Floral development and breeding systems of Dryandra sessilis and Grevillea wilsonii (Proteaceae)

Abstract

Dryandra sessilis (Knight) Domin. and Grevillea wilsonii A.Cunn. co-occur on lateritic soils in the jarrah forest of Western Australia, with their flowering seasons overlapping for several months during winter and spring. Both species are protandrous, with pollen presentation occurring 1–2 days before the stigmas of individual flowers become receptive. Peak receptivity, as indicated by maximum peroxidase secretion, coincides with the maximum opening of stigmatic grooves for D. sessilis or maximum elevation of the stigmatic papillae in G. wilsonii. Pollinators such as western wattlebirds, New Holland honeyeaters and western spinebills almost exclusively visit flowers or inflorescences at times when large amounts of nectar are present, and when self-pollen is available for transfer or stigmas are receptive to the deposition of pollen. By foraging in this manner, yet still moving frequently between plants and inflorescences, honeyeaters that visit D. sessilis should guarantee an effective spread of pollen. The overall level of fruiting success achieved by this species is considerably higher than that for G. wilsonii, a species whose flowers are visited much less often than those of D. sessilis. Allozyme analysis, artificial pollination experiments and the calculation of pollen : ovule ratios indicate that D. sessilis is essentially an obligate outcrosser, at least in locations where plant densities are high, whereas G. wilsonii has a mixed mating system that allows it to set fruits as a consequence of either selfing or outcrossing. Potential fruit-set is low for both species, with post-zygotic incompatibility mechanisms mediated by the availability of nutritional resources thought to be at least partly responsible for the even lower levels of final fruit-set. Total fruit and seed production by individual plants is much greater for D. sessilis than for G. wilsonii, and possibly a reflection of differences in the regeneration requirements of the two species after environmental disturbance such as fire.