Pollen protein synthesis and control of incompatibility in Brassica

Excised but otherwise intact cabbage (Brassica oleracea var. capitata) stigmas release a watersoluble substance which selectively inhibits germination of self-but not cross-pollen. The inhibitory effect on self-pollen germination is dependent on both the concentration of stigma extract and on the time of addition. Low concentrations of stigma extract inhibit when present from the start (zero time additions) of pollen imbibition, whereas high concentrations do not. High concentrations inhibit when stigma extracts are added 1 to 2 minutes after the start of pollen imbibition, but germination is increasingly less inhibited when additions are delayed 2 to 4 minutes. Similar inhibition kinetics are also observed with delayed additions of cordycepin and cycloheximide. Stigma extracts selectively inhibit leucine-(14)C incorporation into proteins of self-pollen. We conclude that germination does not require protein synthesis whereas the regulation of self-incompatibility does.