Double- and Single-Seeded Indehiscent Legumes of Platypodium elegans: Consequences for Wind Dispersal and Seedling Growth and Survival

Abstract

The tropical tree, Platypodium elegans (Leguminosae: Papilionoideae), matures indehiscent wind-dispersed fruits containing one or, much less commonly, two seeds. Relative to single-seeded fruits, double-seeded fruits have greater wet mass, area, wingloading, and rate of descent in still air, and consequently are dispersed shorter distances under field conditions. The proximal seed of double-seeded fruits has a smaller dry mass and usually has lower and slower germination than the distal seed. Its radicle has difficulty emerging from the legume and establishing a root system, and the seedling has lower survival and slower growth. Twin seedlings arising from one fruit grow more slowly than single seedlings from doubleor single-seeded fruits, and both twins rarely survive to one year under growing house conditions. In fruit samples of equal size, more total seedlings emerge from doublethan single-seeded fruits; however, due to their lower probability of survival, double-seeded fruits have no more seedlings at one year under growing house conditions than do single-seeded fruits. Seed predation was not measured in this study. Unless multiseeded fruits more easily escape seed predation, there is no apparent evolutionary advantage to a parent of P. elegans producing multiseeded fruits. Their presence in low numbers appears to result from incomplete elimination or suppression of development of the multiple ovules after pollination. MANY TROPICAL TREES IN THE LEGUMINOSAE have winddispersed fruits. Their flowers commonly have more than one ovule, and they can produce multiseeded, indehiscent fruits. Although a large majority of their fruits are oneseeded, many species also produce a minority of multiseeded fruits. Any change in seed number per fruit has the potential to affect patterns of resource allocation to seeds and fruits, seed predation, dispersal distance, and subsequent germination and seedling survival and growth. For an herbaceous legume using ballistic dispersal, Lee (1984) showed that an increase in seed number per fruit increased the variance, but not the mean, in dispersal distance. Casper and Wiens (1981) have argued that reduced seed number per fruit is achieved by a fixed rate of ovule abortion, and perhaps enhances dispersal by wind in Ctyptantha flava. Following are the results of a study of the tropical canopy tree, Platypodium elegans J. Vogel ((Leguminosae: Papilionoideae). It produces indehiscent, wind-dispersed fruits containing either one or two seeds. The study begins by determining how this variability in seed number affects traits that define the dispersal potential of the fruits, i.e., mass, area, wind-loading, and rate of descent. Next it examines whether double-seeded fruits are dispersed shorter distances than single-seeded fruits. Finally, in a screened greenhouse, the study identifies differences between the two fruit types in seed mass and germination level, and in subsequent survival and growth of seedlings. STUDY SITE AND NATURAL HISTORY The study took place in 1980-1981 in the semideciduous lowland forest on Barro Colorado Island, Panama. Details of this biological preserve are described in Croat (1978) and Leigh et al. (1982). Platypodium elegans is a canopy tree of low abundance on the island. Its flowers have up to four ovules. Seed development usually occurs in the most distal ovule, and never in the two most proximal ovules. The legumes are obliquely oblong, up to 13 cm long and 3.6 cm wide in single-seeded fruits and 16 cm long and 4.5 cm wide in double-seeded fruits. The seed or seeds are in the distal end of the fruit. The portion of the fruit surrounding the seeds is quite thick and woody; by contrast the fruit's wing is highly vascularized and