On the hormonal nature of the stimulatory effect of high incubation temperatures on germination of dormant sorghum (S-bicolor) caryopses

High incubation temperatures (i.e. 30degreesC) stimulate the germination of dormant sorghum grains. To test the hormonal nature of this response, experiments were carried out with two varieties with contrasting dormancy at harvest: Redland B2 (low dormancy, high germination percentages attained under a wide incubation thermal range) and IS 9530 (high dormancy, high germination percentages attained only at 30degreesC). Redland B2 grains with reduced GA content (paclobutrazol -treated) reached high germination temperatures (c 100%) only when incubated at 30degreesC. By contrast, IS 9530 grains with reduced ABA content (fluridone-treated) reached 100% germination at 30, 25, 20 and 15degreesC. Incubation temperatures did not alter embryo responsiveness to ABA, nor did it modify the pattern of changes in embryo ABA content throughout incubation. Low GA(3) concentrations (0.1 muM) were required to totally overcome the inhibition imposed by ABA in embryos incubated at 30degreesC; by contrast, even the highest GA(3) concentrations used (1000 muM) were not able to revert ABA inhibition in embryos incubated at 15degreesC. These results show the hormonal nature of the stimulatory effect of high incubation temperatures, and suggest that this effect is mediated by an increase in tissue responsiveness to GAs.