Читать книгу Physiology of Salt Stress in Plants - Группа авторов - Страница 41

Osmotic shock to the plants in salt stress drops the xylem pressure. The guard cells perceive the drop in xylem pressure and different salinity‐induced signaling cascades, to close the stomata with a purpose to minimize water loss. This closure of the stomata or reduction in the stomatal conductance poses a penalty on the plants by reduced CO₂ assimilation and decreased growth rate of the plants. The stomatal density and the stomatal aperture size regulate the gaseous exchange through the stomata. The halophytes adapted themselves efficiently to control the stomatal aperture and the stomatal density in salt stress with a minimal negative impact on photosynthesis (Zhao et al. 2020). The level of ABA content in the xylem sap and leaves were maintained much lower in halophytes in comparison with the glycophytes (Hedrich and Shabala 2018). More strikingly, the halophyte guard cells have a different sensitivity to the ROS produced during the salt stress, and the Na⁺ regulates the opening and closing of the stomatal aperture in halophytes contrary to the K⁺ in the glycophytes (Chiang et al. 2016). This adaptation provides advantages to the halophytes over glycophytes in maintaining the better gas exchange and the efficient photosynthetic ability to the halophytes in saline soil condition. Halophytes change the architecture of the photosynthetic complexes by omitting the salt‐stress‐sensitive component from the complexes and avoid the ROS‐induced photoinhibition of PSII and PSI (Pagliano et al. 2009; Trotta et al. 2012). For maintaining ROS homeostasis during photosynthesis, halophytes use Na⁺ accumulated in the chloroplast, for the exchange of ascorbate, pyruvate, and phosphate to the chloroplast through different Na⁺ transporters on the chloroplast membrane (Bose et al. 2017). The activity of the enzymes involved in the Calvin–Benson–Bassham cycle could be highly sensitive to the change of the ionic balance and availability in the chloroplast during the salt stress. This example comes from the enzyme FBPase of rice whose in vitro activity was suppressed at very low concentration of the NaCl, whereas the same enzyme from a close halophytic relative of rice, Porteresia coarctata, showed very less inhibition on its activity. The sequence analysis of P. coarctata from rice and P. coarctata exhibited a mutation of a few amino acids in PcFBPase, which resulted in reduced sensitivity of the enzyme to the salt stress (Ghosh et al. 2001).