Effects of Drought or Heat Stress Alone and in Combination on Kentucky Bluegrass

Cool‐season turfgrasses are frequently subjected to heat and drought stresses during summer months. This study was conducted to determine physiological responses of Kentucky bluegrass ( Poa pratensis L.) to drought and heat alone or together, and the effects of drought preconditioning on plant responses to subsequent heat stress. Kentucky bluegrass (cv. Mystic) was subjected to drought and/or heat stress (35°C/30°C, day/night) in growth chambers for 40 d. Canopy photosynthetic rate (P n ) and leaf photochemical efficiency (Fv/Fm) decreased under drought and heat stress. The decline in P n was more severe under heat than under drought stress during the first 12 d of treatment. The reduction in Fv/Fm ratio was more severe under drought stress than under heat stress after 20 d of treatment. The combined heat and drought stresses (H+D) caused more dramatic reductions in P n and Fv/Fm than either heat or drought alone, starting at 3 and 9 d after treatment, respectively. Drought or heat alone, or H+D, significantly reduced root dry weight. However, reduction was more severe under heat alone than under drought stress, particularly in the top 20 cm of soil. Drought preconditioning enhanced plant tolerance to subsequent heat stress but had no influence on plant tolerance to H+D. Drought‐preconditioned plants maintained higher water status, stomatal conductance, and transpiration rate, and had significantly higher P n and root dry weight than non‐preconditioned plants during subsequent heat stress. No significant difference in Fv/Fm was observed between drought‐preconditioned and non‐preconditioned plants under either heat alone or H+D. The results indicated that simultaneous drought and heat stresses were more detrimental than either stress alone. Drought preconditioning could improve Kentucky bluegrass tolerance to subsequent heat stress.