effected by the presence of enantiostyly. Enantiostylous plants outcrossed at a slightly lower rate than nonenantiostylous plants only when the opportunity for geitonogamy was great. These results suggest that the contribution of enantiostyly to selling is minimal.

PATTERNS OF RESOURCE ALLOCATION IN A DIOECIOUS CAREX (CYPERACEAE)

Allocation to vegetative growth and sexual reproduction was investigated throughout the growing season in the dioecious sedge, Carex picta, under natural conditions and following experimental manipulations. Measurements taken on unmanipulated plants showed that the sexes did not differ in the total amount of biomass they allocated to either growth or reproduction. The relatively equal investment in reproduction by the two sexes is contrary to other studies, the majority of which show greater investment in reproduction by females. Two features of the reproductive biology of C. picta may account for the equal investment: the fruit are relatively inexpensive because they are uniovulate and nonfleshy, and the stamens are relatively expensive because C. picta is wind pollinated. In contrast to the lack of differences in the amount of allocation, there were differences between the sexes in the timing of allocation to growth and reproduction: males allocated more to reproduction and less to growth up to the time of flowering, whereas females showed this pattern during the time of fruit maturation. Defoliation and inflorescence removal experiments showed that a trade‐off within plants between growth and reproduction does exist. In addition, the defoliation experiment revealed a difference in the response of the two sexes: defoliated tillers on males showed a reduction in growth, whereas defoliated tillers on females did not. Overall, the data support the idea that differences in the timing of reproductive expenditure are as important as the amount of expenditure in determining many aspects of the life history strategies of the two sexes.

Patterns of Resource Allocation in a Dioecious Carex (Cyperaceae)

Allocation to vegetative growth and sexual reproduction was investigated throughout the growing season in the dioecious sedge, Carex picta, under natural conditions and following experimental manipulations. Measurements taken on unmanipulated plants showed that the sexes did not differ in the total amount of biomass they allocated to either growth or reproduction. The relatively equal investment in reproduction by the two sexes is contrary to other studies, the majority of which show greater investment in reproduction by females. Two features of the reproductive biology of C. picta may account for the equal investment: the fruit are relatively inexpensive because they are uniovulate and nonfleshy, and the stamens are relatively expensive because C. picta is wind pollinated. In contrast to the lack of differences in the amount of allocation, there were differences between the sexes in the timing of allocation to growth and reproduction: males allocated more to reproduction and less to growth up to the time of flowering, whereas females showed this pattern during the time of fruit maturation. Defoliation and inflorescence removal experiments showed that a trade-off within plants between growth and reproduction does exist. In addition, the defoliation experiment revealed a difference in the response of the two sexes: defoliated tillers on males showed a reduction in growth, whereas defoliated tillers on females did not. Overall, the data support the idea that differences in the timing of reproductive expenditure are as important as the amount of expenditure in determining many aspects of the life history strategies of the two sexes.

Self- and Cross-Fertilization in Plants. III. Methods for Studying Modes and Functional Aspects of Self-Fertilization

Procedures are described for partitioning the selfing rate into contributions resulting from the different modes of chasmogamous selfing-geitonogamous, facilitated, prior, competing, and delayed-and for examining several functional aspects of self-fertilization associated with the different modes-reproductive assurance, degree of seed and pollen discounting, and relative abilities of cross- and self-pollen to fertilize ovules. The procedures involve floral manipulations-e.g., exclusion of pollinators, emasculation, and covering of stigmas-and typically require joint estimation of the mating system parameters in manipulated and unmanipulated plants. Two methods of mating system estimation are discussed, the first in which manipulations and selfing rate estimates are made using random parent plants that are resident in the population, and the second using plants with unique genotypes introduced from a separate population. The asymptotic variances of estimators for the different modes of selfing are compared for each method, and advantages and disadvantages of each method are presented. The method of partitioning selfing into modes is illustrated using data from a natural population of the annual plant Impatiens pallida. Selfing in the chasmogamous flowers of this species is shown to result primarily from geitonogamy.

