K+ absorption, thus sustaining the optimal ratio of K+/Na+, is essential to endure salt stress (Amini and Ehsanpour 2005; Ahmad et al. 2016; Shams et al. 2019).
During periods of biological strain in plants, the production of signaling molecules (NO, H2S, and CO) has been observed. There are several reports indicating that NO prompts the development of an essential osmolyte like proline in wheat (Wang et al. 2019), cabbage, and rye grass (Lou et al. 2015) by enhancing the expression of P5C5and down regulating the proDH genes. It has been suggested that NO enhances the accumulation of osmolytes, e.g. proline and glycine betaine, in tomato during cadmium stress when applied externally. Additionally, NO application confines the uptake of Cd and boosts the buildup of nutrients in various portions of tomato plants (Ahmad et al. 2018). The activities of SOD, catalase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase, dehydroascorbate reductase (antioxidant enzymes), and other enzymatic activities of the ascorbate–glutathione cycle are also improved by the application of nitric oxide (Kaya et al. 2020a).
Water deficiency is one of the leading reasons for low agri-productivity and addition of NO has been shown to have an ameliorating effect during water stress in wheat. It has been proposed that SNP applied to wheat seedlings under polyethylene glycol-induced drought stress led to enhanced plantlet growth and high relative water contents and lessened the oxidative destruction to the seedlings under stress (Tian and Lei 2006). In this case, whether NO is protective or toxic to plants was found to be concentration dependent. A low concentration (0.2 mM) inhibits the loss of water, lowers the content of H2O2, and leads to higher antioxidative enzyme activity, which supports the wheat seedling growth, while high concentrations (2 mM) of SNP have the opposite effect.
2.7 Conclusions
Many studies have stated that NO has a presence and role in plant growth, development, and defense responses. It is widely recognized that NO is a second messenger and key regulator in plants, and at low concentrations, it acts as a signal to induce or stabilize the expression of many antioxidative enzymes such as SOD, CAT, etc. It also reacts with ROS, acting as a chain breaker, and exhibits antioxidant properties. It also reacts with lipid alkoxyl and peroxyl radicals and stops the proliferation of radical-mediated oxidation of lipids. Thus, NO helps plants to survive under stressful conditions, but its role is mainly concentration dependent.
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