The effects of short-term salt stress on gas exchange and the regulation of photosynthetic electron transport were examined in Arabidopsis (Arabidopsis thaliana) and its salt-tolerant close relative Thellungiella (Thellungiella halophila). Plants cultivated on soil were challenged for 2 weeks with NaCl. Arabidopsis showed a much higher sensitivity to salt than Thellungiella; while Arabidopsis plants were unable to survive exposure to greater than 150 mM salt, Thellugiella could tolerate concentrations as high as 500 mM with only minimal effects on gas exchange. Exposure of Arabidopsis to sublethal salt concentrations resulted in stomatal closure and inhibition of CO 2 fixation. This lead to an inhibition of electron transport though photosystem II (PSII), an increase in cyclic electron flow involving only PSI, and increased nonphotochemical quenching of chlorophyll fluorescence. In contrast, in Thellungiella, although gas exchange was marginally inhibited by high salt and PSI was unaffected, there was a large increase in electron flow involving PSII. This additional electron transport activity is oxygen dependent and sensitive to the alternative oxidase inhibitor n-propyl gallate. PSII electron transport in Thellungiella showed a reduced sensitivity to 2#-iodo-6-isopropyl-3-methyl-2#,4,4#-trinitrodiphenylether, an inhibitor of the cytochrome b 6 f complex. At the same time, we observed a substantial up-regulation of a protein reacting with antibodies raised against the plastid terminal oxidase. No such up-regulation was seen in Arabidopsis. We conclude that in salt-stressed Thellungiella, plastid terminal oxidase acts as an alternative electron sink, accounting for up to 30% of total PSII electron flow.

Hydroponically grown cucumber plants were exposed to 14-d period of salinity (0, 50, 100 mM NaCl). NaCl caused reduction in the relative water content in the leaves. The Na + content increased and the K + content decreased. The net photosynthetic rate, stomatal conductance and transpiration rate were markedly decreased by all of the salt treatments. Salinity decreased also the maximum quantum efficiency of photosystem 2 (PS 2) determined as the variable to maximum fluorescence ratio, the photochemical quantum yield of PS 2 and the photochemical fluorescence quenching, while the non-photochemical quenching increased. Above results indicate that NaCl affects photosynthesis through both stomata closure and non-stomatal factors.

The effect of salt stress (50, 100 and 150 mM of NaCl) on the activity of superoxide dismutase (SOD, EC. 1.15.1.1), ascorbate peroxidase (APX, EC. 1.11.1.11), glutathione reductase (GR, EC. 1.6.4.2) enzymes and also on the rate of lipid peroxidation in terms of thiobarbituric acid‐reactive substances (TBARS) content and photosynthetic capacity in two wheat (C3 plants) and two maize (C4 plants) varieties was studied. In the non‐salined control plants, the antioxidant enzymes activities were significantly higher for maize than for wheat. Adding salt to the nutrient solution increased the level of antioxidants in leaves of both maize and wheat. The first substantial response to salinity was found for SOD on the 2nd day, whereas changes occurred for APX on the 4th day and for GR on the 4th/5th day of salt treatment. Although SOD activity increased considerably more in wheat (C3), it never reached as high levels as in maize (C4) grown in the same treatment combinations. The total increase in APX activity was similar for wheat and maize, whereas GR activity was higher in leaves of maize. Lipid peroxidation analyses showed an increase in TBARS contents in both plants' species grown under salinity that corresponded to the damage that occurred in secondary oxidative stress. However, as a result of advanced antioxidant defense in maize, the TBARS quantities did not elevate to as high level as in wheat. Chlorophyll fluorescence measurements revealed a considerable decrease in the efficiency of PS II and electron‐transport chain (ETC). Assimilation rate of CO2 decreased in both plant groups; however, in C4 maize, we observed a much better capacity to preserve the photosynthetic apparatus against overproduction of ROS. Results suggest that efficient antioxidant defense plays an important role in maize, the C4 plant, resistance to environmental stresses like salinity or drought.

Sorghum is one of the most drought tolerant crops but surprisingly, little is known about the mechanisms achieving this. We have compared physiological and biochemical responses to drought in two sorghum cultivars with contrasting drought tolerance. These closely related cultivars have starkly contrasting responses to water deficit. In the less tolerant Samsorg 40, drought induced progressive loss of photosynthesis. The more drought tolerant Samsorg 17 maintained photosynthesis, transpiration and chlorophyll content until the most extreme conditions. In Samsorg 40, there was a highly specific down-regulation of selected proteins, with loss of PSII and Rubisco but maintenance of PSI and cytochrome b6 f, allowing plants to maintain ATP synthesis. The nitrogen released allows for accumulation of glycine betaine and proline. To the best of our knowledge, this is the first example of specific reengineering of the photosynthetic apparatus in response to drought. In contrast, in Samsorg 17 we detected no substantial change in the photosynthetic apparatus. Rather, plants showed constitutively high soluble sugar concentration, enabling them to maintain transpiration and photosynthesis, even in extremely dry conditions. The implications for these strikingly contrasted strategies are discussed in relation to agricultural and natural systems.

Abstract:The plastid terminal oxidase (PTOX) has been shown to be an important sink for photosynthetic electron transport in stress tolerant plants. However, overexpression studies in stress sensitive species have previously failed to induce significant activity of this protein.Here we show that overexpression of PTOX from the salt tolerant brassica species Eutrema salsugineum does not, alone, result in activity, but that over-expressing plants show faster induction and a greater final level of PTOX activity once exposed to salt stress. This implies that an additional activation step is required before activity is induced. We show that that activation involves the translocation of the protein from the unstacked stromal lamellae to the thylakoid grana and a protection of the protein from trypsin digestion. This represents an important and novel activation step and opens up new possibilities in the search for stress tolerant crops. Significance statement:Growing concerns about food security and changing climates make the identification of novel stress tolerance traits a priority. Here we show that the activation of the plastid terminal oxidase, via its relocation within the thylakoid membrane, can induce a protective sink for electron transport. We have thus identified a novel and unexpected mechanism for protein activation and opened new avenues for engineering plant stress tolerance.3 /body