Hydrogen peroxide alleviates nickel-inhibited photosynthetic responses through increase in use-efficiency of nitrogen and sulfur, and glutathione production in mustard

The response of two mustard (Brassica juncea L.) cultivars differing in photosynthetic capacity to different concentrations of hydrogen peroxide (H₂O₂) or nickel (Ni) was evaluated. Further, the effect of H₂O₂ on photosynthetic responses of the mustard cultivars grown with or without Ni stress was studied. Application of 50 μM H₂O₂increased photosynthesis and growth more prominently in high photosynthetic capacity cultivar (Varuna) than low photosynthetic capacity cultivar (RH30) grown without Ni stress. The H₂O₂ application also resulted in alleviation of photosynthetic inhibition induced by 200 mg Ni kg^-1 soil through increased photosynthetic nitrogen-use efficiency (NUE), sulfur-use efficiency (SUE), and glutathione (GSH) reduced production together with decreased lipid peroxidation and electrolyte leakage in both the cultivars. However, the effect of H₂O₂ was more pronounced in Varuna than RH30. The greater increase in photosynthetic-NUE and SUE and GSH production with H₂O₂ in Varuna resulted from higher increase in activity of nitrogen (N) and sulfur (S) assimilation enzymes, nitrate reductase and ATP-sulfurylase, respectively resulting in enhanced N and S assimilation. The increased N and S content contributed to the higher activity of ribulose-1,5-bisphosphate carboxylase under Ni stress. Application of H₂O₂ also regulated PS II activity and stomatal movement under Ni stress for maintaining higher photosynthetic potential in Varuna. Thus, H₂O₂ may be considered as a potential signaling molecule for augmenting photosynthetic potential of mustard plants under optimal and Ni stress conditions. It alleviates Ni stress through the regulation of stomatal and non-stomotal limitations, and photosynthetic-NUE and -SUE and GSH production.