PHYSIOLOGIA PLANTARUM 81: 335-342. Copenhagen 1991 Uniconazole-induced themnotolerance in wheat seedlings is mediated by transpirational cooling H. M. Booker, T. J. Gillespie, G. Hofstra and R. A. Fletcher Booker, H. M., Gillespie, T. J., Hofstra, G. and Fletcher, R. A. 1991. Uniconazole- induced thermotoleranee in wheat seedlings is mediated by transpirational cooling. - Physiol. Plant. 81: 335-342. 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 4 8 ^ 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 corre- lated 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 expo- sure to high temperatures in treated seedlings. Furthermore, the treated Glenlea seedlings displayed a reduction in the monomeric form of iight-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. Key words - Heat-shock protein, heat stress, pigments, thermotoleranee, transpira- tional cooling, Triticum aestivum. uniconazole, wheat. H. M. Booker. G. Hofstra (corresponding author) and R. A. Fletcher. Dept of Environmental Biology; T. J. Giilespie, Dept of Land Resource Science, Univ. of Guelph, Guelph, Ontario. Canada NIG 2W1. enous abscisic acid (Creeltnan et al. 1989). Triazoles Introduction modify stomata (Gao et al. 1988) and plant water status The triazoles are plant growth retardants, but they have and reduce total water uptake and tratispiration beyond also been described as plant multi-protectants because that attributed to reduced leaf area (Asare-Boamah et of their ability to protect plants from a broad spectrum al, 1986b, Lurssen 1987). Because triazole treatment of environmentai stresses (Fletcher and Hofstra 1988). produces numerous effects, which also occur in accli- Triazoles were found to inhibit shoot more than root mating plants, it is believed that the triazoles mimic growth; consequently, an increased root to shoot ratio plant hardening, in part (Fletcher and Hofstra 1988), was observed in treated plants (Fletcher and Nath 1984, Stress resistance in plants has been classified by Levitt Grossman et a!. 1987), Similar effects were found in (1980) as the ability to exclude an extemal stress or the plants grown under water deficits or treated with exog- ability to survive an intemal stress. Under field condi- Received 4 September, 1990; revised 1 December, 1990 23 Physiol. Plant. 81. lWl 335