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 susceptibili ty to pre-harvest sprouting was investigated. Fluctuating temperatures were able to stimulate germination of immature caryopses of both vari eties from early stages of development (i.e. 15 d after pollination). Isolated embryos from both Varieties germinated well in water irrespec tive of the thermal regime of incubation. However, the ability of ABA to block germination in Redland B2 (sprouting-susceptible) isolated em bryos was significantly reduced when embryos were incubated under fluc tuating temperatures from 23 DAP onwards. No such effect was found in IS 9530 (sprouting-resistant) embryos. No differences in the pattern w ith which embryonic ABA content decreased during whole grain incubatio n were found in 25 and 35 DAP grains from both varieties incubated und er constant or fluctuating temperatures. Therefore, these results indi cate that alternating temperatures can promote germination through dif ferent mechanisms. One of them is the decrease in embryo sensitivity t o ABA inhibition which appears to be acting in Redland B2 caryopses fr om 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 o ne variety (Redland B2) and throughout seed development in the other ( IS 9530).