Flowering phenology and gender variation in Pennisetum typhoides

Because of the modular structure of pearl millet (an annual grass crop, Poa ceae), different tillers of a plant share the same genotype but are subject ed to different environmental conditions during their maturation. This allo ws investigation of the effects of tiller flowering phenology on allocation to resource-producing photosynthetic biomass, sexual functions, and thus t iller gender. All tillers of plants of two families collected from individu al maternal plants (represented by 33 and 31 plants each) were analyzed. In both families, allocation to aboveground vegetative biomass decreased as f lowering was delayed. On average, late-flowering tillers were 65% smaller t han the first ones to flower. The proportion of biomass allocated to reprod uction significantly increased with the flowering rank of the tillers, sugg esting that translocations of assimilates occurred between early- and late- flowering tillers. In both families, late-flowering tillers produced signif icantly fewer pollen grains per stamen than early-flowering ones, and femal e reproductive allocation (expressed as seed mass per tiller) was also affe cted by flowering phenology. Tillers became increasingly female as flowerin g phenology progressed. This gender variation is possibly adaptive because pollination efficiency is maximized by plant height. Natural selection may favor a shift toward femaleness to maximize reproductive fitness in small, late-developing tillers.