Patterns of reproductive effort in Arabidopsis thaliana: confounding effects of size and developmental stage

We demonstrate for the annual Arabidopsis thaliana, that reproductive effort (RE) varies with plant size but is affected more directly by developmental stage; i.e. RE is "stage-dependent". We evaluated plant size and RE under a constant environment with two harvest criteria, and under a variable environment (a range of densities) with one harvest criterion. Plants of 10 genotypes were grown singly and harvested either at maturity of the first fruit (early) or at complete senescence (late). Plants from a subset of 3 genotypes were grown at 6 additional densities and harvested at maturity of the first fruit. For plants grown singly, later harvest resulted in significantly higher RE (seed biomass / total remaining above ground biomass) and larger plant size than early harvest. Conversely, RE decreased with increasing size for plants grown under a range of densities and harvested at maturity of the first fruit. We argue that these apparently contradictory relationships between RE and size are a consequence of variation in the progress toward full allocation to reproduction (i.e. the developmental stages of individuals) resulting from interactions between environment (density) and chronological age. Plants with 600-fold size variation did not differ significantly in reproductive effort when individuals had the opportunity to conclude their development; i.e. RE was not size-dependent. Because size and developmental stage are often poorly correlated in plants, it may not be possible to interpret variation in RE in terms of size-related effects unless developmental stage is controlled.