Environmental and Experimental Botany 60 (2007) 397–403 Improved salt tolerance of melon (Cucumis melo L.) by the addition of proline and potassium nitrate Cengiz Kaya a,∗ , A. Levent Tuna b , Muhammad Ashraf c , Hakan Altunlu d a Harran University, Agriculture Faculty, Soil Science and Plant Nutrition Department, Sanliurfa, Turkey b Mugla University, Biology Department, Mugla, Turkey c Department of Botany, University of Agriculture, Faisalabad 38040, Pakistan d Mugla University Ortaca Polytechnic, Mugla, Turkey Received 12 April 2006; received in revised form 1 December 2006; accepted 28 December 2006 Abstract 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 solu- tion, (ii) salinity treatment, as for control plus 150 mM NaCl. Salinity treatment was combined with or without either 5 mM supplementary KNO 3 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 Ca 2+ ,K + and N, and chlorophyll a and b contents, accompanied by significant increases in Na + uptake, proline concentration and membrane permeability. Supplementary KNO 3 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 Ca 2+ , N and K + in the leaves of plants subjected to salt stress. © 2007 Elsevier B.V. All rights reserved. Keywords: Amelioration; Exogenous application; Salt tolerance; Fruit; Salinity; Proline; Potassium nitrate 1. Introduction Salinity is a major abiotic stress reducing the yield of a wide variety of crops all over the world (Tester and Davenport, 2003; Ashraf and Foolad, 2007). Although the level of salts in most irrigation waters is below the threshold for the more sensitive crops, salt accumulation in irrigated soils from both irrigation and groundwater sources can increase salinity to lev- els which can reduce growth and yield of even the more tolerant crops. Overcoming salt stress is a main issue in these regions to ensure agricultural sustainability and continued food production (Heuer, 2003). Sodium is the predominant soluble cation in many of the soils of arid and semi-arid areas. However, most plants, espe- cially glycophytes, are very sensitive to high Na + concentrations. ∗ Corresponding author. Tel.: +90 4142470384/2309; fax: +90 4142474480. E-mail address: c kaya70@yahoo.com (C. Kaya). High concentrations of Na + disturb intracellular ion homeostasis which leads to membrane dysfunction, attenuation of metabolic activity, and secondary effects that cause growth inhibition thereby leading to cell death (Rus et al., 2001, 2004; Ashraf, 2004). The ability of plant cells to maintain sodium concentra- tions low in the cytosol is a vital process associated with the ability of plants to grow under high salt regimes (Blumwald, 2000; Ashraf and Harris, 2004). Sodium enters leaf cells and is then pumped into the vacuole before its concentrations are built up in the cytoplasm. The pumping of Na into the vacuole is cat- alyzed by vacuolar Na + /H + antiporter (Blumwald et al., 2000) and the plasma membrane Na + transporter AtHKT1 (Rus et al., 2004). However, Lacan and Durand (1996) proposed that under high Na + concentration, a Na + /H + antiporter worked in reverse, to pump Na + into the cytosol. Plants need to maintain internal water potential below that of soil to maintain turgor and sufficient water for growth. This requires an increase in osmotica, either by uptake of solutes from the soil or by synthesis of metabolically compatible solutes 0098-8472/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2006.12.008