FREEZING-INJURY TO OVULES, POLLEN AND SEEDS IN WINTER RAPE

Winter rapeseed (Brassica napus, cv. Samourai) flowers early in spring and, under field conditions, short freezing periods can occur. Unacclimatized plants were freeze-stressed (- 3 degrees C for 4 h) at different developmental stages of buds, open flowers and seeds. The dissection of pistils from stressed plants showed that freezing results in shrivelled ovules. We assessed freezing injury on the basis of per cent of shrivelled ovules: ovule sensitivity begins early (8 d before anthesis) but increases up to anthesis. Cross-pollination of stressed pistils with non-stressed pollen showed that recording of freeze-injured ovules is a good method for early estimation of the effect of stress on seed yields. On the other hand, stress does not reduce the viability of pollen, except when it was applied at the binucleate pollen stage. Use of frozen pollen x nonstressed pistils has little effect on seed yields. Freeze injury on seeds was assessed by seed filling: seeds are very susceptible just after fertilization until 20 d after fertilization (DAF). Freezing seems to inhibit seed filling. A germination test of stressed seeds during their development indicated that embryo viability is not affected if the stress occurs after 35 DAF. As the embryos develop, resistance to stress increases.