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 pollina- tions 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 ¯owers; 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 ¯owers produced normal and straight pollen tubes while self-pollen tubes had growth abnormalities. Most of the open-pollinated ¯owers 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 sel®ng may occur G. robusta presents a self-incompatibility system and allogamy is its primary breeding behaviour.

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DOI:

http://dx.doi.org/10.1006/anbo.2000.1170

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