Floral sexuality and breeding system in gum karaya tree, Sterculia urens

Comprehensive studies were carried out on phenology, floral sexuality, pollination biology, pollen-pistil interaction, breeding system and fruit and seed set on three populations of gum karaya tree (Sterculia urens). The species is andromonoecious and produces a large number of male and a limited number of bisexual (functionally female) flowers. The numbers of male and bisexual flowers varies not only between trees but also during the flowering period within a tree. Each male flower produces about 5000 fertile pollen grains. Neither in morphology nor in number, is there any difference between pollen grains in the bisexual and male flowers. However, pollen grains of bisexual flowers are completely sterile and incapable of siring any seeds. Their anthers, however, serve to attract pollinators; the emasculated bisexual flowers fail to do so. Thus S. urens is apparently andromonoecious but exhibits cryptic monoecy. That the species is self-incompatible was confirmed by controlled pollinations. The self-incompatibility is of the late-acting type and manifests after the entry of the pollen tube into the ovule. Apis indica is the only pollinator recorded by us and wind plays no role in pollination. The efficacy of pollination is low as only 56% of flowers were estimated to be pollinated. The pollen load on one-third the number of pollinated stigmas was lower than the number of ovules present. Fruit set under open pollination is poor and is highly variable from tree to tree (0.7–3.2%). Apart from pollination constraint, limited resource availability may also contribute to low fruit set.