Investigation of the drought stress on some photosynthetic and morphological indicators of summer savory (Satureja hortensis L.)

Introduction In order to better management of plant performance under conditions of drought stress and study of the condition of plant growth, transpiration, plant stomatal conductance, plant photosynthesis (main determinant of plant growth), photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoid), a completely randomized block design with three replications was conducted in the Faculty of Agriculture of Lorestan University in 2017. The water stress treatments including 100, 80, 60 and 40 Percent of water requirement were applied. Irrigation was performed based on weighing method. Gas exchange measurements and morphological harvesting were done 80 and 90 days after stress. Results showed that water stress treatment has a significant reduction of stomatal conductance, transpiration rate, photosynthesis rate, intercellular CO2 concentration, photosynthetic water use efficiency and carboxylation efficiency. The highest values of the above parameters were related to control treatment and the lowest values were observed in treatment of 40% of water requirement. these values were decreased 68.08%, 59.99%, 61.42%, 37.96%, 20.79% and 37.5%, respectively, compared to the control treatment. As the result showed, the amount of stomatal conductance, transpiration rate, photosynthesis rate are affected by drought stress more than other parameters. Photosynthetic pigments were decreased significantly by increasing drought stress. The highest and lowest of chlorophyll a and b and carotenoid observed in control and heavy drought stress. Finally, drought stress significantly reduced the growth parameters such as plant height, stem diameter, dry weight of the plant. Material and methods The experiment was conducted in the research field of Faculty of Agriculture, Lorestan University, Khorramabad, Iran, through June to October 2017. The experimental site was located at 33029'N latitude, 48022'E longitude and 1125 m altitude above mean sea level. The experimental design was a randomized complete block with three replications. The seeds of summer Savory were sown in December with a density of 150 plants per square meter in plastic pots as micro lysimeters. Plastic pots with a diameter of 0.25 m and a height of 0.3 m were filled with a mixture, containing soil of the research field (near the greenhouse) and animal manure(4(soil):1(animal manure)). The soil texture was SCL. Weighing moisture at the field capacity point (θ_FC) and wilting point (θ_PWP) were obtained 32% and 16% using pressure plate. The treatments were 4 combinations of 4 irrigation water supply including: T1:100% of water requirement (control treatment), T2: 80% of water requirement, T3: 60% of water requirement, and T4: 40% of water requirement. Plants were harvested at flowering stage (95 days after planting). In control treatment, Irrigation was performed, when the soil moisture reached the low level of the readily available water (θ_m). These parameters were obtained from the following formulas (Allen et al., 1998). θ_m=|θ_fc-MAD(θ_fc-θ_PWP)| Where MAD is the maximum allowable depletion that was supposed to be 0.5; θ_FC is the percentage of moisture at the field capacity point; θ_PWP is the percentage of moisture at the wilting point. The water stress treatments were initiated 40 days after germination and continued until the end of the excrement (at flowering stage). After applying stress, considering water requirement in control treatment (T100), Irrigation requirement in T80, T60 and, T40 treatments were 0.8 (T100), 0.6(T100) and 0.4(T100). Photosynthetic parameters including photosynthesis rate, transpiration, stomatal conductance, Co2 sub stomatal were measured by Photosynthesis measurement device (Model LCA4) and chlorophyll content were measured by spectrophotometer based on Arnon's method. For statistical analysis, analysis of variance (ANOVA) and Dunkan’s test were performed using SAS software. Result and discussion Results showed that water stress treatment has a significant reduction on stomatal limiting factors such as stomatal conductance, transpiration rate, photosynthesis rate, inter cellular CO2 concentration, photosynthetic water use efficiency and carboxylation efficiency. Under this kind of condition, Satureja hortensis L. decreased stomatal factors in order to conserve water. Inter cellular CO2 concentration and finally photosynthetic rate decreased. The highest values of the above parameters were related to control treatment and the lowest values for 40% water requirement treatment, which were decreased 68.08%, 59.99%, 61.42%, 37.96%, 20.79% and 37.5%, respectively, compared to the control treatment. As the result showed, the amount of stomatal conductance, transpiration rate, photosynthesis rate are affected by drought stress more than other parameters. Conclusion Results of this research indicated that stomatal conductance, transpiration rate, photosynthesis rate, inter cellular CO2 concentration, photosynthetic water use efficiency and carboxylation efficiency were influenced by drought stress. These parameters decreased significantly as drought stress increased. The highest values of the above parameters observed in control treatment and the lowest values observed in 40% water requirement treatment.