Effects of soil fertility and mycorrhizal infection on pollen production, pollen grain size, and the ability to sire seeds in Cucurbita pepo (Cucurbitaceae)

I studied the effects of soil fertility (two levels of soil nitrogen and two levels of soil phosphorus) and mycorrhizal infection on pollen production, pollen grain size, and the siring ability of pollen from two cultivars of the common zucchini (Cucurbita pepo). Overall, soil fertility and mycorrhizal infection had significant effects on traits affecting the male function of plants (staminate flower production, pollen production per flower, and pollen grain size). There were also differences between the cultivars for traits related to the male function in all three experiments. In addition, pollen grain size decreased toward the end of the growing season. A significant negative relationship between pollen production and pollen grain size was found in the mycorrhiza and soil phosphorus experiments, indicating that there is a tradeoff between pollen production and pollen size. In the phosphorus experiment, pollen produced by plants in high phosphorus soils had a higher phosphate concentration than pollen produced by plants in low phosphorus soils. In the mycorrhiza experiment, phosphate concentration per pollen grain was higher, but not significantly higher, in the mycorrhizal plants than in the non-mycorrhizal plants. In the soil nitrogen and soil phosphorus experiments, pollen mixture experiments revealed that pollen produced by plants in the high soil fertility treatment sired significantly more seeds than pollen from low soil fertility treatment. Moreover, I found that pollen from plants growing in the high nitrogen soils sired a greater percentage of seeds in that region of the fruit (ovary) that had previously been shown to be fertilized by the fastest growing pollen tubes. Thus, the difference in the number of seeds sired by pollen from the two nitrogen treatments is due to differences in pollen performance. I conclude that spatial heterogeneity in soil fertility and the levels of mycorrhizal infection can influence the male function of plants and consequently, the paternity of seeds in a plant population.