Salinity is a major abiotic stress limiting growth and productivity of plants in many areas of the world due to increasing use of poor quality of water for irrigation and soil salinization. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, and molecular or gene networks. A comprehensive understanding on how plants respond to salinity stress at different levels and an integrated approach of combining molecular tools with physiological and biochemical techniques are imperative for the development of salt-tolerant varieties of plants in salt-affected areas. Recent research has identified various adaptive responses to salinity stress at molecular, cellular, metabolic, and physiological levels, although mechanisms underlying salinity tolerance are far from being completely understood. This paper provides a comprehensive review of major research advances on biochemical, physiological, and molecular mechanisms regulating plant adaptation and tolerance to salinity stress.

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and singlet oxygen ( 1 O 2 ) are species of active oxygen derivatives that may attack and damage macromole-Understanding physiological and biochemical factors involved in cules in living cells (Foyer et al., 1994; Halliwel, 1984; heat-stress injury would help improve heat tolerance of cool-season Scandalios, 1993Scandalios, , 1994 Smirnoff, 1993). Plants have degrasses. The objective of this study was to investigate lipid peroxidation of cell membranes in relation to heat-stress tolerance in creeping veloped enzymatic and nonenzymatic scavenging sysbentgrass (Agrostis palustris Huds.) . Two creeping bentgrass cultivars tems to quench active oxygen, and to eliminate the detridiffering in heat tolerance, L-93 (heat tolerant) and Penncross (heat mental effects of active oxygen. Superoxide dismutase sensitive) were grown under two temperature regimes: 22/16؇C (day/ (SOD) and catalase (CAT) break down (O Ϫ · 2 ) and H 2 O 2 , night) and 35/25؇C for 56 d in growth chambers. Photochemical effirespectively (Asada, 1992; Bowler et al, 1992; Elstner, ciency (Fv/Fm) and chlorophyll content of leaves; and electrolyte 1982;Scandalios, 1993Scandalios, , 1994. The ascorbate-glutathileakage (EL); content of the lipid peroxidation product, malondialdeone cycle is an alternative pathway to scavenge active hyde (MDA); and activities of antioxidant enzymes including superoxoxygen (Alscher, 1989; Creissen et al., 1994; Foyer, 1993; ide dismutase (SOD), catalase (CAT), and peroxidase (POD) in Foyer et al., 1994). When plants are subjected to adverse leaves and roots were determined biweekly during heat stress. Leaf Fv/Fm ratio and chlorophyll content decreased, whereas EL and MDA conditions such as high or low temperature, drought, contents of both leaves and roots increased under heat stress in bothand salinity stresses, the scavenging system may lose cultivars, but to a greater extent in Penncross. The activities of SOD its function and the balance between producing and and CAT decreased, whereas POD activity increased in both leaves quenching active oxygen species can be disturbed, reand roots, which occurred to a greater extent for Penncross. The sulting in oxidative damage (Bowler et al., 1992; Hodgincreases in MDA content and POD activity under heat stress were son and Raison, 1991; Price et al., 1989; Zhang and greater for leaves than for roots in both cultivars. These results suggest Han, 1997;Zhang and Kirkham, 1994). Whether lipid that decreased activities of antioxidant enzymes could result in an peroxidation of cell membranes is associated with heatincreased level of lipid peroxidation. Thus, decreased activities of stress injury in cool-season grasses, especially creeping antioxidant enzymes could contribute to damage of cell membranes and to leaf senescence as demonstrated by increased EL and reduced bentgrass under close mowing conditions, is not clear. Fv/Fm, and by decreased chlorophyll content during heat stress. Culti-

Stimulated emission depletion microscopy provides a powerful sub-diffraction imaging modality for life science studies. Conventionally, stimulated emission depletion requires a relatively high light intensity to obtain an adequate depletion efficiency through only light–matter interaction. Here we show efficient emission depletion for a class of lanthanide-doped upconversion nanoparticles with the assistance of interionic cross relaxation, which significantly lowers the laser intensity requirements of optical depletion. We demonstrate two-color super-resolution imaging using upconversion nanoparticles (resolution ~ 66 nm) with a single pair of excitation/depletion beams. In addition, we show super-resolution imaging of immunostained cytoskeleton structures of fixed cells (resolution ~ 82 nm) using upconversion nanoparticles. These achievements provide a new perspective for the development of photoswitchable luminescent probes and will broaden the applications of lanthanide-doped nanoparticles for sub-diffraction microscopic imaging.

Drought and high temperature are two major factors limiting the growth of cool‐season turfgrasses during summer in many areas. The objective of the study was to examine whether the adverse effects of drought and heat alone or in combination on tall fescue (Festuca arundinacea L.) and Kentucky bluegrass (Poa pratensis L.) involve oxidative stress. Grasses were exposed to drought (withholding irrigation), heat (35°C/30°C), and the combined stresses for 30 d in growth chambers. Turf quality (TQ), leaf relative water content (RWC), and chlorophyll content (Chl) decreased with prolonged drought, heat, and combined stresses for both species, but the severity of decline varied with stress type and duration. Transient increases in superoxide dismutase (SOD), ascorbate peroxidase (AP), and glutathione reductase (GR) activities occurred at 6 or 12 d of drought and the combined stresses in both species; however, the activities of all three enzymes decreased with extended periods of drought and the combined stresses. The SOD activity was not affected by heat stress alone. The activities of AP and GR were reduced after 18 d of heat stress for both species, but reductions were less than under the combined stresses. The catalase (CAT) activity continued to decrease to below the control level, beginning at 12 d for drought‐stressed or heat‐stressed plants and 6 d for plants exposed to the combined stresses. Lipid peroxidation occurred after 18 d of stresses in both species, as indicated by the increase in malondialdehyde (MDA) content. The results suggested that injuries of drought, heat, or the combined stresses to both tall fescue and Kentucky bluegrass, as manifested by declines in TQ, RWC, and Chl, could be associated with a decrease in antioxidant enzyme activities and an increase in membrane lipid peroxidation.

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Previous studies suggest that abscisic acid (ABA) stimulates the activities of antioxidant enzymes under normal and chilling temperature and enhanced chilling resistance in Stylosanthes guianensis. The objective of this study was to test whether nitric oxide (NO) is involved in the ABA-induced activities of the antioxidant enzymes in Stylosanthes guianensis due to its nature as a second messenger in stress responses. Plants were treated with NO donors, ABA, ABA in combination with NO scavengers or the nitric oxide synthase (NOS) inhibitor and their effects on the activity of antioxidant enzymes and NO production were compared. The results showed that ABA increased the activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). The effect of ABA on antioxidant enzyme activities was suppressed by the NOS inhibitor, N(omega)-nitro-L-arginine (L-NNA), and the NO scavenger, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide (PTIO). NO content increased after 5 h of ABA treatment. The NO-scavenger, PTIO, and the NOS-inhibitor, L-NNA, inhibited the accumulation of NO in ABA-treated Stylosanthes guianensis. NO donor treatment enhanced the activities of SOD, CAT, and APX. The results suggested that NO was involved in the ABA-induced activities of SOD, CAT, and APX in Stylosanthes guianensis. ABA triggered NO production that may lead to the stimulation of antioxidant enzyme activities.

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