Uniconazole-induced thermotolerance in wheat seedlings is mediated by transpirational cooling

Seed treatment of thermotolerant and sensitive cultivars of wheat ( Triticum aestivum L. cv. Frederick and Glenlea, respectively) with uniconazole reduced shoot and increased root fresh and dry weights. When subjected to 48°C for 6 h, treated seedlings had lower leaf-temperatures and overall higher rates of evapotranspiration. Percent survival 11 days after high temperature incubation in untreated seedlings was 53% in Frederick and 30% in Glenlea, whereas in uniconazole-treated seedlings it was 94% and 80%, respectively. Transpiration resistance markedly increased in the control cultivars after 4 h of high-temperature incubation, but remained relatively unchanged in the treated cultivars. The increased root to shoot ratio exhibited in treated plants may have alleviated guard cell stress, even under high evaporative demand. Chlorophyll fluorescence measurements on leaves of heat-stressed seedlings indicated that increased photosynthetic metabolism in treated seedlings was correlated with lower leaf temperatures. High-temperature stress resulted in a dramatic decline in pigments and proteins in thylakoid extracts of control seedlings but not in extracts from treated seedlings. Integrity of thylakoid pigment protein complexes, as illustrated by reduced relative amounts of free pigment, was maintained after exposure to high temperatures in treated seedlings. Furthermore, the treated Glenlea seedlings displayed a reduction in the monomeric form of Sight-harvesting chlorophyll protein II (LHCP II) compared to control and Frederick seedlings. The heat-tolerant Frederick cultivar showed a greater protective effect from uniconazole treatment than the sensitive Glenlea cultivar. Uniconazole treatment did not affect heat-shock protein (HSP) synthesis in mesocotyl tissue.