Pollination and Resource Constraints on Fruit Set and Fruit Size ofPersoonia rigida(Proteaceae)

Abstract

Flower and immature fruit abscission can be caused by proximate factors preventing the development of all flowers into mature fruits. Two potential limiting factors, pollen supply and carbohydrate availability, were assessed as relative constraints on accumulation of fruit dry matter inPersoonia rigida(Proteaceae). Natural pollen transfer was highly efficient, and no evidence of self-incompatibility was found. Levels of fruit set (67.4% following open-pollination and 50.4–58.8% following hand-pollinations) may be the highest recorded for a species of Proteaceae with hermaphroditic flowers. Fruit abscission occurred principally in a single phase from 4–10 weeks post-anthesis, with a peak during the eighth week post-anthesis, and this early abscission was the result of fertilization failure in some flowers. Manipulation of carbohydrate availability by girdling and/or defoliation of branches provided evidence that carbohydrate supply affected fruit set only when supply was very low. Fruit abscission due to very low carbohydrate availability on defoliated branches was evident during the early abscission phase but was more pronounced during the second half of the fruit development period (>17 weeks post-anthesis), coinciding with the period of greatest dry matter demand by the fruits. Minimal fruit abscission occurred during this later period on leaf-bearing branches. Increases in carbohydrate supply had no effect on fruit set, but fruit size was highly sensitive to carbohydrate availability. Subtending and adjacent leaves were identified as major contributors to the carbohydrate supply of fruits, although some carbohydrate could be obtained from beyond the fruiting branch. Despite the high levels of natural fruit set, both pollen supply and resource availability were indicated as potential constraints on accumulation of fruit dry matter inP. rigida.