3rougTtï°i^d elevated temperature in combination in many areas, limiting cool-season grass growth. Rising atmospheric COg concentration may affect plant adaptation to drought and high temperature. The objective of this study was to investigate the effectiveness of elevated CO^ 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°C or 5°C above the optimal level of 25°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 CO( 400 ML L ') or elevated COg (800 pL L i). At ambient COĉ oncentration, 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^:FJ), net photosynthetic rate {A), stomatal conductance (g^), maximal ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco)-limited rate of photosynthesis (V^^^^), and maximal electron transport-limited rate of photosynthesis {J^^J but increased membrane electrolyte leakage (EL) and dark respiration rate (R^). Elevated temperature to 5°C above the optimal level resulted in the increases in g^., EL, and R^ but had no significant effects on the other physiological parameters. Drought stress for 28 d was more detrimental than increasing temperature by 5''C for tall fescue and the combined stress was more detrimental than either stress alone. Elevated COg 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 COj 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^ for water loss and C consumption through lowering respiration rate.Abbreviations: A, photosynthetic rate; ATP, adenosine triphosphate; initial' conductivity of the incubation solution; C^^^^, conductivity of incubation solution with killed tissues; DW, dry weight; EL, electrolyte leakage; F^:F^, ratio of variable to maximum fluorescence; FW, fresh weight; g^, stomatal conductance; J_^^^, maximal electron transportlimited rate of photosynthesis; R^, dark respiration rate; Rubisco, ribulose-l,5-bisphosphate carboxylase oxygenase; RuBP, ribulose-1,5-bisphosphate; RWC, leaf relative water content; SWC, soil volumetric water content; TW, turgid weight; V^,^^^^, maximal Rubisco-limited rate of photosynthesis.T EMPERATURE is a primary factor limiting growth of cool-season plant ...

γ-Aminobutyric acid is a non-protein amino acid involved in various metabolic processes. The objectives of this study were to examine whether increased GABA could improve heat tolerance in cool-season creeping bentgrass through physiological analysis, and to determine major metabolic pathways regulated by GABA through metabolic profiling. Plants were pretreated with 0.5 mM GABA or water before exposed to non-stressed condition (21/19 °C) or heat stress (35/30 °C) in controlled growth chambers for 35 d. The growth and physiological analysis demonstrated that exogenous GABA application significantly improved heat tolerance of creeping bentgrass. Metabolic profiling found that exogenous application of GABA led to increases in accumulations of amino acids (glutamic acid, aspartic acid, alanine, threonine, serine, and valine), organic acids (aconitic acid, malic acid, succinic acid, oxalic acid, and threonic acid), sugars (sucrose, fructose, glucose, galactose, and maltose), and sugar alcohols (mannitol and myo-inositol). These findings suggest that GABA-induced heat tolerance in creeping bentgrass could involve the enhancement of photosynthesis and ascorbate-glutathione cycle, the maintenance of osmotic adjustment, and the increase in GABA shunt. The increased GABA shunt could be the supply of intermediates to feed the tricarboxylic acid cycle of respiration metabolism during a long-term heat stress, thereby maintaining metabolic homeostasis.

Abscisic acid (ABA), salicylic acid (SA) and γ-aminobutyric acid (GABA) are known to play roles in regulating plant stress responses. This study was conducted to determine metabolites and associated pathways regulated by ABA, SA and GABA that could contribute to drought tolerance in creeping bentgrass (Agrostis stolonifera). Plants were foliar sprayed with ABA (5 μM), GABA (0.5 mM) and SA (10 μM) or water (untreated control) prior to 25 days drought stress in controlled growth chambers. Application of ABA, GABA or SA had similar positive effects on alleviating drought damages, as manifested by the maintenance of lower electrolyte leakage and greater relative water content in leaves of treated plants relative to the untreated control. Metabolic profiling showed that ABA, GABA and SA induced differential metabolic changes under drought stress. ABA mainly promoted the accumulation of organic acids associated with tricarboxylic acid cycle (aconitic acid, succinic acid, lactic acid and malic acid). SA strongly stimulated the accumulation of amino acids (proline, serine, threonine and alanine) and carbohydrates (glucose, mannose, fructose and cellobiose). GABA enhanced the accumulation of amino acids (GABA, glycine, valine, proline, 5-oxoproline, serine, threonine, aspartic acid and glutamic acid) and organic acids (malic acid, lactic acid, gluconic acid, malonic acid and ribonic acid). The enhanced drought tolerance could be mainly due to the enhanced respiration metabolism by ABA, amino acids and carbohydrates involved in osmotic adjustment (OA) and energy metabolism by SA, and amino acid metabolism related to OA and stress-defense secondary metabolism by GABA.

Elevated CO2 concentration may promote plant growth while high temperature is inhibitory for C3 plant species. The interactive effects of elevated CO2 and high temperatures on C3 perennial grass growth and carbon metabolism are not well documented. Kentucky bluegrass (Poa pratensis) plants were exposed to two CO2 levels (400 and 800 μmol mol−1) and five temperatures (15/12, 20/17, 25/22, 30/27, 35/32°C, day/night) in growth chambers. Increasing temperatures to 25°C and above inhibited leaf photosynthetic rate (Pn) and shoot and root growth, but increased leaf respiration rate (R), leading to a negative carbon balance and a decline in soluble sugar content under ambient CO2. Elevated CO2 did not cause shift of optimal temperatures in Kentucky bluegrass, but promoted Pn, shoot and root growth under all levels of temperature (15, 20, 25, 30, and 35°C) and mitigated the adverse effects of severe high temperatures (30 and 35°C). Elevated CO2-mitigation of adverse effects of high temperatures on Kentucky bluegrass growth could be associated with the maintenance of a positive carbon balance and the accumulation of soluble sugars and total nonstructural carbohydrates through stimulation of Pn and suppression of R and respiratory organic acid metabolism.