Metabolic rate of angiosperm seeds: effects of allometry, phylogeny and bioclimate

Energetics is considered a fundamental 'currency' of ecology and the way that metabolic rate (MR)-the rate of energy expenditure on biological processes-scales relative to the size of the organism can be both an adaptive benefit and a constraint in mediating the energetic demands of ecological processes. Since few investigations have examined this relationship for angiosperm seeds, we measured standard metabolic rate (SMR) of 108 species' seeds, spanning a broad suite of species. We used fluorescence-based closed-system respirometry at temperatures between 18°C and 30°C, based on optimal germination conditions, and Q₁₀ corrected to 20°C. The allometric relationship for SMR as a function of seed mass was 0.081 × M^0.780 with ordinary least squares regression and 0.057 × M^0.746 with phylogenetic generalized least squares regression. This relationship is consistent with the pervasive metabolic allometry documented for both vegetative plants and domesticated cultivars (n = 14) which had higher SMR residuals than wild species (seven weeds and 87 Australian native species). For native species, seed SMR was strongly related to measures of increasing environmental aridity (annual mean temperature and precipitation, and precipitation in the wettest quarter), consistent with seeds from arid environments having a high MR to supply energy needed to germinate rapidly. By comparing SMR of seeds for diverse angiosperm species, we provide insights into inter-relationships of physiology, distribution, climate and domestication on seed ecology and suggest that energetics represents a valuable addition to established functional trait libraries for seed biology.