FLUCTUATING TEMPERATURES HAVE DIFFERENT EFFECTS ON EMBRYONIC SENSITIVITY TO ABA IN SORGHUM VARIETIES WITH CONTRASTING PREHARVEST SPROUTING SUSCEPTIBILITY

The effect of fluctuating temperatures on the germination of immature caryopses of two Sorghum varieties presenting contrasting susceptibility to pre-harvest sprouting was investigated. Fluctuating temperatures were able to stimulate germination of immature caryopses of both varieties from early stages of development (i.e. 15 d after pollination). Isolated embryos from both Varieties germinated well in water irrespective of the thermal regime of incubation. However, the ability of ABA to block germination in Redland B2 (sprouting-susceptible) isolated embryos was significantly reduced when embryos were incubated under fluctuating temperatures from 23 DAP onwards. No such effect was found in IS 9530 (sprouting-resistant) embryos. No differences in the pattern with which embryonic ABA content decreased during whole grain incubation were found in 25 and 35 DAP grains from both varieties incubated under constant or fluctuating temperatures. Therefore, these results indicate that alternating temperatures can promote germination through different mechanisms. One of them is the decrease in embryo sensitivity to ABA inhibition which appears to be acting in Redland B2 caryopses from 23 DAP onwards; the other one seems to be independent of ABA level and sensitivity and is active at very early stages of development in one variety (Redland B2) and throughout seed development in the other (IS 9530).