Physiological and biochemical responses of Stevia rebaudiana Bertoni to nutri-priming and foliar nutrition under water supply restrictions

Poor germination capacity of stevia is a major problem in its cultivation. Moreover, the water restrictions stress (WRS) are negatively affected germination, growth, yield, and secondary metabolites of stevia (Stevia rebaudiana Bertoni). In the present study, three separate experiments (growth-chamber, greenhouse, and field) were designed to assess the responses of stevia to nutri-priming and foliar nutrition with paclobutrazol (PBZ), nano potassium fertilizer (NKF), and salicylic acid (SA) treatments under water restriction stress (WRS). Water restriction stress included 0, − 3, − 6, and − 9 bar induced by PEG-8000 in germination and irrigation after 90 ± 2, 75 ± 2, 50 ± 2, and 25 ± 2% field water capacity (FC) in greenhouse and field conditions. Based on the results, WRS affected germination indices, physiological characteristics, growth and yield attribute, and Steviol glycoside (SVgly) biosynthesis negatively. It decreased germination percentage and rate, seedling vigor index, leaf number and yield, and photosynthetic pigment content, while increased the activity of catalase, peroxidase, and superoxide dismutase, and level of proline. Following the seed nutri-priming and foliar nutrition, morpho-physiological traits improved under stress and non-stress conditions. In the germination experiment, the integrated application of NKF, PBZ, and SA enhanced germination indices under the high PEG amount. Additionally, the foliar nutrition, especially PBZ and NKF, resulted in synthesising more photosynthetic pigments and proline, and rising antioxidant enzyme activity, and consequently increasing growth indices and WRS tolerance. The SVgly content and yield were maximised under partial WRS (75% FC) and high WRS stress reduced the secondary compounds of stevia significantly. Further, a significant enhancement was observed in the metabolite production after foliar treating with NKF and PBZ compared with the control. The results suggested the more sensitivity of stevia to WRS stress during germination. The integrated application of NKF, PBZ, and SA as a nutri-priming treatment can promote the poor germination performance of this plant and protect it from the negative effects of WRS. However, the use of NKF and PBZ changed physiological processes, leading to an improvement in stress tolerance, growth, yield, and secondary metabolite synthesis. Finally, the role of PBZ on yield attributes and metabolite production in this plant requires more extensive studies.