Osmotic adjustment and root growth associated with drought preconditioning-enhanced heat tolerance in Kentucky bluegrass

Prior exposure to drought stress (drought preconditioning) affects turfgras s tolerance to subsequent heat stress. The study was designed to examine wh ether these effects for Kentucky bluegrass (Poa pratensis L.) are associate d with osmotic adjustment and root growth. Plants were subjected to two cyc les of drying and rewatering, and turf quality was then allowed to recover to the well watered control level before being exposed to 21 d of heat stre ss (35 degreesC/30 degreesC) in growth chambers. Compared with nonprecondit ioned plants, drought-preconditioned plants had 13 and 21% higher turf qual ity, 6 and 10% higher leaf relative water content, and 17 and 48% higher os motic adjustment at 14 and 21 d of heat stress, respectively. Total ion (K, Ca2+, Na+, Ma(2+), Cl-, and P) concentration of cell sap increased during heat stress and was 11 to 16% higher in drought-preconditioned plants than nonpreconditioned plants. The concentration of KI accounted for 59 to 65% of total ion solutes in both groups of plants during heat stress. Soluble c arbohydrate content (WSC) of leaves increased during heat stress and was ab out 21 and 44% higher in drought-preconditioned plants than nonprecondition ed plants at 14 and 21 d, respectively. Heat stress decreased root dry weig ht (DW) and WSC, but significant higher DW and WSC content of roots in the 40-60 cm soil layer were observed for preconditioned plants than nonprecond itioned plants before and after heat stress. The results demonstrated that drought preconditioning enhanced heat tolerance in Kentucky bluegrass, whic h could be related to the maintenance of higher osmotic adjustment associat ed with accumulation of ion solutes and water soluble carbohydrates and dev elopment of extensive roots deeper in the soil profile.