Physiological recovery of Kentucky bluegrass from simultaneous drought and heat stress

Drought and heat are two major factors limiting growth of cool season grasses. Rapid recovery from the combination of those stresses is important for the persistence of perennial turfgrasses. The study was designed to examine physiological factors associated with the persistence and recovery of Kentucky bluegrass (Poa pratensis L.) exposed to combined drought and heat stress following rewatering and/or temperature drop. Two cultivars differing in drought and heat tolerance, 'Midnight' (tolerant) and 'Brilliant' (sensitive), were exposed to drought and heat stress (35degreesC) simultaneously in a growth chamber until most plants became brown and completely desiccated (14 d). Plants were then subjected to three recovery treatments: (i) rewatered but exposed to heat stress (rewatering); (ii) returned to optimum temperature (20degreesC) but unwatered (cooling), and (iii) rewatering and cooling. Leaf photochemical efficiency (Fv/Fm), chlorophyll content, and activities of superoxide dismulase (SOD) and catalase (CAT) declined, while electrolyte leakage (EL) and lipid peroxidation increased rapidly during the combined stress. The adverse impact of the combined stress was more severe for Brilliant than for Midnight. Following rewatering or in combination with cooling, all parameters except chlorophyll content fully recovered for Midnight. However, for Brilliant, most of the parameters did not recover completely; Fv/Fm recovered partially. There was no recovery for any parameters of either cultivar when plants were returned to the optimum temperature but still unwatered. The results suggested that simultaneous drought and heat stress could cause permanent physiological damage for Kentucky bluegrass, particularly for the stress-sensitive cultivar. Rewatering was essential for physiological recovery from the combined stress, regardless of temperature conditions. Rapid resumption of Fv/Fm, cell membrane stability, and antioxidant activities were important factors contributing to the recovery of Kentucky bluegrass.