Читать книгу Physiology of Salt Stress in Plants - Группа авторов - Страница 31

Salt stress affects chloroplast more severely than another organelle, as the ROS increases rapidly due to chlorophyll triplet and oxygen molecules produced due to the photolysis of water, leading to its destruction. The salt stress leads to increased Na⁺ and Cl⁻ concentrations and decreased K⁺ concentration inside chloroplasts resulting in the swelling of thylakoid and unstacking of grana, thereby altering the photosynthetic structure (Santos 2004). The regulation of K⁺ and Cl⁻ across the thylakoid membrane is important in regulating the pH difference, which regulates the photoprotective nonphotochemical quenching mechanism and optimal photosynthesis (Finazzi et al. 2015). Any sort of disorder in this mechanism due to insufficient K⁺ during salt stress in the chloroplast may hamper its photoprotective mechanism. Accumulation of anions in the chloroplast affects both the acceptor and donor sides of photosystem II (PSII) by binding to a specific site on PSII (Jajoo et al. 1994, 2005). When the anions permeate the membrane during the salt stress, the influx of protons to balance the resultant charge causes structural reorganization in thylakoid membranes resulting in energy redistribution. All these salt stress components ultimately upset the ionic balance within chloroplasts, resulting in poor photosynthetic performance and reduced growth and yield. A shift in anion balance has shifted the solar energy distribution from PSII to PSI, affecting the function of the water‐splitting complex and electron transport to and from the plastocyanin (Singh‐Rawal et al. 2011). At the same time, soluble cations also cause alteration in the excitation energy transfer in the photosynthetic system at physiological temperatures by affecting several primary processes in thylakoids: initial energy distribution or “spillover” from PSII to PSI and rate constant of nonphotochemical quenching (Wong et al. 1980).

During salt stress, the maximum quantum yield of PSII (Fv/Fm), photochemical quenching coefficient, and electron transport rate decreases, whereas nonphotochemical quenching increases (Yang et al. 2020), possibly due to unstacking of the grana. The reduced photochemistry and carbon fixation lead to an increase in ROS production which are potent in damaging chloroplast and its membrane (Hernández et al. 1995). To cope with this situation, the level of enzymatic (catalase, ascorbate peroxidase, and glutathione reductase) and nonenzymatic (ascorbate, total carotenoids, phenolics, and flavonoids) antioxidants increases (Taïbi et al. 2016). The ROS production, especially H₂O₂, inhibits the synthesis of D1 protein (Murata et al. 2007). Salt stress modifies the Q_B plastoquinone binding site of D1 at the acceptor side. It stabilizes oxygen‐evolving complex (OEC) in S2 state at the donor side by salt‐induced migration of the PsbO subunit of PSII to the lumen (Sasi et al. 2018). This change in the PSII and OEC slows the water‐splitting process, which may be part of the plant’s defense strategy. The enzyme plastoquinol terminal oxidase (PTOX) in halophyte E. salsugineum chloroplast minimizes the damage by diverting electrons from plastoquinol to oxygen and producing water molecules (Stepien and Johnson 2009).

Salt has an essential role in maintaining enzyme activity. If the salt concentration is below optimum, the charged amino acid side chains of the enzyme will attract each other, thus denaturing it. At the same time, if the salt concentration is too high, regular interaction of charged groups will be blocked, new interactions will occur, and again the enzyme will denature. Some of the enzymes involved in the Calvin–Benson–Bassham cycle share their functional activity with the chloroplast glycolytic pathway. Therefore, one of the direct effects of ionic stress is the denaturation of enzymes involved in the Calvin–Benson–Bassham cycle, leading to the transduction of signals for the upregulation or downregulation of several genes encoding enzymes involved in photosynthesis. We will discuss the effect of salt stress on their activity in the following glycolytic section.