Evaluation of Drought Stress Tolerance among some of Grape Cultivars Using Physiological and Biochemical Studies

Introduction Drought is one of the most important environmental stresses. It limits crop production in the world and has adverse impacts on growth of plants and their metabolic processes. By changing some of the metabolic processes, drought stress changes the behaviour of plants and, eventually, makes them resistant to some stresses. Given the water crisis in Iran, and since most of the available water is used in the agriculture sector, there is a need to change the current cropping pattern. The substitution of low-water-use for high-water-use plants could be an important water management strategy. Every physiological and biochemical factor involved in water conservation in plants is an effective factor in introducing tolerant varieties.Materials and MethodsThe required chemicals ( 80% acetone, 95% ethanol, ninhydrin, glacial acetic acid, benzene, proline, pure glucose, anthrone, sulfuric acid, potassium phosphate, polyvinylpyrrolidone [PVP], EDTA, hydrogen peroxide and guaiacol) were purchased from the Merck Company. This study was carried out in the spring and summer of 2019 in the Research Greenhouse of Islamic Azad University of Bojnourd. The rooted cuttings of 18 dominant grape varieties in the region (Kolahdari, KajAngoor, Fakhri, sefid Beryan, Divaneh, Sahebi, La’l, Siyah, Shiregi, Garmeh, Khalili, SanjariKajAngoor, Keshmeshi, Ghareghat, Maskeh, Asgari, Flame Seedless and White Seedless) were planted in plastic pots with a diameter of 35 cm and a height of 40 cm. The soil was a mixture of blown sand, leaf litter, and garden soil in the 1:1:1 ratio. Before the experiment, all pots were irrigated to reach field capacity.The factorial experiment was conducted using completely randomized design with three replications. The first factor was irrigation cessation (15-day drought stress) and control (irrigation to field capacity level). The second factor was variety (18 varieties).To apply the stress condition, irrigation cessation continued until leaf wilting signs were appeared. Based on previous experience, the duration of tolerance to drought stress was approximately 2 weeks. During the experiment, the minimum and maximum greenhouse temperatures were, respectively, 18.5 °C and 34 °C and natural light was used.Results and Discussion The results of analysis variance showed that drought stress significantly increased the amount of electrolyte leakage (6.29), and activities of peroxidase (0.056 katal per ml) and catalase (0.92 katal per ml). It also decreased leaf relative water content (61.95%), relative chlorophyll content (16.85) and contents of chlorophyll a (3.45 mg/g), chlorophyll b (1.12 mg/g) and carotenoids (2.84 mg/g).Grape varieties respond differently to drought stress and, in general, water scarcity reduces their leaf RWC and chlorophyll contents. According to Schutz and  Fangmier (2001), a decrease in chlorophyll content under stress conditions is because of an increase in the production of oxygen radicals in the cell. These free radicals can cause peroxidation and decomposition of the pigments. The intensity and greenness of the leaves reduce with decreasing the chlorophyll concentration and accelerating the process of aging.Reduced greenness of the leaves under long-term stress conditions may be partially due to reduced nitrogen flow into the tissues and changing activities of such enzymes as nitrate reductase. Since nitrogen is a constituent of a chlorophyll molecule, its deficiency in plants may slow down the formation of chlorophyll. Lawlor and Cornic (2002) showed the effectiveness of carotenoid, as an auxiliary pigment, in protecting thylakoid membranes and preventing chlorophyll photo-oxidation. Drought stress increases the activity of the peroxidase and catalase enzymes in both drought-sensitive and -resistant varieties; however, the activity of antioxidative enzymes is significantly higher in the stress-resistant varieties.The studied cultivars were divided into the three groups include of tolerant (White Seedless, Garmeh, Maskeh, Flame Seedless, Fakhri, Khalili and Divaneh), semi-susceptible (Kolahdari, Sefid Beryan, Sahebi, Laal, Shiregi, Kaj Angoor Sanjari and Asgari) and sensitive (Siah, Ghare-Ghat, Kaj Angoor Sanjari and Keshmeshi) cultivars to drought stress. Among the studied cultivars, White Seedless had the highest levels of relative leaf water (77.81%), relative chlorophyll content (28.62), carotenoids (4.81 mg/g) and the lowest amount of electrolyte leakage (31.5) and Garmeh the highest chlorophyll a (6.64 mg/g) and chlorophyll b (2.12 mg/g) contents and peroxidase (0.0618 katal per ml) and catalase activities (0.959 katal per ml).Conclusion The grape plant adaptation to drought stress is the result of changes in many morphological, physiological, and biochemical mechanisms, which cause changes in the rate of electrolyte leakage, leaf RWC, proline content, soluble solids, speed of photosynthesis, enzymatic activities, etc. The results showed that the white seedless variety had the highest leaf RWC, relative chlorophyll content, carotenoid content, and the lowest electrolyte leakage. Besides, Garmeh variety with the highest chlorophyll a and b, peroxidase, and catalase contents is amongst the most resistant varieties.