Effects of Elevated CO2 on Physiological Responses of Tall Fescue to Elevated Temperature, Drought Stress, and the Combined Stresses

Drought and elevated temperature often occur alone or in combination in many areas, limiting cool-season grass growth. Rising atmospheric CO2 concentration may affect plant adaptation to drought and high temperature. The objective of this study was to investigate the effectiveness of elevated CO2 in mitigating the negative effects of drought or elevated temperature alone or a combination of these stresses on physiological processes in a perennial grass species. The effects of these treatments on water relations, photosynthesis, and respiration were determined in tall fescue (Festuca arundinacea Schreb. cultivar Rembrandt). Grass plants were subjected to the following treatments in growth chambers: heat stress (30 degrees C or 5 degrees C above the optimal level of 25 degrees C), drought stress by maintaining soil water content at 50% of field capacity, or the combined two stresses for 28 d. Stressed and unstressed control plants were exposed to a constant level of either ambient CO2 (400 mu L L-1) or elevated CO2 (800 mu L L-1). At ambient CO2 concentration, drought and the combined stress for 28 d caused significant decline in leaf relative water content (RWC), photochemical efficiency (ratio of variable to maximum fluorescence [F-v:F-m]), net photosynthetic rate (A), stomatal conductance (g(s)), maximal ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco)-limited rate of photosynthesis (V-cmax), and maximal electron transport-limited rate of photosynthesis (J(max)) but increased membrane electrolyte leakage (EL) and dark respiration rate (R-d). Elevated temperature to 5 C above the optimal level resulted in the increases in g(s), EL, and R-d but had no significant effects on the other physiological parameters. Drought stress for 28 d was more detrimental than increasing temperature by 5 degrees C for tall fescue and the combined stress was more detrimental than either stress alone. Elevated CO2 mitigated the degree of change in all physiological factors under drought or heat stress and resulted in increases in A (162%) and RWC (19%) and a reduction in EL (21%) under the combined stress. These results suggest that elevated CO2 could improve tall fescue tolerance to drought and elevated temperature by enhancing plant water status, cellular membrane stability, and photosynthesis capacity and by suppressing g(s) for water loss and C consumption through lowering respiration rate.