Variation in magnesium and nitrogen content in seeds of antelope bitterbrush ( Purshia tridentata, Rosaceae)

One‐hundred and sixty‐three seeds from 38 shrubs of antelope bitterbrush ( Purshia tridentata ) from four different sites were individually analyzed for N and Mg content. Seed nutrient content has previously been shown to affect seedling growth, competitive ability, and adult biomass and seed production, though estimations of nutrient concentration in seeds have always been based on bulk samples of more than one seed. The results from antelope bitterbrush show that individual seeds vary in N and Mg content (coefficient of variation = 15.9 and 10.1%, respectively), with most of the variation being attributable to seed size (over 70%). Larger seeds not only had greater absolute quantities of N and Mg, but also a greater concentration of N. Differences between seeds from the same shrub explained the second largest amount of variation (over 20%), while differences between shrubs and site of growth, though significant, accounted for much less of the variation (just over 5%). Soil N and Mg were not closely related to seed N or Mg at a site, suggesting that decisions on how much N or Mg to allocate to seeds are not entirely based on supply. Shrubs that were browsed most intensively the winter prior to seed production had seeds with higher concentrations of Mg. Differences in seed mineral content between shrubs suggest the possibility for natural selection to operate, though research to determine heritability of this character will be necessary to confirm how much of the observed variation is attributable to plasticity in seed filling.