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

Prior exposure to drought stress (drought preconditioning) affects turfgrass tolerance to subsequent heat stress. The study was designed to examine whether these effects for Kentucky bluegrass (Poa pratensis L.) are associated with osmotic adjustment and root growth. Plants were subjected to two cycles 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 stress (35 degrees C/30 degrees C) in growth chambers. Compared with nonpreconditioned plants, drought-preconditioned plants had 13 and 21% higher turf quality, 6 and 10% higher leaf relative water content, and 17 and 48% higher osmotic adjustment at 14 and 21 d of heat stress, respectively. Total ion (K(+), Ca(2+), 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 K(+) accounted for 59 to 65% of total ion solutes in both groups of plants during heat stress. Soluble carbohydrate content (WSC) of leaves increased during heat stress and was about 21 and 44% higher in drought-preconditioned plants than nonpreconditioned plants at 14 and 21 d, respectively. Heat stress decreased root dry weight (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 nonpreconditioned plants before and after heat stress. The results demonstrated that drought preconditioning enhanced heat tolerance in Kentucky bluegrass, which could be related to the maintenance of higher osmotic adjustment associated with accumulation of ion solutes and water soluble carbohydrates and development of extensive roots deeper in the soil profile.