EFFECTS OF SOIL-NITROGEN ON POLLEN PRODUCTION, POLLEN GRAIN-SIZE, ANDPOLLEN PERFORMANCE IN CUCURBITA-PEPO (CUCURBITACEAE)

To determine the effects of soil nitrogen on pollen production, pollen size, and pollen performance, two cultivars of zucchini (Cucurbita pe po) were grown under two nitrogen regimes in an experimental garden. T he two cultivars were true breeding for alternative alleles for a one gene trait, ovary color. The soil nitrogen treatment had a significant effect on most measures of reproductive output through the female fun ction. The nitrogen treatment did not affect the number of staminate f lowers per plant but did have an effect on the number of pollen grains per staminate flower and the mean pollen grain size. A pollen mixture experiment revealed that pollen produced by plants in the high nitrog en treatment sired significantly more seeds than pollen from low nitro gen plants. Moreover, we found that the high nitrogen pollen sired eve n a greater percentage of seeds in the region of the fruit (ovary) pre viously shown to be fertilized by the fastest growing pollen tubes. Th us, the difference in the number of seeds sired by pollen from the two nitrogen treatments is due to differences in pollen performance. We c onclude that spatial heterogeneity in soil nitrogen can influence the paternity of seeds in a plant population.