SEEDLINGS FROM LARGE SEEDS TOLERATE DEFOLIATION BETTER - A TEST USING PHYLOGENETICALLY INDEPENDENT CONTRASTS

The characteristic seed size of a plant species may reflect evolution toward an optimal compromise between size and number of seeds produced. The extent to which large seed size conveys an advantage to a seedling probably varies according to circumstances, and this may explain at least some of the great variation in seed size among species. Here we test whether large-seeded species are better able than small-seeded species to withstand defoliation at the seedling stage. We compared growth and survival of seedlings of 40 species of Australian angiosperms after removing 95% of their photosynthetic tissue shortly after emergence. We accounted for effects of phylogeny by selecting pairs of species that were ''phylogenetically independent contrasts.'' That is, the phylogenetic path connecting the two species of any pair was independent of the path connecting the two species of any other pair we used. We also selected the species so that the two members of each pair differed at least 10-fold in seed mass. Therefore, each pair provided an independent case for examining the consequences of evolutionary divergence in seed size. We found that the larger seeded species survived loss of photosynthetic tissue better than the smaller seeded species in 14 of the 16 pairs for which there was a significant difference in survival. The extent to which growth of survivors was reduced by loss of photosynthetic tissue also differed in 8 pairs, with the large-seeded species being less affected in 6 of these cases. We conclude that large seed size is generally associated with greater ability of seedlings to cope with loss of photosynthetic tissue. Consequently, large seed size may be favored in any circumstances in which seedlings are likely to experience carbon deficit early in development.