Effect of drought stress and nitrogen fertilizer on the growth and yield of stevia (Stevia rebaudiana Bertoni) in the second year of growth

Stevia is a perennial herb with sweet leaves, about 300 times sweeter than sucrose and today cultivated in many parts of the world. Drought stress and nitrogen deficiency are the most important environmental stresses that limit the production of this plant and have adverse effects on plant growth and development and other metabolic processes. The effect of drought stress on the plant depends on the type of plant, species, intensity, duration and growth stage of the plant. With the onset of drought decrease leaf water and stomatal conduction, and as it intensifies, loses permeability of the leaf cell membrane and damages. On the other hand, with the closure of the stomata due to lack of water is difficult and the entry of carbon dioxide into the leaves and in general, photosynthesis is reducing. Plant nutrition in drought conditions is also of special importance and proper nutrition in this situation can increase growth and stress tolerance. This element is the most important component of photosynthetic pigment proteins and has a great effect on leaf size and area.

Therefore, in order to investigate the effect of drought stress and nitrogen fertilizer on the growth and yield of stevia, an experiment was conducted in greenhouse of Campus of Agricultural and Natural Resources, Razi University, Kermanshah, during 2016. Factorial experiment was conducted in a Completely Randomized Design (CRD) with three replications. Treatments included drought stress at three levels (no stress, mild stress and severe stress) and nitrogen fertilizer at five levels (0, 50, 100, 150 and 200 kg.ha-1) from urea source. Irrigation time for non-stress, mid and severe stress were 50, 65 and 80%, respectively, respectively from drainage of usable moisture in the soil. The size of the Cultivation space was 50 × 50 × 50 cm (length, width and height) and they were made of cement. The irrigation system was drip irrigation. the first and second harvest were on the May 23 and September 6, 2016, respectively. Traits were examined including yield and yield components, photosynthetic pigments, relative water content (RWC), water use efficiency (WUE), soluble sugars, proline, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT). Data were analyzed using SAS and MSTAT-C software and the means were compared using the LSD test at a probability level of 5%.

The results showed that the drought stress and nitrogen had a significant effect on biomass, leaf dry matter, shoot dry matter, chlorophyll a and b, carotenoids, relative water content, water use efficiency, soluble sugars, proline and enzyme Superoxide dismutase, peroxidase and catalase. The highest amount of leaf dry matter in non-stressed treatments and applying 400 kg.ha-1 N in the first and second cutting were 20.4 and 17.1 g.pl-1, respectively, and the lowest amount of leaf dry matter in severe stress treatments and applying 400 kg N ha-1 the first and second cutting were 6.81 and 5.85 g.pl-1, respectively. With increasing amounts of nitrogen increased chlorophyll a, b, a + b and carotenoids in the first and second cutting. The amount of chlorophyll a, b, a + b and carotenoids was higher in the second cutting, but in carotenoids the values were equal in both cutting. Also, with increasing drought stress decreased the RWC in the first and second cutting and in the non-drought stress in the first and second cutting were 68.4% and 70.4%, and in the severe stress were 55.0% and 58.3%, respectively. Mean comparison of the interactions (stress × nitrogen) on WUE and proline showed that in conditions non-stress and using 200 kg ha-1 nitrogen had the highest WUE. WUE in the first and second cutting were 1.94 and 0.65 g.kg-1, respectively. In this experiment, the amount of superoxide dismutase in the first and second cutting increased with increasing drought stress, but on the contrary, the amount of peroxidase in the second cutting decreased with increasing drought stress. In addition, with the increase of N, the superoxide dismutase in the first and second cutting and also the amount of peroxidase increased in the second cutting.

In general, it can be concluded that stevia production in the second year has a good yield under non-drought stress. In addition, the use of nitrogen in conditions non-stress increased leaf yield. Leaf yield under mid stress with nitrogen application was not very significant. Under drought stress reduced leaf yield and was more severely with nitrogen utilization. Stevia is a summer plant with high water uses. It is recommended to pay attention to the amount of water available in summer and usually have a shorter irrigation cycle than other summer crops in the region, which is economically viable.