POLLEN-STIGMA INTERACTIONS IN BRASSICA

SUMMARY The osmotic potential (tpx) of the stigmatic papillar cells of Brassica oleracea is —14-8 bars. In laboratory conditions each cell transpires water at rates within the range from 3 X 10" 5 to 5 X 10~ 5 mm 3 h~'. A small increase in transpiration rate is detected followingcross-(compatible) but not self-(incompatible)pollination. No significant changes in Tpn occur following pollinations of either compatibility. Electron microscopy reveals an active papillar cytoplasm apparently secreting proteins into the cell wall via small vesicles. The cuticle is discontinuous and freeze-fracture techniques indicate that channels transverse the cell wall, suggesting a possible pathway for the movement of protein molecules of high molecular weight from the cytoplasm to the stigma surface. Analysis of electron-microscopic autoradiographs of mature, self-incompatible papillae following pulse-chase experiments with L-[ 3 H]leucine and treatment with cycloheximide shows that protein molecules secreted into the cell wall may return to the cytoplasm at a later stage. The significance of these results is discussed in terms of current models of the pollen-stigma interaction in Brassica.