Investigation the Different Levels of Drought Stress on Almond Cultivars

Introduction  Water shortage is very frequent in many countries, and, together with the rising demand for industry, growth of human population, climate change and specifically the trend towards irrigated agriculture, has led to widespread problems of water scarcity, especially in Middle East countries. This situation imposes the need to optimize water use in all human activities. Among the different productive uses of water, agriculture is by far the main water user in most water scarce regions and, consequently, any potential improvement in the use of the available water resources may play a significant role toward achieving a more sustainable use of water. Plant responses to water deprivation are usually monitored through selected morphological and physiological parameters which have been proven to be good indicators of drought in different studies. Some of the most important standards for evaluating plant genotypes under drought stress are measurements of morphological parameters such as height, leaf characters and root growth.   Materials and Methods  To compare the growth response of different almond cultivars to different levels of water stress, an experiment was conducted as a split plot in the base of randomized complete block design with three replications in the Agricultural and Natural Resources Research Center of Chaharmahal and Bakhtiari Province in two growing season 2019-2020 and 2020-2021. Different irrigation periods based on the percentage of usable soil moisture between filed capacity to wilting point, including 70% filed capacity (control or no stress), 50% filed capacity (mild stress), 30% filed capacity (medium stress) and 10% of field capacity (severe stress) were considered as the main factor of the experiment. The sub-factor included 14 commercial cultivars of almonds (Mamaei, Rabi, Saba, Araz, Eskandar, Aidin, Shahrood 6, 7, 8, 10, 12, 13 and 21 and GN vegetative rootstock), all of which were grafted on GN rootstock. In this study, uniformly grafted seedlings in terms of age, stem diameter and height were selected and planted. In the second year after planting the seedlings, in order to apply drought stress, tubes for hygrometer (TDR) were installed in each experimental plot and based on soil moisture content, irrigation cycle was determined for different treatments.   Results and Discussion  In both years, three months after applied water stress growth traits and nutrient concentrations in the leaves of treated seedlings were measured. Based on the results of analysis of variance, the morphological traits of almond seedlings were significantly affected by cultivar type and drought stress level. In all almond cultivars, the highest height was belonged to seedlings that were grown in non-stress conditions and with increasing the drought stress intensity, the height of almond seedlings was decreased. Under severe drought stress, GN and Mamaei cultivars had the highest (183.93 cm) and the lowest (94.60 cm) height, respectively. Seedling height in GN, Shahrood 12, Saba and Shahrood 10 cultivars showed the lowest decrement under severe drought stress. In all cultivars, drought stress caused a significant reduction in the length and width of the seedlings crown, and the greatest decreasing was recorded in severe drought stress (10% FC). Under severe drought stress, cultivar GN had the largest crown and cultivars Rabi, Shahrood 7 and Eskandar had the smallest crown. Increasing the drought stress intensity significantly reduced the branches growth of seeding in terms of number and length of sub-branches. As the intensity of drought stress increased, the length of sub-branches decreased however the number of intermediates in sub-branches increased. In non-stressed condition, the cultivar GN had the longest branch (55.95 cm), which was significantly higher than the other studied almond cultivars. The shortest branches were also observed in Saba (29.94 cm) and Eskandar (29.47 cm) cultivars. Increasing drought stress caused a significant reduction of leaf area in all studied cultivars and the highest decreasing was observed under severe drought stress. The GN (37.76 cm²) and Shahrood 10 (31.81 cm²) had the highest leaf area in non-stress and drought stress conditions. Under severe drought stress (10% FC) cultivar Shahrood 6 showed the lowest leaf area. The results of this study showed that increasing the intensity of dehydration significantly reduced the amount of nitrogen, phosphorus, manganese and zinc in the leaves of the studied cultivars of almonds, however, the amount of potassium and iron in stressed plants increased under drought stress. Based on the results of the present study, under severe drought stress the GN, Shahrood 8 and Shahrood 12 cultivars in terms of growth indices including seedling height, stem diameter, canopy growth, branch growth and concentration of macro and micro elements was superior compared with the other studied cultivars.   Conclusion  Based on the results of this study, drought stress significantly reduced growth indices and nutrient concentrations, although the reaction of almond cultivars to different levels of drought stress was different. In this study, among the studied almond cultivars GN, Shahrood 8 and Shahrood 12 cultivars in terms of growth characters including seedling height, stem diameter, canopy growth, branch growth and concentration of macro and micro elements showed higher tolerance to different level of drought stress. These cultivars less affected by the high intensities of dehydration. Therefore, GN, Shahrood 8 and Shahrood 12 cultivars can be used in future studies to evaluate the possibility of cultivating these cultivars in areas with water deficit.