The Effect of Humic Acid on the Growth and Physiological Indices of Pistachio Seedling (Pistacia vera) under Drought Stress

This study was carried out to evaluate the effect of soil application of humic acid on growth and physiological indices of pistachio seedlings ("Badami Zarand" rootstock) under drought stress. This experiment was arranged as a completely randomized design with three replications in greenhouse conditions. The experimental treatments consisted of humic acid (0, 30 and 60 gr in 4 kg-1 soil in pot) and irrigation intervals (7, 20 and 30 days). The studied  growth parameters included of stem height, leaf number, leaf area, stem diameter, biomass of root and shoot and physiological indices in leaves such as relative water content (RWC), relative water loss (RWL), leaf water content (LWC), excised leaf water retention (ELWR), excised leaf water loss (ELWL) and relative water protection (RWP). In this experiment, the results showed that relative water content reduced and relative water loss increased in leaf in irrigation interval of 30 days. The results indicated that the application of humic acid increased the vegetative growth of pistachio seedling in comparison with the control under high irrigation interval (p<0.05). The results also showed that relative water content and relative water protection significantly increased with the application of humic acid (p<0.05). These results indicated that the pistachio seedlings were sensitive to water stress and the application of humic acid under drought stress could be an appropriate management strategy to improve the growth of seedlings in pistachio orchards in arid and semi-arid areas of Iran.