EFFECT OF SALINITY AND NITROGEN NUTRITION ON FREE PROLINE AND SEED OIL CONTENT OF MEDICINAL PUMPKIN (CUCURBITA PEPO SUBSP. PEPO CONVAR. PEPO VAR. STYRIACA)

Field and greenhouse studies were conducted in 1998 and 1999 to study the effect of salinity and nitrogen nutrition on the amount of free proline in the leaves and the seed oil content of common pumpkin. Salinity treatments were applied at four levels (0, 2.5, 5, and 10 g.l of NaCl) and nitrogen nutrition treatments were applied at five levels (0, 75, 150, 225, and 300 kg.ha of NH4NO3). In this research leaf dry weight, N concentration of different plant parts, leaf proline content, K:Na ratio of the leaf and root, and seed oil content of the plants were determined. Salinity stress was applied at two stages i.e., by soaking the seeds in the solution for 36 h before planting, and during growth stage from four leaf stage until flowering. Nitrogen fertilizer was applied at planting, fourth leaf stage, and flowering. The results indicated that in the two years of experiment, salinity stress increased the amount of free proline in the leaves compared to the control. The highest amount of proline was obtained when salinity and nitrogen levels were at 2.5 g.l of NaCl and 225 kg.ha of nitrogen. The highest seed oil content was obtained at salinity of 2.5 g.l of NaCl. Leaf dry weight and leaf and root K:Na ratios decreased as salinity decreased. Leaf N content increased as salinity and nitrogen increased. Oil seed content was highest at 2.5 g.l of NaCl and 75 kg.ha of nitrogen. INTRODUCTION Salinity stress may affect plant growth and development (Alfocea et al., 1993; AlRawahy et al., 1992). Plants grown under environmental stresses such as, saline conditions cope with stress through avoidance, exclusion of salts, and physiological mechanisms (Solaiman and Doss, 1992). Soil salinity is one of the important characteristics of arid and semi arid regions in the world (Cornillon and Palloix, 1997). In addition, soil salinity is caused by the application of high rate of fertilizer, rising water tables and use of water that passed from saline area or saline water for crop irrigation (Bohnert and Shen, 1999; Gunes et al., 1996; Al-Rawahy et al., 1992; Knight et al., 1992). Salinity may affect plant growth and development through (1) high salt concentration in the soil solution cause high osmotic pressure that result in low soil water potential and (2) high concentration of potential toxic ions, such as Na and Cl (Bohnert and Shen, 1999; Gunes et al., 1996). Soil salinity created by inorganic salts in the growing medium suppress or retard plant growth through osmotic effects of specific ion, inadequate uptake of essential nutrients, or by changing of several process such as nitrogen and carbon dioxide assimilation, protein synthesis and water absorption. Plant response to stress (tolerance or avoidance) depending on the nature of the salts present and the growth stage (Alfocea et al., 1993; Gunes et al., 1996). Drought or salt stress resulted to osmotic stress increase the cellular concentration of osmolyts or compatible solutes that interpreted as adjustment (Bohnert and Shen, 1999; Guerrier, 1998; Hua et al., 1997; Volkmal et al., 1997; Voetberg and Sharp, 1991). There are some organic and inorganic compounds such as prolin and betain glycin that accumulate in the osmotic affected plants or tissues (Hua et al., 1997; Solaiman and Doss, 1992). Proline is one of Proc. WOCMAP III, Vol 2: Conservation Cultivation & Sustainable Use of MAPs Eds.: A. Jatisatienr, T. Paratasilpin, S. Elliott, V. Anusarnsunthorn, D. Wedge, L.E. Craker and Z.E. Gardner Acta Hort. 676, ISHS 2005 46 the compatible solutes that varies in the amount or concentration with the type and level of salinity (Bohnert and Shen, 1999; Solaiman and Doss, 1992). Salinity stress decrease plant growth by affecting nitrogen absorption required for cell growth. Nitrogen is severely affected by salt stress and yield reduction in many plants due to N deficiency is caused by high external Cl concentration (Grattan and Grieve, 1999; Pereze-Alfocea et al., 1993). The purpose of this study was to determine the effect of various NaCl levels and ammonium nitrate on the growth, free proline content, K:Na ratio in leaves and roots, plant dry weight and seed oil content of medicinal pumpkin. MATERIALS AND METHODS The research conducted in Resarch Station of Ferdowsi University of Mashhad (RSFUM) in 2000-2001 season crop. Medicinal pumpkin plants were grown in a glasshouse under natural light conditions. Soil properties were: EC=1.1 dS.m; pH=8.2; Soil texture: sandy loam; nitrogen content=0.02%; the temperature 26/22°C. Salt treatments were applied using half strength Hoglands solution at four levels: 0, 2.5, 5, and 10 g.l of NaCl. The seeds were soaked for 36 h and then rinsed with distilled water and sowed in 10 l trays (3 seed per tray) that filled with sandy loam soil and irrigated with tap water. The NaCl was added to the half-strength hoglands solution and applied again from appearance of fourth leaf to female flower formation. Nitrogen was applied at 0, 75, 150, 225, and 300 kg.ha prior to seed sowing, at the fourth leaf stage and at flowering stage. Nitrogen as ammonium nitrate was applied so that 0, 25, 55, 85, and 110 g was achieved per plant. There were three replications for each treatment of NaCl and nitrogen that were evaluated in a split-plot complete block design. Where salinity treatments were the main plots and nitrogen treatments were the sub plots. Data obtained from the different analyses and measurements were analyzed by ANOVA and the treatment means were compared by MSTATC package program for Duncans Multiple Range Test (p≤0.01,0.05). Proline measurement for fully developed leaves was determined spectrophotometricaly (Shimadzu Uv-2000) using the ninhydrin method described by Bates et al. (1973), and a proline standard obtained from Aldrich. Total nitrogen content of leaves, stems, and roots was determined at the flowering stage. Total N was determined by the Kjeldahl method (Jones et al., 1991). Dry weight of leaves, stems, and roots were determined by sampling at flowering stage. Developing and mature leaves samples were collected from each plant treated with NaCl and nitrogen fertilizer at the beginning of the flowering stage. Fresh and dry weight were taken for leaf, stem and root samples. Na and K content of the leaf analyzed with the flame photometer by the Versente method (Soliman and Doss, 1992) and K:Na ratio determined. The oil of the seed was extracted with organic solvent by soxhelet instrument, and oil seed content in each treatment determined (%) after harvesting and seed extraction and drying. RESULTS AND DISCUSSION Dry Weight of Leaves Salinity affected the fresh and dry weight of leaves indicating that growth was inhibited with an increase in salinity (p < 0.01). Dry weight was reduced by salinity. It was more pronounced for plants grown at higher NaCl concentration (Table 1). Effects of nitrogen fertilizer on the growth of plants (leaf dry weight) resulted in different rate of growth i.e. growth was increased at 75, 150 kg.ha of nitrogen as compared to the other treatments and control (Table 2). These results corresponds to reports that dry matter of tomato (Bloarin et al., 1993; Solaiman and Doss, 1991; Al-Rawahy et al., 1992) pepper (Gunes et al., 1996) peanut and cotton (Leidi et al., 1992) were reduced by salinity stress. However, most salinity and nitrogen interaction field studies were conducted on soils deficient in N, and therefore the addition of N improved growth and/or yield when the degree of salinity was not severe (Grattan and Grieve, 1999).