Is the benefit of larger seed provisioning on seedling performance greater under abiotic stress?

Seed size is a relevant predictor of seed survival, seedling phenotype and species recruitment and it is generally accepted that larger seeds give rise to seedlings with improved performance. In pioneer tree species the success in seedling survival is strongly determined by resource availability, which largely varies at diverse spatial scales. Here, using a pioneer Mediterranean pine species as a model (Maritime pine, Pinus pinaster Ait.), we hypothesized that the positive effect of seed provisioning on seedling establishment and early performance may be stronger under environmental harassment, and that the progeny of lineages investing in bigger seeds would be favored in such stressful conditions. We explored the effect of seed mass on seedling traits related to germination, survival, growth and biomass partitioning by sowing pre-weighted seeds of known genetic background and culturing the seedlings under a range of abiotic stress conditions by experimentally modifying phosphorus (four levels, 5260 seeds) and water availability (three levels, 975 seeds) in two independent experiments. Phenotypic variation in seed mass was extremely large, with more than a tenfold change between the smallest and the largest seed. Seed mass variation showed a strong genetic component with large and significant variation both between and within populations. As expected, seed mass was a positive predictor of germination time and early seedling performance. However, contrary to our expectations, populations from more stressful sites produced smaller seeds, and the positive effect of seed mass on seedling performance was not magnified under resource limitation. Our results thus does not support that early seedling performance is favored by greater seed provisioning under resource limiting conditions.