Nitrogen-richness and protection of seeds in Australian tropical rainforest: a test of plant defence theory

Some species invest more in protecting their seeds or embryos than others, yet the reasons behind this disparity have not been clearly identified. In order to test the hypotheses that species with marked protection in the fruit have high nitrogen concentration ([N]) in the seeds, and species with marked protection in the seeds have high [N] in the embryo-cum-endosperm fraction (EE), we determined the [N] in the seeds and EE of 194 species from lowland tropical rainforest in NE Queensland. For species in which the fruit provides marked protection of the seed there is a significant negative linear relationship between N and log seed mass. There is a similar relationship for 'unprotected' species, but the intercept on the y-axis is significantly and markedly higher for the group of 'protected' species. This relationship holds true when genera rather than species are considered We are also able to make taxonomically controlled comparisons within six families and within four genera: all six familial comparisons and two of the generic comparisons support the finding for the collection of species as a whole. We then compare the [N] in the EE of species in which the fruit does not offer marked protection, but in which there is a range of investment in the seed wall fraction. Species with > 60% dry mass in the wall have significantly higher [N] than species with less massive walls. Possible advantages of having nitrogen-rich seeds are discussed.