Pollen load size, reproductive success, and progeny kinship of naturally pollinated flowers of the tropical dry forest tree Pachira quinata (Bombacaceae)

Several studies have demonstrated, using controlled pollinations, that the number and identity of pollen grains deposited onto a flower's stigma affec t the reproductive success of plants. However, few studies have shown this relationship under conditions of natural pollination. Using the tropical dr y forest tree Pachira quinata, we evaluated the relationship between the nu mber of microgametophytes per pistil and the number of sires with respect t o the production of fruits and seeds in a natural population of Pachira qui nata. Our study demonstrates that fruit and seed production are directly re lated to the number of microgametophytes per pistil in natural populations of P, quinata, Only 6% of the marked flowers developed into mature fruits a nd 10% of the marked flowers initiated fruits but later aborted them, A mea n of 23 pollen grains were required to produce a seed. Flowers with > 400 p ollen grains on the stigma always developed into mature fruits, whereas flo wers that received < 200 grains never matured fruits. Half of the pollen gr ains transferred to a flower stigma germinated and developed pollen tubes t o the base of the style. The number of pollen grains on a stigma explained 34% of the variation in seed number per fruit. and the number of seeds prod uced per fruit is positively correlated with the size of the seeds. The pop ulation of P. quinata studied is predominantly outcrossing, and seeds withi n fruits are sired by one or a few donors. The total seed crop within trees was sired by three to five donors. Our study examined the implications of the variation in size of microgametophyte loads per pistil with respect to the breeding system and the paternity of progeny under natural conditions. The competitive ability of pollen and pollen tube attrition are important f actors regulating fruit production in P. quinata.