Seed size: phylogeny and adaptation in two closely related Proteaceae species-pairs.

We studied seed size, seed nutrient status and seedling growth of two closely related fynbos Proteaceae species-pairs growing on juxtaposed soils of different nutrient and moisture status. Seeds had a greater mass and higher phosphorus and nitrogen contents for species occurring on limestone (higher nutrient and moisture contents) than those on the colluvial sands (lower nutreints and moisture). This trend was found within, but not across genera, stressing the importance of phylogeny in interpreting adaptations. It would be difficult to test for the effects of either nutrients or moisture separately, since the same advantage of enhanced seedling size, and hence survival in a stressed environment, applies to both factors. The increased root: shoot ratios (using lengths) of the Leucadendron species relative to the Protea species are interpreted as an attempt to overcome a phylogenetic constraint that results in smaller seed size in the former genus.