Low temperature has negative effects on seed performance and seedling growth in soya bean, reducing stand establishment. It was hypothesised, that seed treatment with plant hormones could improve seed imbibition and performance at low temperature. Therefore, the effects of 24-epibrassinolide (EBL) and gibberellic acid (GA3) on soya bean seed germination were studied. Seeds of ‘CM60’ and ‘CM0701-24’ were imbibed with 0.25 or 0.50 ppm EBL, 50 or 100 ppm GA3, or distilled water at 10, 15, 20 and 25°C. The seed imbibition patterns of the two soya bean varieties followed a similar trend. Low temperature delayed seed imbibition resulting in longer time to reach the end of phases 1 and 2 of seed germination as compared with high temperature. Soya bean seeds treated with 50 or 100 ppm GA3 showed increased germination index, shoot and root length and reduced mean germination time. Thus, 50-100 ppm GA3 could be used as a pre-soaking seed treatment to improve soya bean seed and seedling performance under low temperature.

Gibberellin (GA3) and brassinosteroid (EBL) interactions can stimulate germination, extend seedling growth and induce low temperature stress tolerance. The effects of GA3 and EBL on seed quality, α-amylase and total dehydro­genase activities of soya bean seeds at different vigour and temperature levels were studied. The results showed that medium vigour seeds dressed with 100 ppm GA3, 0.25 ppm EBL plus 50 ppm GA3, and 0.50 ppm EBL plus 100 ppm GA3 had significantly higher germination (GP) and seedling vigour rates (SVR) than those of untreated seeds. Similarly, 0.50 ppm EBL resulted in higher SVR of low and medium vigour seeds when compared to the control under low temperature. All soya bean seeds dressed with 100 ppm GA3 had higher GP, vigour, shoot and root lengths, and SVR under both temperatures than those of the untreated seeds, and mean germination time decreased. At 15°C, all seeds dressed with 0.50 ppm EBL plus 100 ppm GA3 had higher α-amylase activity than the untreated seeds, whereas seeds with low and medium seed vigour dressed with 100 ppm GA3 and 0.25 ppm EBL plus 50 ppm GA3 had higher total dehydrogenase activity than the untreated seeds.