PLANT SIZE EFFECTS ON ALLOCATION TO MALE AND FEMALE FUNCTIONS IN PEARL-MILLET, A HERMAPHRODITIC WIND-POLLINATED SPECIES

In hermaphroditic higher plants, reproductive fitness can be achieved through male and female functions. The relative allocations to reprodu ction by maternal and paternal investments define the gender of each i ndividual. In wind-pollinated species, the theory predicts that gender should evolve with plant size, the largest plants investing the most in male function. More specifically, the tallest plants should exhibit the highest degree of maleness, because pollen dispersal is most effe ctive when the release point is high. We searched for potential variat ion in gender with plant size in pearl millet (Pennisetum typhoides), where each plant produces several shoots. Size was estimated by using four parameters, namely height, stem diameter, total leaf area, and ve getative weight, for shoots and for plants. Gender was estimated as th e pollen to ovule (P:O) ratio for each shoot or plant. We found that a llocation to male function is very variable, both between shoots withi n a plant and between plants. Most of the difference observed is corre lated with differences in the weight of the shoots or plants, the heav iest ones having the highest P:O ratios. The theoretical prediction th at maleness should increase with plant height was not fulfilled. The p arameter used to estimate plant size seems to have a great influence o n the ability to detect gender variation with size. The allocation to reproduction (defined as the weight of seeds, stamens, floral parts, a nd rachis of the spike) represented a decreasing proportion of total p lant weight; also, the heavier the plant, the higher the proportion of this allocation devoted to male function. Since pearl millet is a cul tivated species, this may be due to the fact that the domestication pr ocess, by selecting for larger plants, involuntarily selected for male ness by favoring the largest plants.