Anabel Robredo

The oxidative stress caused by salinity in two barley cultivars is mitigated by elevated CO2.

Changes in antioxidant metabolism because of the effect of salinity stress (0, 80, 160 or 240 mM NaCl) on protective enzyme activities under ambient (350 micromol mol(-1)) and elevated (700 micromol mol(-1)) CO(2) concentrations were investigated in two barley cultivars (Hordeum vulgare L., cvs Alpha and Iranis). Electrolyte leakage, peroxidation, antioxidant enzyme activities [superoxide dismutase (SOD), EC 1.15.1.1; ascorbate peroxidase (APX), EC 1.11.1.11; catalase (CAT), EC 1.11.1.6; dehydroascorbate reductase (DHAR), EC 1.8.5.1; monodehydroascorbate reductase (MDHAR), EC 1.6.5.4; glutathione reductase (GR), EC 1.6.4.2] and their isoenzymatic profiles were determined. Under salinity and ambient CO(2), upregulation of antioxidant enzymes such as SOD, APX, CAT, DHAR and GR occurred. However, this upregulation was not enough to counteract all ROS formation as both ion leakage and lipid peroxidation came into play. The higher constitutive SOD and CAT activities together with a higher contribution of Cu,Zn-SOD 1 detected in Iranis might possibly contribute and make this cultivar more salt-tolerant than Alpha. Elevated CO(2) alone had no effect on the constitutive levels of antioxidant enzymes in Iranis, whereas in Alpha it induced an increase in SOD, CAT and MDHAR together with a decrease of DHAR and GR. Under combined conditions of elevated CO(2) and salinity the oxidative damage recorded was lower, above all in Alpha, together with a lower upregulation of the antioxidant system. So it can be concluded that elevated CO(2) mitigates the oxidative stress caused by salinity, involving lower ROS generation and a better maintenance of redox homeostasis as a consequence of higher assimilation rates and lower photorespiration, being the response dependent on the cultivar analysed.

Lipoic acid and redox status in barley plants subjected to salinity and elevated CO2

Future environmental conditions will include elevated concentrations of salt in the soil and an elevated concentration of CO(2) in the atmosphere. Because these environmental changes will likely affect reactive oxygen species (ROS) formation and cellular antioxidant metabolism in opposite ways, we analyzed changes in cellular H(2)O(2) and non-enzymatic antioxidant metabolite [lipoic acid (LA), ascorbate (ASA), glutathione (GSH)] content induced by salt stress (0, 80, 160 or 240 mM NaCl) under ambient (350 micromol mol(-1)) or elevated (700 micromol mol(-1)) CO(2) concentrations in two barley cultivars (Hordeum vulgare L.) that differ in sensitivity to salinity (cv. Alpha is more sensitive than cv. Iranis). Under non-salinized conditions, elevated CO(2) increased LA content, while ASA and GSH content decreased. Under salinized conditions and ambient CO(2), ASA increased, while GSH and LA decreased. At 240 mM NaCl, H(2)O(2) increased in Alpha and decreased in Iranis. When salt stress was imposed at elevated CO(2), less oxidative stress and lower increases in ASA were detected, while LA was constitutively higher. The decrease in oxidative stress could have been because of less ROS formation or to a higher constitutive LA level, which might have improved regulation of ASA and GSH reductions. Iranis had a greater capacity to synthesize ASA de novo and had higher constitutive LA content than did Alpha. Therefore, we conclude that elevated CO(2) protects barley cultivars against oxidative damage. However, the magnitude of the positive effect is cultivar specific.

Atmospheric CO2 concentration influences the contributions of osmolyte accumulation and cell wall elasticity to salt tolerance in barley cultivars.

Future environmental conditions will include elevated concentrations of salt in the soils and elevated concentrations of CO(2) in the atmosphere. Soil salinization inhibits crop growth due to osmotic and ionic stress. However, plants possess salt tolerance mechanisms, such as osmotic and elastic adjustment, to maintain water status. These mechanisms, which enhance the uptake and accumulation of ions and the synthesis of compatible solutes, require substantial energy expenditure. Under elevated CO(2), the carbon and energy supplies are usually higher, which could facilitate the energetically expensive salt tolerance mechanisms. To test this hypothesis, the factors involved in osmotic and elastic adjustments in two barley cultivars (Hordeum vulgare cv. Alpha and cv. Iranis) grown under several salt concentrations and at ambient or elevated [CO(2)] were evaluated. Under ambient [CO(2)] and salt stress, both cultivars (1) decreased the volumetric elasticity modulus (epsilon) of their cell walls, and (2) adjusted osmotically by accumulating ions (Na(+) and Cl(-)) from the soil, confirming barley as an includer species. The contributions of sugars and other unidentified osmolytes also increased, while the contribution of organic acids decreased. Under elevated [CO(2)] and salt stress, epsilon decreased less and osmotic adjustment (OA) was greater than at ambient [CO(2)]. In fact, the greater OA under elevated [CO(2)] was positively correlated with the contributions of sugars and other unidentified compounds. These results indicate that barley is likely to be successful in more salinized soils due to its capacity for OA under elevated [CO(2)].

Influence of water stress on photosynthetic characteristics in barley plants under ambient and elevated CO2 concentrations

We evaluated the combined effects of elevated CO2 and water availability on photosynthesis in barley. Soil and plant water content decreased with water stress, but less under elevated CO2 concentration (EC) compared with ambient CO2 concentration (AC). During water stress, stomatal conductance, carboxylation rate, RuBP regeneration, and the rate of triose phosphate utilisation (TPU) were decreased but less when plants grew under EC. Drought treatments caused only a slight effect on maximum photochemical efficiency (variable to maximum fluorescence ratio, Fv/Fm), whereas the actual quantum yield (ΦPS2), maximum electron transport rate (Jmax) and photochemical quenching (qP) were decreased and the non photochemical quenching (NPQ) was enhanced. Under water deficit, the allocation of electrons to CO2 assimilation was diminished by 49 % at AC and by 26 % at EC while the allocation to O2 reduction was increased by 15 % at AC and by 12 % at EC.

Does Elevated CO2 Mitigate the Salt Effect on Photosynthesis in Barley Cultivars

The Lower Decreases Of Pigment Content And Assimilation Rates And The Higher Rates Of Instantaneous Water Use Efficiency Observed In Plants Grown Under Salinity And Elevated Co2 Would Indicate A Better Photosynthetic CapaCity Than Their Counterparts At Ambient Co2.