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 id 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.