Alterations in IAA, ABA and Polyamine Pool Levels of Two Winter Wheat Cultivars Caused by Drought Stress—Interactions with Melatonin

The present study investigates the potential of the plant growth regulator melatonin to alleviate the effects of moderate soil drought on two Bulgarian wheat cultivars (drought-tolerant and less-drought-tolerant) by modulating indole-3-acetic acid (IAA), abscisic acid (ABA) and polyamine (PA) levels. Drought induced ABA and decreased IAA content, while a specific response reflecting wheat tolerance was observed in the levels of the polyamines spermine, spermidine and putrescine. The less-drought-tolerant cultivar Fermer accumulated supernatant-conjugated PAs, while in the drought-tolerant cultivar Gines, the pellet-bound fraction increased. After four days of recovery, the decreased levels of IAA and pellet-bound fractions of PAs, along with the still-increased ABA content in cv. Fermer implied drought stress had a residual effect. In total, 75 µM melatonin (added to the irrigation water 24 h before or after drought) alleviated drought-induced alterations in IAA and ABA, with a more pronounced effect in cv. Fermer. The less-drought-tolerant variety responded to drought through a melatonin-modulated reorganization and accumulation of PAs, particularly in their conjugated forms, whereas cv. Gines maintained a comparatively stable polyamine pool with smaller treatment-induced changes. Overall data indicated that the outcome of melatonin treatment on the levels of IAA, ABA and the polyamine pool under drought is rather complex and depends on the specificity of the cultivars’ tolerance and the timing of melatonin application.