SELECTIVE FRUIT ABORTION IN GREVILLEA-BARKLYANA (PROTEACEAE)

The breeding system of Grevillea barklyana F. Muell. ex Benth. was studied using experimental hand pollinations. Fruit set was generally low (less than 5%), and equivalent for autogamy, self-pollination, outcross-pollination and inflorescences left exposed to natural pollination. Pollen tubes were produced in only a low proportion of flowers in the autogamy treatment, with few pollen tubes per flower and short pollen tubes in styles at both 24 and 48 h after anthesis. Selfing and cross-pollination produced equal numbers of pollen tubes in an equivalent and high proportion of flowers (ranging from 40% to 80%). However, pollen tubes resulting from selfing were shorter than from outcrossing after 24 and 48 h. Hand pollination experiments in which half of the flowers on inflorescences were pollinated with self pollen and the other half with outcross pollen generally produced a slightly skewed pattern of fruit set, with a higher proportion of the outcrossed flowers setting fruit. The following processes could contribute to the selective development of outcrossed fruit in this self-compatible species: (1) pollen tubes from outcrossed pollinations reach ovules before those growing from self pollen; (2) fruits that start to develop first obtain first call on limited resources; (3) fruits that are initiated later abort when other fruits on an inflorescence are developing.