Mitigation of Stress Induced by Salinity and Nickel in Pea (Pisum sativum L.) By Exogenous Application of Synthetic Proline and Proline-enriched Lolium perenne (L.) Leaf Extract

The plants of pea (Pisum sativum L.) were grown under NaCl and/or NiCl₂ stress, to evaluate whether pure proline and proline-enriched Lolium perenne (L.) leaf extract could efficaciously mitigate the drastic effects of nickel and salinity stress on pea. Both stress factors (salinity and nickel) significantly inhibited growth, chlorophyll contents, photosynthetic activity, stomatal conductance, intercellular carbon dioxide (CO₂) level, number of stomata, stomatal size, water-use efficiency, relative water content (RWC), and the membrane stability index (MSI). However, the proline and glycine betaine contents in leaves and roots, lipid peroxidation, electrolyte leakage, and activities of antioxidant enzymes (superoxide dismutase, peroxidase, catalase, ascorbate peroxidase, guaiacol peroxidase, and glutathione reductase) were significantly increased. Exogenously applied proline and L. perenne (LP) leaf extracts significantly overcame the nickel and/or salinity-induced toxic effects on growth, RWC, and various photosynthetic attributes. However, follow-up treatment with proline and LP leaf extract detoxified the stress caused by NiCl₂and/or NaCl, by suppressing lipid peroxidation and electrolyte leakage, accelerating the antioxidant activities, and improving the MSI, leaf/root proline, and glycine betaine contents. Natural proline (LP leaf extract) proved to be better than pure proline for improving growth, gas exchange parameters, proline, glycine betaine, RWC, and antioxidant activities. As LP leaf extract was enriched with a substantial amount of proline along with many other essential nutrients, it was found to be as efficacious as pure proline in ameliorating growth, some major physiological attributes, and non-enzymatic and enzymatic antioxidant activities in the pea, under nickel and/or salinity stress. Thus, it could be used as an alternative inexpensive source of proline, to be used as a mitigating agent for safeguarding plants against the deleterious effects of nickel and/or salinity stress.

Keywords: Salt stress, nickel stress, Lolium perenne, proline, antioxidant, photosynthetic attributes