Nitric oxide (NO), an endogenous signaling molecule in animals and plants, mediates responses to abiotic and biotic stresses. Our previous work demonstrated that 100 microM sodium nitroprusside (SNP, an NO donor) treatment of maize seedlings increased K(+) accumulation in roots, leaves and sheathes, while decreasing Na(+) accumulation (Zhang et al. in J Plant Physiol Mol Biol 30:455-459, 2004b). Here we investigate how NO regulates Na(+), K(+) ion homeostasis in maize. Pre-treatment with 100 muM SNP for 2 days improved later growth of maize plants under 100 mM NaCl stress, as indicated by increased dry matter accumulation, increased chlorophyll content, and decreased membrane leakage from leaf cells. An NO scavenger, methylene blue (MB-1), blocked the effect of SNP. These results indicated that SNP-derived NO enhanced maize tolerance to salt stress. Further analysis showed that NaCl induced a transient increase in the NO level in maize leaves. Both NO and NaCl treatment stimulated vacuolar H(+)-ATPase and H(+)-PPase activities, resulting in increased H(+)-translocation and Na(+)/H(+) exchange. NaCl-induced H(+)-ATPase and H(+)-PPase activities were diminished by MB-1. 1-Butanol, an inhibitor of phosphatidic acid (PA) production by phospholipase D (PLD), reduced NaCl- and NO-induced H(+)-ATPase activation. In contrast, applied PA stimulated H(+)-ATPase activity. These results suggest that NO acts as a signal molecule in the NaCl response by increasing the activities of vacuolar H(+)-ATPase and H(+)-PPase, which provide the driving force for Na(+)/H(+) exchange. PLD and PA play an important role in this process.

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tase and CAT are the most effective antioxidant enzymes in scavenging AOS (Bowler et al., 1992).Turf quality of Kentucky bluegrass (Poa pratensis L.) often declines during summer when temperatures exceed its optimum range.Various cultural practices such as irrigation, fertiliza-This study was designed to determine whether application of salicylic tion, and soil cultivation have been extensively investiacid (SA) to the shoots and soil could improve heat tolerance of gated to improve turf performance in cool-season turf-Kentucky bluegrass, and to investigate whether SA-induced heat tolgrass during summer stress. However, limited information erance is related to changes in antioxidant activities. Effects of SA is available on the use of hormones or plant growth reguat different concentrations (0, 0.1, 0.25, 0.5, 1, and 1.5 mmol) on heat lators in heat stress management for cool-season turftolerance were examined in Kentucky bluegrass exposed to 46؇C for 72 h in a growth chamber. Influences of SA on the production of grasses and results vary with chemistry types. Kentucky active oxygen species (AOS), superoxide anion (O 2 . Ϫ ), and hydrogen bluegrass plants treated with trinexapac-ethyl, a synperoxide (H 2 O 2 ), and activities of antioxidant enzymes, superoxide thetic gibberellic acid inhibitor, were less heat tolerant dismutase (SOD), and catalase (CAT), were examined. Among SA than untreated plants (Heckman et al., 2002). Humic concentrations, 0.25 mmol was most effective in enhancing heat tolersubstances and seaweed extracts that contain phytohorance in Kentucky bluegrass, which was manifested by improved regrowth potential following heat stress of 72 h and maintenance of mones were reported to improve turfgrass stress tolerleaf water content at 77% during the 12-h stress period similar to that ance in various turfgrass species (Zhang et al., 2003a, under normal temperature conditions. The O 2 . Ϫ generating rate in-2003b). Other organic compounds such as ascorbic acid creased significantly at 6 h of heat stress, and SOD activity increased were used as free radical scavengers to alleviate oxidasignificantly at 2 h but decreased to the control level at 6 h of heat tive stress in various species (Zhang and Kirkham, stress in SA-untreated plants. The SA application suppressed the 1996). Salicylic acid has been defined as a new potential increase of O 2 . Ϫ generating rate and enhanced SOD activity significantly at 2 and 6 h of heat stress, respectively. The SA application decreased plant hormone (Raskin, 1992a) and found to play an H 2 O 2 level significantly at 2 and 12 h of heat stress, and increased important role in disease resistance (Raskin, 1992b) and CAT activity significantly within 12 h of heat stress. The results suggest

H(2)O(2) is an essential signal in absicic acid (ABA)-induced stomatal closure. It can be synthesized by several enzymes in plants. In this study, the roles of copper amine oxidase (CuAO) in H(2)O(2) production and stomatal closure were investigated. Exogenous ABA stimulated apoplast CuAO activity, increased H(2)O(2) production and [Ca(2+)](cyt) levels in Vicia faba guard cells, and induced stomatal closure. These processes were impaired by CuAO inhibitor(s). In the metabolized products of CuAO, only H(2)O(2) could induce stomatal closure. By the analysis of enzyme kinetics and polyamine contents in leaves, putrescine was regarded as a substrate of CuAO. Putrescine showed similar effects with ABA on the regulation of H(2)O(2) production, [Ca(2+)](cyt) levels, as well as stomatal closure. The results suggest that CuAO in V. faba guard cells is an essential enzymatic source for H(2)O(2) production in ABA-induced stomatal closure via the degradation of putrescine. Calcium messenger is an important intermediate in this process.

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