Effects of drought or heat stress alone and in combination on Kentucky bluegrass

Cool-season turfgrasses are frequently subjected to heat and drought stress es during summer months, This study was conducted to determine physiologica l responses of Kentucky bluegrass (Poa pratensis L,) to drought and heat al one or together, and the effects of drought preconditioning on plant respon ses to subsequent heat stress. Kentucky bluegrass (cv, Mystic) was subjecte d to drought and/or heat stress (35 degrees C/30 degrees C, day/night) in g rowth chambers for 40 d. Canopy photosynthetic rate (P-n) and leaf photoche mical efficiency (Fv/Fm) decreased under drought and heat stress. The decli ne in P-n was more severe under heat than under drought stress during the f irst 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 a nd Fv/Fm than either heat or drought alone, starting at 3 and 9 d after tre atment, respectively. Drought or heat alone, or H + D, significantly reduce d root dry weight. However, reduction was more severe under heat alone than under drought stress, particularly in the top 20 cm of soil. Drought preco nditioning enhanced plant tolerance to subsequent heat stress but had no in fluence on plant tolerance to H + D, Drought-preconditioned plants maintain ed higher water status, stomatal conductance, and transpiration rate, and h ad significantly higher P-n and root dry weight than non-preconditioned pla nts during subsequent heat stress. No significant difference in Fv/Fm was o bserved 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 he at stress.