Improved salt tolerance of melon (Cucumis melo L.) by the addition of proline and potassium nitrate

A pot experiment was carried out under glasshouse conditions with melon (Cucumis melo) cv. “Tempo F1” in a mixture of peat, perlite and sand (1:1:1) to investigate the effects of external proline and potassium nitrate applications to salinity-treated (150 mM) plants with respect to fruit yield, plant growth, some physiological parameters and ion uptake. Treatments were—(i) control (C): plants receiving nutrient solution, (ii) salinity treatment, as for control plus 150 mM NaCl. Salinity treatment was combined with or without either 5 mM supplementary KNO3 or 10 mM proline. The salt treatment (150 mM NaCl) led to significant decreases in plant growth, fruit yield, relative water content (RWC), stomatal density, uptake of Ca2+, K+ and N, and chlorophyll a and b contents, accompanied by significant increases in Na+ uptake, proline concentration and membrane permeability. Supplementary KNO3 and proline treatments significantly ameliorated the adverse effects of salinity on plant growth, fruit yield and the physiological parameters examined. This could be attributed to the effects of all the external supplements in maintaining membrane permeability, and increasing concentrations of Ca2+, N and K+ in the leaves of plants subjected to salt stress.