Costs of Reproduction in a Willow: Experimental Responses Vs. Natural Variation

We report on the interaction between reproduction and subsequent reproductive and vegetative growth in female plants of a dioecious, early successional, tall shrub, the feltleaf willow (Salix alaxensis). We measured natural variation among individuals in flowering, and its correlation with subsequent shoot growth and flowering. We also experimentally curtailed reproductive allocation in two ways: (1) by removing inflorescence buds in April when plants were dormant, and (2) by covering inflorescences to prevent pollination and fruit development. We hypothesized that these treatments would cause manipulated plants to retain more resources and to deploy them subsequently in shoot growth or reproduction. However, treatments had no significant effect upon shoot elongation, which decreased slightly after floral buds were removed. Treatments did significantly increase the fraction of buds differentiating as inflorescences instead of shoots. We compared the experimental outcome with the natural, phenotypic variation among plants. Among plants of either the treated or the unmanipulated groups, there was a significant negative correlation between current reproduction and subsequent growth; the corresponding regressions did not differ between the two groups. Thus, natural phenotypic variation suggested a tradeoff between reproduction and growth, but a direct test by experiment revealed only a tradeoff between current and future reproduction. Furthermore, the negative correlation seen in unmanipulaed plants was unaffected by treatments curtailing reproductive investment. We conclude that this correlation is not directly related to allocation contraints, and does not provide reliable evidence about costs of reproduction. The increased commitment to flowering that occurred after experimental reduction of current reproductive investment may be evidence for a cost of reproduction, although the response need not have been caused directly by resource allocation constraints.

Phosphorus limitation ofCymodocea nodosa growth

Experimental nutrient (N, P, and N+P) additions to shallow (ca. 1 m)Cymodocea nodosa (Ucria) Aschers. stands growing in patches and in a continuous meadow in a Mediterranean Bay (Alfacs Bay, NE Spain) in 1988 demonstrated mid-summer growth to be strongly P-limited, as suggested by the high N:P ratios (>35) in unmanipulated plants. P additions resulted in increased leaf P content, reduced N:P ratios, and enhanced shoot growth and turnover in both populations. These effects promoted in turn a stand response, leading to increased biomass and, therefore, increased areal productivity in the plots receiving P. The stand response was largely attributable to a doubling of shoot density, indicating enhanced rhizome growth and branching.

A Field Tests of Interspecific Competition on Oviposition of Gall‐Forming Sawflies on Willow

We performed a manipulative field experiment to test for competitive effects of the stem galling sawfly (Euura lasiolepis) on three other gall—forming sawflies on arroyo willow (Salix lasiolepis). Within a willow clone, three branches were left untreated, three branches had E. lasiolepis females placed on them in nylon screen bags during that species' oviposition period, and three branches were bagged during that same period but had no sawflies introduced. When oviposition by E. lasiolepis was completed, bags were removed, and three other sawflies were permitted to oviposit on the branches. We found no significant interspecific competitive effects on the densities of three species in response to 9— and 17—fold increase in densities of galls of the stem—galling sawfly on branches of field plants in 1985 and 1986, respectively. The densities of stem gallers on the experimental branches were high, but were near densities observed on some unmanipulated field plants. The density of the petiole galler in 1985 was higher on branches with the stem galler than was the density of the leaf galler. This suggested a facilitation effect, which was not repeated in 1986. The leaf folder had significantly greater densities than the leaf galler or petiole galler. However, bagging of branches resulted in greatly reduced densities of the leaf folder compared to unbagged control branches in both years. The petiole galler and leaf galler were not affected by bagging in either year, probably due to their later oviposition phenology. Within treatment branches, where stem gall densities were high, these three sawfly species did not significantly avoid oviposition on shoots with stem galls, and in 1986 there were generally positive co—occurrence patterns of each of these species with stem galls on shoots. We conclude that interspecific competitive effects on gall density was not important under these field conditions for the leaf galler and the petiole galler, but bagging effects on the leaf folder density could have reduced the likelihood of detecting competitive effects by the stem galler on this species. Differences in the results of this study with an earlier experiment may be related to compensatory ability of the larger field plants and differences in stem gall densities.

