Apical pattern of fruit production in the racemes of Ceratonia siliqua (Leguminosae : Caesalpinioideae): Role of pollinators

Fruit production and arrangement within the raceme were studied in two dioe cious populations of Ceratonia siliqua (Leguminosae: Caesalpinioideae), an arboreal species that produces caulogenous racemes (emerging only from the old branches) with numerous flowers. Fruit production per raceme was low an d similar between years and populations and even between individuals. Durin g flowering, there were considerable flower losses from predation and lack of pollination. A mean of nine flowers per raceme began the transformation into fruits, of which 77% aborted. The final fruit production per raceme in creased significantly following hand pollination, but was always very much lower than the availability of Rowers in the raceme. The results suggest th at fruit production of each raceme is limited by both availability of resou rces and a deficient pollination. In racemes setting fruit arrangement foll ows a definite pattern that remains constant between years and populations: fruit production was significantly higher in the apical zone of the raceme and lower in the basal zone. The pollinators of C. siliqua (flies and wasp s) showed a clear preference for beginning their visits at the apex of a ra ceme. As a result, the pollen load deposited on the stigmas decreased from apex to base of the raceme. In most of the flowers situated in the central and basal zone of the raceme, the number of pollen grains deposited on thei r stigmas was lower than the number of their ovules. The high number of see ds in developed fruits suggests that the plant selectively aborts flowers t hat receive a smaller pollen load. The results indicate that the final patt ern of fruit arrangement within the raceme is a direct result of pollinator activity.