Effects of Competition on the Growth of Lotus corniculatus L. Seedlings Produced by Random and Natural Patterns of Fruit Abortion

Abstract

Inflorescences of Lotus corniculatus abort about half of their immature fruits. Compared to random patterns of fruit abortion (accomplished by handthinning fruits on some inflorescences prior to any abortion), natural patterns of fruit abortion (inflorescences that self-thinned their fruits) produced mature fruits that contain significantly more seeds. Moreover, progeny from self-thinned inflorescences had significantly more leaves and significantly greater stem length at 30, 60 and 90 days after germination and they had significantly greater dry weight at 90 days than progeny from hand-thinned inflorescences. Because this study controlled for both effects of seed size and the maternal parentage of seedlings in competition, it is likely that differences in vigor between progeny from handand self-thinned inflorescences are due to differences in the paternal parentage of seeds. INTRODUCTION Darwin (1883; p. 398-400), Janzen (1977), Charnov (1979) and Lee (1984) hypothesized, using different lines of reasoning, that plants could improve the average quality of their seed crops through nonrandom (selective) fruit abortion. Their hypothesis rests on the common observation that many plant species regularly abort a large portion of their pollinated flowers and immature fruits (see Lloyd, 1980; Stephenson, 1981; Sutherland and Delph, 1984) because they are physiologically incapable of providing the necessary resources to mature all young fruits (see Stephenson, 1981). Support for their hypothesis comes from evidence showing that fruit abortion is often nonrandom (Stephenson, 1981; Stephenson and Bertin, 1983; Lee, 1984; Winsor et al., 1987). For example, in many species fruits with below average number of seeds are the most likely to abort (Lee, 1984; Winsor et al., 1987). ln a recent study, we found that Lotus corniculatus (Leguminosae) selectively aborts fruits on the basis of seed number. Moreover, L. corniculatus seeds produced by natural patterns of fruit abortion (selective fruit abortion) are more likely to germinate, are more vigorous as seedlings when grown singly in greenhouse pots, and have greater reproductive output as adults under experimental garden conditions than do seeds produced by random patterns of fruit abortion (accomplished by hand-thinning fruits prior to abortion) (Stephenson and Winsor, 1986). Here we examined the growth of progeny produced by natural and random patterns of fruit abortion under competitive conditions. MATERIALS AND METHODS Lotus corniculatus is a weedy herbaceous perennial that is common in disturbed habitats and meadows throughout much of eastern North America (Gleason and Cronquist, 1963). Each spring, the root system sends up one to several ramets (shoots) that form a 'Present address: Division of Biological Sciences, The University of Michigan, Ann Arbor