Floral structure, stigma receptivity and pollen viability in relation to protandry and self-incompatibility in silky oak (Grevillea robusta A. Cunn.)

The reproductive biology of Grevillea robusta growing under exotic conditions in Kenya and Australia is reported. The species showed both protandry and a self-incompatibility mechanism. The stigma was wet and papillate with a distinct groove in the middle. The anthers dehisced prior to anthesis, when the perianth opened. Stigmatic receptivity began 1 d after anthesis, with the greatest pollen germination rates and longest pollen tubes obtained 2 d after anthesis. Nectar secretion commenced with pollen dehiscence and was abundant at anthesis. Most stigmatic grooves opened widely 1-2 d after anthesis and stigmas showed taller papillae and abundant secretion. Controlled pollinations gave a greater fruit set from cross-pollination (5.9 % in April and 17.5 % in July) than open-pollination (0.1 % in April and 3.3 % in July). No fruit set from self-pollination was obtained in April, and very few fruit set for geitonogamous (two out of 1622; 0.1%) or for autogamous (one out of 2707 flowers; 0.04 %) pollination treatments in July. Following self-pollination, growth of pollen tubes was poorer than in other treatments, and was generally arrested in the upper style. Cross-pollinated flowers produced normal and straight pollen tubes, while self-pollen tubes had growth abnormalities. Most of the open-pollinated flowers were found without pollen or with only self-pollen on their stigmas indicating that the amount of cross-pollen reaching the stigma under open-pollination may be a factor limiting seed production. Flowers shed soon after the fertilization phase were those with ungerminated pollen or no pollen. Although a very low rate of selfing may occur, G. robusta presents a self-incompatibility system and allogamy is its primary breeding behaviour. (C) 2000 Annals of Botany Company.