Limitations to Fruit Production in Ionopsis utricularioides (Orchidaceae)

Abstract

The proportionally low fruit set observed in many epiphytic tropical orchids usually has been attributed to insufficient pollination; for example, the tropical orchid Ionopsis utricularioides, a twig epiphyte, offers no floral rewards, is rarely visited, and has low fruit set. To determine what factors limit fruiting in this species, we assessed effects of resources, pollinator service, pollen parentage, and pollen quantity using greenhouse and field studies in which individual plants were followed over two flowering seasons. Plants were self-compatible, and pollination frequency substantially affected fruit set. Hand-pollination of field experimentals increased total inflorescence fruit sets from 6 to 19 percent, even though over one-third of such pollinations failed to yield fruit. For flowers pollinated on the same day, fruit failure was related to leaf length but not to position in the inflorescence, flower age, or pollen quantity (one vs two pollinia). High fruit set resulted in reduced growth and a higher probability of foregoing inflorescence production the following year. This last result suggests that resource availability may limit lifetime fruit production, even though pollination limitation occurs within a single season. We also explored some predicted consequences of resource-limited reproduction. First, we found no evidence for mate choice in the sense of selective abortion of fruits depending on selfed verses outcrossed pollen parentage. Second, our plants produced many more flowers than could mature fruit. Although fruit production on a per-flower basis decreased with inflorescence size, pollinarium removal increased with number of open flowers. This suggests that pollen donation by surplus flowers increases the fitness of those plants with larger inflorescences. However, most inflorescences are small, which suggests trade-offs between conflicting selective pressures. Third, we speculate that resource limitation of fruit production may have influenced the evolution of nectardeception pollination in many orchids. PLANTS OFTEN PRODUCE FAR fewer fruits than flowers. Resource shortages frequently limit fruit set (Stephenson 1981), but fecundity may also be reduced by adverse environmental conditions (Wyatt 1976, Coulter 1979, Lee & Bazzaz 1982), genetic incompatibility between maternal and paternal genomes (Schlising 1976, Wyatt & Hellwig 1979), paternal identity (Bertin 1982, Waser & Price 1983), seed predation (Boucher & Sork 1979, Arnold 1982, Haddock & Chaplin 1982), inadequate pollination (Bierzychudek 1981), or combinations of factors (Augspurger 1981, Heithaus et al. 1982, Hainsworth et ail. 1984). Fruit set in epiphytic tropical orchids often is low, probably in part because of infrequent pollinator visits (Darwin 1885, Dressler 1968, pers. obs.). Schemske (1980), Janzen et al. (1980), Ackerman and Montalvo (1983, pers. obs.), and Ackerman and Montero Oliver (1985) demonstrated that artificial pollination increased fruit set in natural populations of several such species. They suggested that fruit set may be pollinator limited, but also discussed complicating factors which might limit fruit production over the lifetime of these perennial plants. In Puerto Rico we examined factors limiting fruit production in an epiphytic, nectarless orchid, L. utricularioides (Sw.) Lindl. Our major questions were as follows. (1) Does pollination limit fruit set? We supplemented natural pollination on experimental plants in the field and compared subsequent fruit set of entire inflorescences to that of open-pollinated controls. An increase in fruit set indicates pollination limitation within a single season. (2) Do resources limit present or future fruit production? We followed individual plants for two flowering seasons to determine if fruit set in one year affected subsequent vegetative growth and inflorescence production. We also provided field-collected plants with water and nutrients in the greenhouse to determine effects on subsequent plant size and flower production. (3) Do pollen loads or paternal identity influence fruit set? We conducted self-compatibility experiments and measured the effects of two sizes of pollen loads on fruit set. These studies were supplemented with field observations of herbivory, disease, and natural pollination frequency. Proximate causes and ultimate consequences of limited fruit set are important in understanding many evolutionary questions (Stephenson 1981). We address the following implications of resource limitation. (1) Plants may be able to choose which ovules or fruits they mature according to I Received 28 May 1985, revision accepted 24 October 1985. 2 Order of authorship determined by a coin toss. 24 BIOTROPICA 19(1): 24-31 1987 This content downloaded from 157.55.39.248 on Thu, 28 Jul 2016 05:41:35 UTC All use subject to http://about.jstor.org/terms identity of pollen parents (Janzen 1977, Waser & Price 1983, Willson & Burley 1983), especially if resources are limiting. We tested for one level of female choice by measuring whether plants selectively mature fruits of outcrossed over selfed flowers. (2) If resources are limiting, then selection should favor a reduction in flower number unless surplus flowers contribute to fitness through pathways other than female fecundity. We explore some of the possible functions of surplus flowers. (3) Resource limitation may influence the evolution of nectar-deception, perhaps the most prevalent form of deceit pollination in orchids, in which flowers appear to contain nectar but, in fact, do not (Ackerman, 1985).