Effect of silicon in improving drought tolerance of stevia (Stevia rebaudiana Bertoni) under moisture stress

Abiotic stresses adversely affect agricultural productivity worldwide. In the arid and semi-arid areas, water is one of the most basic ecological units, which determines the growth and development of plants. Therefore, efficient use of water resources should be considered in order to achieve desirable plant growth. Aside different effective factors, minerals such as silicon are the most essential affecting the growth and development of plants. Silicon has beneficial effects on many crops, especially under biotic and abiotic stresses. Silicon can affect biochemical, physiological, and photosynthetic processes, consequently ameliorates drought stress. However, the effects of silicon on stevia (Stevia rebaudiana L.) plants under drought stress has not well known. Stevia is a branched bushy shrub of the Asteraceae family and originated from South America, and its cultivation has spread worldwide. Stevia is well known for its high content of sweet components. The purpose of this research was to determine the effects of different silicon concentrations on physiological and morphological characteristics of stevia under drought stress conditions. The treatments were four levels of drought stress including: 25%, 50%, 75%, 100% pot capacity and three levels of silicon concentrations consisted of 0, 1, 1.5 mM, which arranged in a factorial experiment based on completely randomized design with three replications. After the plants establishment, drought treatments were imposed. After 40 days of applying drought treatments, silicon was twice sprayed with 7 dyas interval. The leaves of plants were sampled one week after the last spraying and transferred to the laboratory and the amount of soluble sugars, proline and chlorophyll concentrations were measured. In order to analysis data, analysis two-way ANOVA were used and Duncan test compared means after normality test. Statistical analysis was performed using SPSS software, and Excel software was used for drawing the charts. Applying experimental treatments had only significant effect (p<0.01) on plant height, shoot fresh weight, canopy diameter, and canopy volume, while had no significant effect on shoot dry weight and number of leaves. The effect of silicon foliar application was significant on plant height, canopy diameter and canopy volume at 1% probability level and on shoot fresh weight and shoot dry weight at 5% probability level. The interaction between stress and silicon only was significant (p<0.01) on shoot dry weight, diameter and volume canopy, but had no significant effect on other traits. The results also showed that in no spraying, 1 and 1.5 mM silicon treatments under 25% FC significantly increased soluble sugars compared to 100% PC, so that the maximum amount of soluble sugars found in 25% FC and 1.5 mM silicon. Since the sugars are made from compatible osmolytes and its accumulation lead to osmotic adjustment, keep the cellular turgor and stability of protein. Thus increasing in soluble sugars such as sucrose, glucose and fructose as a result of environmental stresses, particularly drought are considered as a strategies of plants to tolerate adverse environmental conditions. Increasing drought stress to 25% PC significantly increased proline of stevia by 74% compared to the three levels of 100%, 75% and 50% PC. The results of several studies showed the positive effect of drought stress on proline, so that proline content is greater in drought stress than normal conditions. Increasing proline content in stress conditions might be due to increasing in activity of enzymes are responsible for the synthesis of this material, reducing the proline oxidation to glutamate and/or decreasing in consumption of proline in the process of making proteins. The results of evolution of different treatments of drought stress on chlorophyll content of stevia plant showed that the effect of drought stress was significant (p<0.01) on chlorophyll b, so that the most of chlorophyll b was observed in 75% PC treatment, which had no significant difference to 50% and 25% PC treatments, and the lowest was observed in 100% PC. Increasing inconsiderably chlorophyll b in drought stress conditions showed that chlorophyll pigments are somewhat tolerance to water loss. The results of this experiment showed that height, shoot fresh weight of, diameter and volume of canopy, root volume, dry and root fresh weight of, and chlorophyll b were significantly decreased with increasing in drought stress level, however, using of both different silicon concentrations, especially 1 mM modulated damages of drought stress on stevia. Generally, the results of this research revealed the positive role of silicon in decreasing the negative effects of drought stress in stevia.