Self-incompatibility and post-fertilization maternal regulation cause low fecundity in Aegle marmelos ( Rutaceae).

Outbreeding confers an evolutionary advantage, and flowering plants have evolved a variety of contrivances for its maximization. However, neither fruit set nor seed set is realized to its fullest potential for a variety of reasons. The causes of low flower to fruit and seed to ovule ratios were investigated in naturally occurring bael trees ( Aegle marmelos) at two sites for three seasons. The study established that the mass effect of synchronized flowering attracted a variety of insect pollinators to the generalist flowers; Apis dorsata was the most efficient pollinator. The seed to ovule ratio was low despite high natural pollination efficiency ( c. 2400 pollen per stigma). Although pollination-induced structural and histochemical changes in the style allowed many (9.5 ± 2.1) pollen tubes to grow, only cross-pollen tubes could grow through the style. Gametophytic self-incompatibility, manifested in the stylar zone, resulted in a significantly slower growth rate of self-pollen tubes. The occurrence of obligate self-incompatibility, coupled with increased self-pollen deposition (geitonogamy), caused a significant number of flowers to abort. Fruit retention in the trees declined from 40% to 12% as a result of abortion of fruits at different stages of development. The number of mature fruits on a tree was negatively correlated ( r = −0.82) with their size. It is inferred that low natural fecundity in A. marmelos is primarily a result of obligate self-incompatibility and strong post-fertilization maternal regulation of allocation of resources to the developing fruits. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172, 572-585. [ABSTRACT FROM AUTHOR]