Allometric relationships between masses of seed functional components

Compared with biomass allocation patterns in plant vegetative organs, traits scaling has been much less studied in seeds. By quantifying intraspecific variations in masses of seed functional components at multiple ecological levels, we aim to provide the first understanding of the ways in which masses of seed dispersal, defence and reserve components scale against each other, and to examine the variations in the composition of seed components across populations along a south–north gradient. In a wind-dispersed Asteraceae annual, Geropogon hybridus, pappus, seed coat and seed reserve were weighed for individual seeds across three populations at its southern range margin. The variations of seed component masses were partitioned at the population, plant and seed levels. The scaling relationships were quantified between seed component masses, using standard major axis regressions. The variations in seed component masses had a generally balanced distribution across the three hierarchical levels of population, plant and seed. Consistently at both the seed and the plant levels, the mass scaling of seed components showed an isometry between pappus and seed coat (slope CI including one), and a negative allometry between seed reserve and seed coat (slope CI below one). From south to north, the scaling slopes within populations increased, and meanwhile the mass ratios of pappus and seed coat in a whole seed decreased whereas the mass ratio of seed reserve increased. The findings demonstrate that biomass investment in dispersal scales proportionally with that in defence. This study also suggests that smaller seeds invest proportionally more biomass in protective tissues than do larger seeds, in agreement with traditional ideas that small seeds may have advantages in terms of physical defence. Overall, our quantification of the within-seed biomass allocations demonstrates strong correlations between seed functional components, and improves our understandings of plant reproductive strategies under changing climate.