Factors Affecting Fruit-Set in Telopea Speciosissima (Proteaceae): The Importance of Pollen Limitation

The principal aim of this study was to determine whether seed set in Telopea speciosissima R.Br. (Proteaceae) (waratah), a shrub with large, conspicuous inflorescences, is limited by inadequate pollination. To overcome the major methodological shortcoming mentioned above, whole-plant samples were used, and the impact of increased fruiting in one year on flowering in the following year for the hand-pollinated plants was also investigated. Having found that pollination was limiting fruit-set in the sample under study, our secondary aim was to examine the importance of number of inflorescences per plant determining fruit-set among unmanipulated plants

ANDROMONOECY AND VARIATION IN PHENOTYPIC GENDER OF PASSIFLORA INCARNATA (PASSIFLORACEAE)

We studied the perennial vine Passiflora incarnata Linnaeus (Passifloraceae) in Alachua County, Florida, from May to August 1983 to determine the breeding system and investigate seasonal changes in phenotypic gender of individual plants. Passiflora incarnata is an obligate outcrosser, incapable of self‐fertilization. The major pollinators were carpenter bees (Xylocopa sp.). The proportion of flowers setting fruit was not limited by pollinator service, but the weight of individual fruits and number of seeds/fruit was increased over naturally pollinated flowers by manually performing cross‐pollination. Morphological differences in style position among flowers caused some flowers to function primarily as males and others to function as hermaphrodites. Although some of the flowers that functioned as males set fruit when manually cross‐pollinated, the proportion of these male flowers capable of setting fruit was lower than the proportion of hermaphroditic flowers setting fruit when manually cross‐pollinated. Further, some male flowers had atrophied ovary and styles and were completely incapable of setting fruit. Passiflora incarnata is thus functionally andromonoecious. The relative production of male versus hermaphroditic flowers varied among individuals and over the course of the flowering season. Unmanipulated plants in the population became increasingly male‐biased in floral sex ratios as the reproductive season progressed. We attempted to modify phenotypic gender in experimental plants by limiting the ability of plants in some treatment groups to set fruit. Treatment groups significantly affected production of hermaphroditic flowers, but production of male flowers was not affected by treatment. Treatment did not significantly affect fruit weight, number of seeds per fruit or the percentage of hermaphroditic flowers that successfully matured fruit. These results suggest that andromonoecy in P. incarnata is a mechanism for adjusting allocation of reproductive effort to male and female function, and that maternal investment in this species is regulated primarily by varying production of hermaphroditic flowers.

Ant Constancy at Passiflora Extrafloral Nectaries: Effects on Caterpillar Survival

I examined the role of ants in controlling the magnitude and distribution of caterpillar survival at Corcovado National Park, Costa Rica. Ants of 29 species were observed visiting the extrafloral nectary glands of Passiflora vitifolia and P. quadrangularis. Plants attended by ants had ants present 60% of the time. Approximately equal numbers of plants attended by ants and plants without ants were treated with Tanglefoot to exclude walking predators from a single branch on each plant. Heliconius ismenius caterpillars were placed on the treated plants and an equivalent set of unmanipulated controls, and their growth and survivorship were measured. Forty—eight percent of caterpillars died over a 2—d period on unmanipulated plants with many ants, while only 20, 22, and 23% died on unmanipulated plants with few ants, Tanglefoot—treated branches with many ants outside the Tanglefoot barrier, and Tanglefoot—treated branches with few ants outside the Tanglefoot barrier, respectively. This suggests that ants were the only significant walking predator of Heliconius ismenius caterpillars in this habitat, and that °75% of the deaths observed on control plants were on plants with ants. After 4 d all caterpillars were replaced by a second set of caterpillars. Measurements of these latter individuals revealed that mortality was 62, 46, and 24% on control plants on which caterpillars previously survived for 0, 2, and 4 d, respectively. No trend was observed on branches with ants excluded. This indicates that the effects of ant predation are locally stable, i.e., that ant constancy causes plants to be heterogeneous in their suitability as host plants. Caterpillars thus are much more likely to survive on some plants than on others. Analysis of caterpillar growth rates did not reveal any effects on survival; i.e., slow—growing caterpillars survived as well as fast—growing ones.

BREEDING SYSTEM, POLLINATION, AND POLLINATOR LIMITATION IN A PERENNIAL HERB, AMIANTHIUM MUSCAETOXICUM (LILIACEAE)

Observations of insects on Amianthium muscaetoxicum suggested that at least 16 species can serve as pollinators, although the chief pollinators appear to be beetles. Five species from three coleopteran families appear to be the most important pollinators. Experimental manipulations suggest that Amianthium is nearly self‐incompatible and that fecundity and fruit set levels are at least partly pollinator limited. Outcrossed plants have higher fruit set and fertility levels than unmanipulated plants, and unmanipulated plants display higher levels than self‐pollinated plants. Self‐pollinated plants produce a few very small seeds, of which only a small percentage are viable. Fruit set levels in unmanipulated plants in one population were consistent over a 2‐yr period.