Plant growth retardants (PGRs) affect growth and secondary metabolite biosynthesis in Stevia rebaudiana Bertoni under drought stress

Beyond the inhibitory action against the gibberellin biosynthesis, some plant growth retardants (PGRs) can play an important role in regulating plant responses to abiotic stress through the induction of different tolerance mechanisms. The aim of the present study was the exploitation of the potential of PGRs in enhancing the resistance to drought stress in Stevia rebaudiana Bert. Therefore, the effects of three PGRs on stevia plants grown under drought stress condition were investigated. Stevia plants were first subjected to water stress and, second, treated with PGRs to detect PGRs effect on biometric, productive and phytochemical characteristics of drought stressed-plants. The control plants were uniformly irrigated at 3-day intervals, while water-stress conditions were imposed by watering the plants at 12-day intervals. Subsequently, the Chlorocholine chloride (CCC, as Copalyl diphosphate synthase inhibitor and Kaurene synthase inhibitor), Paclobutrazol (PBZ, as Kaurene oxidase inhibitor) and Daminozide (DAM, as anti-gibberellins) were applied in drought stressed-plants. The CCC and DAM were sprayed on stevia shoots, while PBZ was drenched. The obtained results showed that leaf dry weight of stevia plantswas significantly reduced by drought stress, but this parameter increased as a consequence of CCC and PBZ treatments. Drought stress also caused a significant reduction in total steviol glycoside (SVglys) content. This reduction was more pronounced in drought stressed-plants treated with CCC, while PBZ was able to counteract the SVglys reduction,with SVgly content similar to that observed in the control. Similarly, PBZ was able to increase the soluble sugar production and total antioxidant capacity in the leaves of stressed-stevia plants. These findings suggested that CCC and, in particular, PBZ had a protective effect on stevia growth under drought stress by induction of antioxidant defenses and soluble sugar production. CCC seems to inhibit gibberellin biosynthesis, preventing the SVglys production, while DAM and PBZ, as gibberellin inhibitors, didn't have a negative effect on SVglys production in drought stressed-plants. This observation seems to emphasize their role in limiting the rate of target enzymes of CCC in SVglys biosynthetic pathway.Moreover, the induction of glucose production, as a substrate for SVglys biosynthesis, could be a convincing evidence for SVglys promotion in PBZ treated-plants.