Effect of Drought Stress on Growth and Physiological Parameters in Four Pistachio Rootstocks (Pistacia vera L.)

Introduction Pistachio is one of the most important horticultural crops in Iran. Most pistachio orchards are located in arid and semi-arid regions of Iran, which have a lack of irrigation water. Drought stress is considered as one of the most important limiting factors for the production of plants in dry areas. Pistachio (P. vera L.) is one of the drought-resistant fruit trees due to its deep roots, but it has been reported that water stress reduces the yield and dry quality of this product. The degree of resistance to drought stress in pistachios depends on the type of rootstock, so the evaluation of pistachio species from the point of view of the rootstock is important.   Material and Methods In order to evaluate the resistance of four pistachio rootstocks to drought stress, a factorial experiment in a completely randomized design with two drought factors at three levels (3, 6 and 9 days irrigation intervals) and a rootstock at four levels, 'Badami- e- Riz-e-Zarand', 'Ghazvini', Eurycarpa (Pistacia euricarpa), and an interspecific hybrid (P. vera × P. terebinthus) with three replications were performed under greenhouse conditions. Hybrid rootstocks produce in a breeding program using hybridization of P. vera and P. terebinthus. At the end of the experiment, the growth parameters including the number of leaves, leaf area, stem height, as well as stem diameter at a height of one centimeter above the soil surface, leaf fresh and dry weight, shoot fresh and dry weight, and root fresh and dry weight, as well as physiological and biochemical parameters including chlorophyll fluorescence (FV/FM), SPAD index, PI, Cha, Chb, total chlorophyll and carotenoids, RWC, proline, soluble carbohydrates, and phenolic compounds and concentrations of Calcium, Magnesium, Potassium and Iron elements in leaves were measured.   Results and Discussion The results showed that drought stress reduced growth and physiological indices so that the lowest reduction in shoot dry weight was related to the hybrid rootstock and the highest reduction was related to 'Ghazvini'. It has been reported that the height and diameter of the stem of pistachio seedlings decrease significantly with the increase in irrigation period. This decrease can be attributed to the reduction of the cell turgor pressure under stress. In response to drought stress, the content of proline and soluble carbohydrates in the leaves of the studied rootstock increased. The results of this study are in line of other studies on pistachio. The highest content of proline and the lowest content of soluble carbohydrates were observed in 'Badami- e- Riz-e-Zarand’. The highest content of soluble carbohydrates belonged to Eurycarpa and hybrid rootstocks. The reason for the increase of soluble carbohydrates in the present study can be due to the decrease in the amount of photosynthesis. Drought stress exerted an influence on the nutrient concentrations within both shoots and roots, leading to an increase in potassium content. The highest concentrations of potassium in shoots and roots were noted in the Eurycarpa and hybrid rootstocks, respectively. This elevation in potassium concentration in both roots and shoots could be attributed to active absorption of this element. Plants enhance potassium absorption by expending energy to sustain stomatal conductance, cell turgor pressure, and osmotic regulation.   Conclusion Drought stress is considered one of the most important environmental stresses, the first effects of which appear in plants in the form of reduced growth and disturbance in physiological parameters. Based on the present study, most of the measured parameters were subjected to drought treatment and the investigated rootstocks showed different reactions, so that hybrid and Eurycarpa rootstocks were less affected by drought stress than other rootstocks. According to the results of the present study, it can be postulated that Eurycarpa and hybrid rootstocks have a high potential for drought resistance.