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 adistinct groove in the middle. The anthers dehisced prior to anthesis, when the perianth opened. Stigmatic receptivitybegan 1 d after anthesis, with the greatest pollen germination rates and longest pollen tubes obtained 2 d afteranthesis. Nectar secretion commenced with pollen dehiscence and was abundant at anthesis. Most stigmatic groovesopened 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 forgeitonogamous (two out of 1622; 0.1%) or for autogamous (one out of 2707 ¯owers; 0.04 %) pollination treatmentsin July. Following self-pollination, growth of pollen tubes was poorer than in other treatments, and was generallyarrested in the upper style. Cross-pollinated ¯owers produced normal and straight pollen tubes, while self-pollentubes 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 bea factor limiting seed production. Flowers shed soon after the fertilization phase were those with ungerminated pollenor no pollen. Although a very low rate of sel®ng may occur, G. robusta presents a self-incompatibility system andallogamy is its primary breeding behaviour.

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DOI:

https://doi.org/10.1006/anbo.2000.1170

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