Exogenous proline and proline-enriched Lolium perenne leaf extract protects against phytotoxic effects of nickel and salinity in Pisum sativum by altering polyamine metabolism in leaves

Authors: MUHAMMAD ADNAN SHAHID, RASHAD MUKHTAR BALAL, MUHAMMAD ASLAM PERVEZ, TAHIRA ABBAS, MUHAMMAD ANJUM AQEEL, MUHAMMAD MANSOOR JAVAID, FRANCISCO GARCIA-SANCHEZ

Abstract: Pea plants (Pisum sativum L.) were exposed to NaCl and/or NiCl_2 stress to investigate whether pure proline and Lolium perenne L. (LP) leaf extract could efficaciously protect against the phytotoxicity generated by nickel and/or salinity stress in pea. Both stress factors (salinity and nickel) significantly inhibited growth, total chlorophyll content, photosynthetic activity, stomatal conductance, intercellular CO_2 level, relative water content (RWC), and membrane stability index (MSI). However, superoxide, hydrogen peroxide, proline, glycine betaine, total free amino acids, total soluble sugars, total phenolic compounds, tocopherol contents, lipid peroxidation, and electrolyte leakage were significantly increased. Similarly, free, soluble-conjugated, and insoluble-bound polyamine contents [spermine (Spm), spermidine (Spd), and putrescine (Put)] and activities of polyamine-biosynthetic enzymes (arginine decarboxylase, ornithine decarboxylase, and S-adenosylmethionine decarboxylase) in leaves were also increased in response to nickel and/or salinity stress. Exogenously applied proline and LP leaf extracts significantly overcame the nickel- and/or salinity-induced toxic effects on growth, RWC, and various photosynthetic attributes. The follow-up treatment with proline and LP leaf extract detoxified the stress caused by NiCl_2 and/or NaCl by suppressing the lipid peroxidation and electrolyte leakage, accelerating the activities of polyamine-biosynthetic enzymes, and improving the MSI, leaf polyamines (Spm, Spd, and Put), and organic osmolytes (free proline, glycine betaine, total free amino acids, total soluble sugars, total phenols, and tocopherol contents). Natural proline (LP leaf extract) proved better than pure proline in improving growth, photosynthetic activity, RWC, organic osmolytes, and polyamine metabolism. Since LP leaf extract is enriched with substantial amounts of proline along with many other essential nutrients, it was as efficacious as pure proline in improving growth, some major physiological attributes, and polyamine metabolism in pea under nickel and/or salinity stress. Thus, it can be used as an alternative inexpensive source of proline for its utilization as a mitigating agent for protecting plants against the phytotoxic effects of nickel and/or salinity stress.

Keywords: Salt stress, nickel stress, proline, polyamine, photosynthesis, osmolytes

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