Factors influencing seed weight in Impatiens capensis (Balsaminaceae)

I analyzed withinand between-plant variation in seed weight from one greenhouse and four natural populations of Impatiens capensis Meerb. Most variation in seed size in natural populations occurred within populations and within plants. Weights of seeds from cleistogamous flowers on greenhouse plants increased with height on the plant and proximity to the main stem, a result that may be related to the local supply of photosynthate. Mean seed weight in the natural populations declined during seed set, then increased again. Seeds from chasmogamous flowers were significantly larger in one of the four populations. Maternal parents differed significantly in both mean seed weight and the relative size of the two types of seeds. Combined genetic and general environmental effects accounted for 26% of the total phenotypic variance, which provides an outside estimate of the heritability of this character. COMPARISONS OF SEED WEIGHT between plant species have revealed an extreme range of variation, while seed weight within a species remains roughly constant despite great variation in overall plant size (Harper, Lovell and Moore, 1970; Harper, 1977). Salisbury (1942) and Baker (1972) investigated variation in seed weight between species and found larger seeds in species colonizing dry and/or shady habitats. They suggested that increased resources were required to initiate growth in these habitats, and that within a habitat, demands of various dispersal mechanisms also affected seed size (Salisbury, 1975). Smith and Fretwell (1974) modelled this general relationship and predicted that increased seed size should be favored under harsh conditions. Comparisons of seed size within a species have shown more subtle variation within and between populations. Some of this variation is attributable to the same selective forces that influence seed size between species. Thus, populations growing at higher plant densities, with increased shade and root competition, often produce larger seeds (Solbrig and Simpson, 1974; Werner and Platt, 1976). Such variation is adaptive because even these relatively minor differences in seed size within a species are capable of producing substantial differ1 Received for publication 17 July 1981; revision accepted 21 January 1982. I thank B. Klehr for her assistance in this study. R. Sweeney, J. Hickman, and an anonymous reviewer contributed many helpful suggestions on the manuscript. T. Mitchell-Olds pointed out how the data could provide an outside estimate of heritability. ences in seedling performance (Black, 1958; Henson and Tayman, 1961; Stickler and Wassom, 1963; Cope, 1966; Harper et al., 1970; Williams, Black and Donald, 1968; Gross and Soule, 1981). Demands of wind or animal dispersal or predation (Janzen, 1977; Hare, 1980) may also favor adaptive changes in seed size within species. Maun and Cavers (1971) investigated variation in seed within individual plants of Rumex crispus, and found considerable variability, indicating an appreciable degree of phenotypic plasticity (environmental variance) for this trait. The seeds produced at the tips of the distal branches within the panicle were smaller than seeds on more proximal branches near the base of the inflorescence. Within-species variation in seed weight, then, has both genetic and environmental components. Differences between populations and individuals reflect both of these components, but variation within a plant can only reflect environmental variance. This environmental variance could result from either developmental instability or genetically based adaptive variability, but these are very difficult to distinguish (Hickman, 1979). Only in species incapable of adjusting the number of seeds after their initiation would it be obvious that changes in physiological status would lead to nonadaptive variation in final seed weight. I here report the results of an investigation into variation in seed weight in Impatiens capensis Meerb. (= I. biflora Walter = I. fulva Nuttall [Fernald, 1950]), orjewelweed, an herbaceous annual that produces both cleistogamous and chasmogamous flowers. Jewelweed is capable of colonizing a wide variety of hab-