Modification of the Pollen-Stigma Interaction in Brassica oleracea by Water

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Abstract
The presence of a film of distilled water on the stigma surface of freshly opened flowers results in complete inhibition of pollen following both incompatible and compatible pollinations in self-incompatible (SI) genotypes of Brassica oleracea, SI genotypes of B. campestris and one self-compatible (SC) genotype of B. campestris. The application of water to the stigmas after pollination also resulted in a marked reduction in pollen germination and tube penetration. An increase in the time intervals between the application of pollen onto the stigma and the water treatment progressively reduced this inhibition. Pollen germination was also completely inhibited when stigmas from freshly-opened flowers of SI B. campestris and B. oleracea genotypes were washed in water, dried and pollinated with pollen grains of either compatibility. The ability of stigmas to induce pollen germination and tube growth was restored over a period, the length of which was dependent on the incompatibility (S) genotype. Stigmas of B. napus (SC) and SC mutants of SI B. campestris were found to be affected by washing, but stigmas of a SC variety of B. campestris and the immature stigmas from buds of B. oleracea were found to be considerably less affected. Microscopic examination of pollen placed on washed stigmas reveals that grains, irrespective of their compatibility, fail to hydrate normally. When induced to hydrate by raising atmospheric humidity, pollen grains on washed stigmas did germinate, but most of the tubes failed to penetrate the papillar wall and very few entered the style. It is proposed that the water treatment mobilises components of the pellicle which reorganize to block the activity of molecules, present in both SC and SI individuals, responsible for establishing full contact between the pellicle and pollen grain coating.