The effect of drought stress on some photosynthetic characteristics of kochia (Kochia scoparia L.) in a greenhouse trial

Introduction Drought is undoubtedly one of the most important environmental stresses limiting the productivity of crop plants around the world. It has substantial influence on the growth condition, morphological structure and physiology and biochemistry of plants. Deficit irrigation strategy and planting crops with low water requirements and low expectations, for example, Kochia is very important under drought stress. Kochia is a salt- and drought-tolerant species that can be grown on drought soils, yielding fodder in quantities approaching that produced by alfalfa. Farmers in some arid areas of the world have already begun to cultivate Kochia as a salt-tolerant forage crop on lands where other crops are difficult to grow; accordingly, Kochia has been called ‘‘the poor man’s alfalfa’’. Drought stress decreases the rate of photosynthesis. Limiting factors in photosynthesis under environmental stresses is the two groups, stomatal limiting factors (Transpiration, stomatal conductance and co2 sub-stomatal) and non- stomatal limiting factors (chlorophyll content, chlorophyll fluorescence and etc.). Plants grown under drought condition have a lower stomatal conductance in order to conserve water. Consequently, CO2 fixation is reduced and photosynthetic rate decreases, resulting in less assimilate production for growth and yield of plants. Diffusive resistance of the stomata to CO2 entry probably is the main factor limiting photosynthesis under drought. Furthermore, drought stress lead to oxidative stress in the plant cell. Drought stress caused a large decline in the chlorophyll a content, the chlorophyll b content, and the total chlorophyll content in most plants. The decrease in chlorophyll under drought stress is mainly the result of damage to chloroplasts caused by active oxygen species. One of the most often employed parameters is maximum quantum yield of PSII (Fv/Fm), which gives the information about the proportion of the light absorbed by chlorophyll in PSII that is used in photochemical processes that it decreased under stress conditions. The aim of this greenhouse study was investigating some stomatal and non- stomatal limiting factors in photosynthesis rate and biomass of Kochia under drought stress conditions. Materials and Methods To comparison the photosynthetic parameters of kochia under drought stress, a pot experiment was carried out in completely randomized design with three replications in University of Ferdowsi in 2013. There were 9 treatments included control (no drought), light drought stress during the vegetative phase, heavy drought stress during the vegetative phase, light drought stress during the reproductive phase, heavy drought stress during the reproductive phase, light drought stress during the vegetative phase and heavy drought stress during the reproductive phase, heavy drought stress during the vegetative phase and light drought stress during the reproductive phase, light drought stress during total growth period and heavy drought stress during total growth period. Photosynthetic parameters including photosynthesis rate, transpiration, stomatal conductance, Co2 sub stomatal (by IRGA, Model LCA4), PWUE, SPAD (by SPAD, 502, Japan), chlorophyll content (with spectrophotometer model 6305 by Arnon's method), chlorophyll fluorescence components (chlorophyll fluorescence meter model OSL-FL) and biomass were measured. For statistical analysis, analysis of variance (ANOVA) and FLSD test were performed using SAS ver. 9.1 software. Results and Discussion Results showed that chlorophyll a, chlorophyll b and total chlorophyll content and SPAD under drought stress conditions were lower than the control treatment. Effect of drought stress was significant on chlorophyll fluorescence indices so that maximum quantum yield of photosystem II declined significantly due to increasing of initial fluorescence (Fo) and decreasing of maximum fluorescence (Fm). Transpiration, stomatal conductance and co2 sub-stomatal did not differ significantly from control. Photosynthetic rate also decreased in drought stress at different growth stages. The most and lowest of biomass observed in control and heavy drought stress during growth period, respectively. Conclusions Finally, this study showed that stomatal parameters (Transpiration, stomatal conductance and CO2 sub-stomatal) shown the same reaction under stress and non- stress conditions. Even under heavy drought stress, this plant is able to hold open stomata and maintain the continuity of CO2 absorption that it refers to the high resistance of Kochia. Therefore stomatal limiting factors didn’t have role in limit photosynthesis. Unlike stomatal limiting factors, non- stomatal factors had a significant positive correlation with photosynthesis rate and a decrease in each of them reduces photosynthesis rate.