Effect of Salicylic Acid Foliar Application on Barley Water use Efficiency in Cut-Off Condition

Drought stress has osmotic and physiological effects on plants, resulting in reduced growth, metabolic disorders and oxidative stress in plants. Different plants use several strategies to cope with drought stress. External application of different compounds such as organic solvents and minerals is a solution to increase drought tolerance in plants. Salicylic acid is recognized as a plant hormone and its role in defense mechanisms against biological and non-biological stressors is well defined. The purpose of this experiment was to evaluate the effect of salicylic acid on reducing the effects of drought stress on yield, water use efficiency and some physiological characteristics of the barely. This experiment was carried out as factorial experiment based on randomized complete block design with three replications in the Kizor fields located in the city of Sabzevar in 2017. The altitude of this area is 1195 m above sea level and is 36° 5 N latitude and 44° 3' E longitude. The treatments included irrigation at two levels of cut-off irrigation (irrigation cut from booting to harvesting stage) (Z39) and non-cut-off irrigation and salicylic acid spraying treatment at four levels of zero (control), two, four and six mM at stage of shoots on barley. Data were analyzed with the SAS software ver 9.1 and the means were compared with LSD test. The results showed that in both conditions of non-cut-off irrigation and cut-off irrigation, the highest number of spikes, number of seeds, seed yield and biological yield per m2 were obtained in two to six mM of salicylic acid, respectively, and the lowest was related to control treatment. The lowest water use efficiency (0.38) was obtained under non-cut-off irrigation conditions without salicylic acid and with the consumption of salicylic acid up to six mM, water use efficiency increased. Also, salicylic acid spraying in cut-off irrigation conditions compared to non-cut-off irrigation led to increase water use efficiency. In both conditions, non-cut-off irrigation and cut-off irrigation, the highest and lowest grain yields were obtained from 6 mM salicylic acid and control treatments (without salicylic acid spraying), respectively. Under non-cut-off irrigation and cut-off irrigation, spraying with 6 mM of salicylic acid increased the grain yield by 44% and 48% compared to control (without salicylic acid spraying). It seems that the effect of salicylic acid spraying in cut-off irrigation conditions was more effective than non-cut-off irrigation conditions. The cause of higher water use efficiency in cut-off irrigation conditions in comparison with non-cut-off irrigation was high waste of water due to evapotranspiration and deep penetration in non-cut-off irrigation treatments. Spraying with salicylic acid by stimulating the growth of hair and sub-roots plays an effective role in increasing water use efficiency. In cut-off irrigation conditions, salicylic acid consumption has reduced the amount of electrolyte leakage so that the spray at a concentration of 6 mM of salicylic acid compared to the control (without salicylic acid spraying) decreased electrolyte leakage about 38 %. The reduction of membrane damage due to the use of salicylic acid is related to the production of antioxidants. Spraying with salicylic acid increases the stability of the membrane against oxidative stresses. Water deficit at first affecting the structure of the cell membrane, increased the permeability of the membrane relative to the ions and macromolecules and at second stage decreased the photosynthesis by decreasing the chlorophyll index, which finally reduced the yield and its components. Also, salicylic acid alleviate drought stress by reducing ion leakage, increasing proline concentration and chlorophyll index resulting to improved wheat grain yield under cut-off irrigation conditions compared to non-cut-off irrigation.