Pollen limitation and its effect on seed germination

In nature, fruit and seed production in many plants have been shown to be pollen limited. Pollen limitation is demonstrated when open-pollinated plants that are hand-supplemented (P_s) with outcross pollen produce more fruits and/or seeds than open-pollinated controls that are not hand-pollinated (P_o). There are three categories of results in such studies: P_s > P_o, P_s = P_o and P_s < P_o, in which case pollen limitation indices are positive, zero and negative, respectively. In an index widely used to calculate pollen limitation, 1 - (P_o/P_s), the bounds for P_s ≥ P_o are 0 to + 1, whereas the bounds for Ps < Po are 0 to -∞. The first aim of this review was to show how the pollen limitation index can be modified so that the bounds of P_s < P_o are 0 and -1, whereupon the index gives equal weight to the best performer (P_s or P_o) and worst performer (P_s or P_o). In addition to seed quantity, pollen supplementation can affect seed quality, including germinability. Thus, our second aim was to summarize the results of studies that have also tested the effect of pollen limitation on seed germination. In short, the 30 case studies in 15 families, 16 genera and 18 species that we identified show that seed germination percentage increased, was not affected or decreased by pollen supplementation in 12, 11 and seven cases, respectively. The effect of pollen limitation on seed germination, which can be quite large, has not been considered in developing population growth models to determine the effect of pollen limitation on λ.