Allocation to reproduction in pearl millet: Correlations between male and female functions

We have described variation in allocation to male and female floral function in relation to plant size in an attempt to detect expected life history trade-offs in pearl miller (Pennisetum typhoides, Poaceae). Traits quantifying allocation to male and female functions (mean number of pollen grains per anther, number of ovules, number of seeds, and mass of seeds per spike) were measured among tillers within plants and among plants. Trait values varied significantly both among plants and among tillers. Variation in male and female reproductive allocation did not result from differences in flowering phenology among tillers. Positive correlations between allocation to male function (expressed as mean pollen grain number per anther) and female function (expressed as mass or size of seeds) were recorded at tiller and plant level. Since in pearl millet male and female allocation are strongly positively correlated with the size of the plant, new estimates of allocation to sexual functions were used that were independent of possible size variation observed among plants. If allocation to sexual function were expressed per g dry mass, there were significant positive correlations between allocation to the two sexual functions when female function was expressed in terms of number of ovules or number of seeds per unit of biomass. On the contrary, there were significant negative correlations between sexual functions when female function was expressed in terms of mass of seeds produced per unit of biomass. Because ovule abortion rate was negligible in the plants studied and because of strong developmental constraints on the floral structure of pearl millet (a spikelet is constituted of a male flower with three stamens and a hermaphroditic Bower with one ovule and three stamens), positive correlations between number of seeds (or ovules) produced per unit of dry mass and male function were unavoidable. However, plants that produced many pollen grains per unit of aboveground dry mass also produced significantly lower mass of seeds (per unit of aboveground dry mass) than plants that invested less into male function. Allocation of large amounts of resources into male function may impair the amount of resources available later for allocation to female function, i.e., mass of seeds.