The effect of foliar application of ascorbic acid and calcium on the morpho-physiological characteristics of guar under drought stress conditions

Guar (Cyamopsis tetragonoloba) is an annual plant from the legume family and tolerant to salinity and drought, which can be used as an alternative product in low water plains. Ascorbic acid is one of the antioxidants involved in the defense mechanisms of plants against various stresses, including drought, which plays a role in ROS detoxification. Calcium is also an essential element for plant growth, which, despite being immobile, has many electrochemical, structural and catalytic functions in plants. It has been reported that calcium ions can increase drought tolerance in plants by participating in the drought signal transduction process and stimulating abscisic acid synthesis. Therefore, this research was conducted with the aim of investigating the effect of foliar spraying of ascorbic acid and calcium on some morpho-physiological traits of guar under drought stress conditions. In order to investigate the morpho-physiological responses of guar to foliar application of ascorbic acid and calcium under drought stress conditions, a factorial experiment was conducted with two factors in the form of a randomized complete block design with three replications in the research greenhouse of Saravan Higher Education Complex in 2020. The experimental treatments included three levels of drought stress (70, 50 and 30% of field capacity) and three levels of foliar application (no foliar application, foliar application with calcium carbonate and ascorbic acid with a concentration of 3‰). The results showed that in 50 and 30% of the field capacity, compared to the treatment of 70% of the field capacity, plant height, number of pods per plant, number of seeds per pod, pod length, leaf dry weight and total protein showed a decreasing trend, but traits of reducing sugar, Polyphenol oxidase (PPO) enzyme activity and root length showed an increasing trend. The increase of reducing sugars is considered a mechanism to deal with drought in plants because reducing sugars are osmolytes that play a role in plant cells as regulators of the osmotic potential of the intracellular environment. Similarly, the increase in PPO activity during drought stress is justified by the increase in radical oxygen species generated during this stress, because the increase in the activity of antioxidant compounds as a defense mechanism is essential to maintain cellular processes and prevent the damaging effects of free radicals on plant physiological processes. Foliar application with ascorbic acid did not significantly change plant height and number of pods per plant compared to non-sprayed plants but increased leaf dry weight, total protein, reducing sugar and root length by 22%, 11%, and 21%, respectively. While, calcium foliar application significantly increased plant height, number of pods per plant and leaf dry weight by 16%, 24%, and 42%, respectively compared to non-foliar application and 8.5%, 12%, and 17.5% compared to foliar application with ascorbic acid, but total protein, reducing sugar, root length and dry weight of root remained statistically unchanged in calcium spraying respect to foliar application with ascorbic acid. However, both ascorbic acid and calcium application than no-foliar application showed a significant increase in the amount of total protein, reducing sugar, root length and dry weight of root and a decrease in PPO enzyme activity. The decrease in PPO activity can be considered as a result of reducing the harmful effects of drought stress caused by the application of calcium and ascorbic acid. In general, it seems that the availability of moisture in less than 70% of the field capacity has caused the formation of mechanisms to deal with drought stress in the guar, but foliar application with ascorbic acid and calcium can somewhat to be effective in modulating the effects of drought stress on this plant.