EFFECT OF SEED SIZE ON SEEDLING SUCCESS IN THREE SPECIES OF SESBANIA (FABACEAE)

Variation in seed size may produce variation in seedling fitness, but the relationship is not simple. Differences in seed size within and among species may not have the same effects. We examined effects of differences in seed size within and among three species of Sesbania, S. macrocarpa, S. drummondii, and S. vesicaria, on seedling emergence and growth in the greenhouse and the field. Of the three species, the largest-seeded species, S. vesicaria, produced the largest, longest-lived seedlings in both the greenhouse and the field. Even though plant size differed, annual S. macrocarpa produced the same seed mass as annual S. vesicaria in the greenhouse. Within-species effects were less clear. In the greenhouse, S. vesicaria seedlings grown from large seeds remained largest until maturity, but the other species did not exhibit this effect. Some persistent within-species effects of seed size differences on height were observed in the field in 1981, but not in 1980, suggesting that field conditions increase the importance of seed size differences. Unscarified S. drummondii seeds germinated before seeds of the two annual species. Within species, larger seeds of the annuals and smaller seeds of the perennial germinated first. Differences among the species in the importance of seed size to seedling fitness may allow the species to have different patterns of regulation of reproduction in response to stress. Sesbania vesicaria showed the largest within-species effects of seed size and has the lowest plasticity in seed size, suggesting that patterns of plasticity have been selected such that the most important component of yield varies least. WHILE THEORETICAL CONSIDERATIONS predict that the production of larger seeds will yield larger, more competitive seedlings (Salisbury, 1942; Harper, Lovell, and Moore, 1970; Stebbins, 1971; Smith and Fretwell, 1974), studies of the relationship between seed size and seedling growth and survivorship have given mixed results (e.g., Twamley, 1967; Carelton and Cooper, 1972). Seed size may affect initial seedling growth, but not final seed yield (Black, 1959; but see Gross and Soule, 1981). Furthermore, the effects of seed size may be apparent only under competitive conditions (Black, 1958; Stanton, 1984) and earlier germination of small seeds may compensate for initial differences in seed size (Black and Wilkinson, 1963). Thus, although a number of studies of the effect of seed size on seedling success have been performed, our understanding of this relationship remains incomplete. ' Received for publication 30 July 1985; revision accepted 21 October 1985. David Braun weighed plant parts, Scott Shapiro sorted roots, and Kevin Ford and Mary Jean Hattrup assisted in the germination experiment. Tim Holtsford, Norm Ellstrand, Norma Fowler, and Don Levin provided helpful comments on an earlier draft of this manuscript. Financial support was provided by a research grant and a fellowship awarded to DLM from the graduate school of The University of Texas, Austin. Permission to work at Lake Somerville was provided by the U.S. Army Corps of Engineers, Ft. Worth District. 2 Present address: Department of Biology, Univ. of New Mex., Albuquerque, NM 87131. A number of factors may make the relationship between seed size and seedling fitness complex. First, the relationship between seed size and seedling quality may not be linear (Marshall, Fowler, and Levin, 1985a), so that all differences in seed size may not have uniform effects. For example, growth rates may differ among seedlings of varying size (Stebbins, 1976). Second, differences in seed size among species may represent physiological differences in addition to differences in size and, thus, have different effects than variation in size within species. Third, the nature of the size differences in seedlings-whether differences in leaf area, stem length or root length are involved-may determine the conditions under which increased seed size will have an impact on fitness. Fourth, the most important effect of seed size may be its influence on the kind of microsite in which germination and seedling establishment is possible (e.g., Gross and Werner, 1982; Gross, 1984) rather than its influence on seedling growth. Fifth, field conditions, which may involve microhabitat variation and competition, could either mask or exaggerate seed size effects seen in the greenhouse (Fenner, 1978; Gross, 1984). Finally, even if seed size and seedling size are correlated, plasticity in offspring size, rather than a constant size, may be adaptive in a variable habitat (Bradshaw, 1965; Capinera, 1979). Attempts to understand the importance of