Study the effect of depth and duration of flooding stress and nitrogen spraying on growth characteristics of Stevia (Stevia rebaudiana B.) in Rasht conditions

Introduction Stevia rebaudiana Bertoni in an ancient perennial shrub of South America, produces diterpene glycosides that are low-calorie sweetener sand about 300 times sweeter than saccharose (Savita et al., 2004). Stevia extracts, besides having therapeutic properties, contain a high level of sweetening compounds, known as steviole glycosides, which are thought to possess antioxidant, antimicrobial and antifungal activity. Stevia cultivation and production, especially in Iran, would further help those who have to restrict carbohydrate intake in their diet; to enjoy the sweet taste with minimal calories. In Iran, the best area that has the most similarity of favorable climate condition for Stevia cultivation is the north of Iran. However, poor soil aeration associated with excessive moisture usually influences plant establishment and growth negatively in this part of Iran. Heavy textured soils in northern regions of Iran are more susceptible to waterlogging after heavy rain, and hypoxia is likely to limit Stevia growth. Thus waterlogging is considered as one of the important limiting factors for Stevia cultivation in such regions. On the other hand, Oxidation-reduction systems in the soil, which are generally stable as long as the soil is bathed in oxygen, become unstable when the oxygen supply is restricted. Nitrate is stable in well-aerated soils but become unstable when the oxygen supply of the soil is cut off. Nitrogen is subjected to loss through denitrification because of the ability of facultative anaerobes to substitute nitrate for oxygen. Advantage can be taken from the reducing conditions in waterlogged soils to increase the utilization of nitrogen fertilizer. The objectives of the present experiment were to investigate the effects of waterlogging stress and the alleviation of the waterlogged damage by the application of nitrogen fertilizer in Stevia. Material and methods A greenhouse pot experiment was conducted in University of Guilan, in 2013. The experimental design was factorial using completely randomized design layout with three replications. Treatments included four waterlogging levels; (0, -5, -10 cm from soil surface and normal irrigation as control), in periods of 2 and 4 days of waterlogging and two levels of nitrogen (6‰ and no nitrogen spraying as control) by foliar spraying from source of urea. Because the economic organ of Stevia is its leaves samples were taken before flowering stage. Results and discussion Results showed that measured shoot traits including leaf percentage, leaf area density, specific leaf weight, leaf dry weight, leaf area, plant height, number of lateral branch, number of leaves were significantly different among treatments. Nitrogen application improved plant height (14%) (Bakhshandeh et al., 2016) but deceased leaf area density (14%) compared with control. Interaction of waterlogging duration and nitrogen fertilizer significantly influenced leaf percentage, specific leaf weight and leaf dry weight. During water logging period increase from two to four days, nitrogen application improved leaf percentage by 4.5%, specific leaf weight by 11%, and leaf dry weight by 14%. In addition, interaction between water logging level and nitrogen application significantly influenced specific leaf weight, number of lateral branch and leaf dry weight. For specific leaf weight in -5 cm water logging level, nitrogen fertilizer application resulted in 12% increase compared with the same water logging level but without nitrogen fertilizer. Nitrogen application under -10 cm water logging level improved the number of lateral branches by 9% compared with complete water logging level without nitrogen application. Also for leaf dry weight, nitrogen application under -10 cm water logging effect resulted in 11% increase compared with no nitrogen application. Generally, the results of this experiment show that nitrogen application under water logging stress improved growth features of Stevia although water logging stress reduce growth characteristics of Stevia. Nitrogen application under the highest waterlogging level improved leaf dry weight by 10% compared with no nitrogen application treatment. Because economic part of Stevia is its leaves, it is likely that nitrogen application under water logging stress improve this trait (Rashid et al., 2013). Conclusion Results of this experiment showed that although level and duration of flooding stress caused a reduction in stevia leaf production but nitrogen application can ameliorate the negative effects of the stress on this trait. Since leaf dry weight is the economical yield of stevia, N application will improve yield if this plant under flooding stress