Investigation of drought and salinity stresses on leaf area index, some physiological traits, and water productivity of sesame in hot and arid region of Yazd province

Introduction:   In the irrigated agriculture of Iran, the quantitative and qualitative limitations of water resources and consequently, drought and salinity stresses, always threaten crop production. This problem exists in most regions of the country. Sesame is one of the oilseed and industrial crops that is somewhat resistant to drought and salinity and its placement in the cultivation pattern can provide some part of the country's need for sesame production and oilseed crops. Sesame is known as the queen of oil seeds because of its high percentage and quality of oil and high properties for health. The aim of this study was to investigate the water productivity, leaf area index, and some physiological traits of sesame Oltan variety including canopy cover temperature, Chlorophyll index, and water potential under simultaneous drought and salinity stresses.   Materials and methods:   A factorial experiment in randomized complete block design was conducted in Research Farm of Ardakan University during the 2019 to 2020 growing season with three replications. Experimental factors were four salinity levels of irrigation water including 1.5, 4, 7, and 10 dS.m-1 and three drought levels with different irrigation intervals as 4, 8 and 12 days, respectively. During the growing season, leaf area index, canopy cover temperature, and chlorophyll index were measured for four times (35, 50, 65 and 85 days after sowing). Leaf water potential was also measured in the mid-growing season. Water productivity for grain production was also determined. Statistical analysis was done using SPSS version 22 software and averages were compared with Duncan's test at 5% probability level.   Results and discussion:   Results showed that the irrigation interval had a significant effect on the leaf area index, leaf water potential, grain yield, and water productivity. Irrigation water salinity had a significant effect on all of the studied traits, the interaction of the drought and salinity stresses was not only significant on SPAD index. Increasing the irrigation interval from 4 to 12 days and increasing water salinity from 1.5 to 10 dS m-1 decreased the maximum leaf area index of sesame by 49% and 58%, respectively. Also, an increase in salinity from 1.5 to 10 dS.m-1 resulted in 18% increase in leaf temperature, 40% decrease in SPAD index, and 17% decrease in leaf water potential. The increase in drought stress also had no significant effect on leaf temperature, while it caused 14% increase in SPAD index and 15% decrease in leaf water potential. Increasing the irrigation interval from 4 to 8 and 12-day caused a 67 and 68% significant decrease in grain yield, respectively. There was no significant difference in the grain yield of 8 and 12-day irrigation interval. Increasing the water salinity from 1.5 to 4, 7, and 10 dS.m-1 decreased the grain yield by 35, 80, and 96%, respectively. Increasing the irrigation interval from 4 to 8 and 12 days caused a 55 and 51% decrease in water productivity, respectively. This index decreased significantly (95%) by increasing water salinity from 1.5 to 10 dS.m-1.   Conclusion:   Water productivity and maximum leaf area index decreased significantly by increasing water salinity and irrigation interval. An increasing trend in leaf temperature was observed with increasing salinity, which is due to the decrease in plant transpiration under exposure to salinity stress. The SPAD index showed a decreasing trend with the increase in salinity, while the increase in drought stress (increasing the irrigation interval) caused an increase in this index. Sesame leaf water potential also decreased by increasing salinity and drought stresses. Generally, results showed that leaf area index and studied physiological traits of sesame were more affected by water salinity stress than drought stress. The results of interaction also showed that in all investigated traits, except for leaf water potential, no significant difference was observed between 8 and 12-day irrigation interval at all salinity levels. In 1.5 and 4 dS.m-1 salinity levels, the difference in grain yield and water productivity were significant in 4 and 8-day irrigation interval, while there was no significant difference in 7 and 10 dS.m-1 salinity levels. Although the highest grain yield and water productivity were observed in the 4-day irrigation interval and 1.5 dS m-1 salinity level treatment, based on the results of physiological traits, grain yield, and water productivity of sesame, especially under water shortage conditions, it is recommended to use 12-day irrigation interval instead of 8-day because there was no significant difference in the water productivity.