DROUGHT RESISTANCE OF Sorghum bicolor. 4. HORMAL CHANGES IN RELATION TO DROUGHT STRESS IN FIELD-GROWN PLANTS

Abstract

This study was undertaken to investigate the nature of hormonal changes in relation to drought stress in two cultivars of Sorghum bicolor L. Moench. Two cultivars, M–35 and NK300, were grown in a field plot protected by a rain shelter. Plants in one soil compartment were stressed by withholding water while those in another (controls) were irrigated frequently. Levels of the plant hormones abscisic acid (ABA), phaseic acid (PA) and indole-3-acetic acid (IAA) measured by high-performance liquid chromatography (HPLC) were determined in the youngest leaves of control and stressed plants at intervals throughout the growth cycle. Plant height, senescence, and leaf water status were also determined. Leaf water potential (ψw) and solute potential (ψs) were reduced in both cultivars by drought stress; values for M–35 plants were lower than NK300. Leaf senescence was higher in M–35 plants and was promoted by stress in both cultivars. Cultivar M–35 behaved as a drought-tolerant plant whereas cultivar NK300 behaved more like a drought avoider. ABA levels were higher in M–35 control plants than in corresponding NK300 plants and levels in both cultivars followed seasonal changes in leaf water potential. Under drought stress, ABA levels increased between 1.5 and 2 times in both cultivars with the largest increases occurring during the vegetative stage in M–35 and during the flowering stage for NK300. PA levels in both cultivars were higher in stressed than in control plants. PA levels in M–35 plants were relatively low and constant throughout the life cycle, whereas in NK300, levels were high until shortly before flowering. IAA levels were higher in NK300 than in M–35 plants, particularly during the vegetative stage. Under drought stress, IAA levels were reduced in both cultivars with a more pronounced reduction in NK300. The high level of ABA in the more drought-tolerant cultivar M–35 was associated with low leaf ψw and ψs and high leaf senescence. On the other hand, in the drought avoider, NK300, high levels of IAA and PA were associated with high leaf ψw and ψs. It is concluded that these cultivars, which differ in their response to drought stress, can be distinguished by their leaf hormone levels.