Stevia response to salt stress: agronomic, physiological and biochemical evaluation using a floating system technique

Today, there is a need to increase the European area under stevia cultivation, due to the rapid progress of stevia market size. It has been estimated that, in Europe, soil salinity affects 1 million hectares, mainly in the Mediterranean regions, where salinity in irrigation water and in soil represents one of the most limiting abiotic factors affecting crop productivity. Consequently, expanding knowledge on basic and applied aspects of stevia productivity under salt stress conditions is of pivotal importance. For this reason, an experimental trial in a floating system under controlled conditions was carried out to evaluate the effect of salt (NaCl) on the quality and quantity of the stevia production. From May to the end of June 2016, four NaCl concentrations (0, 12.5, 25 and 50 mM), each with three replications, have been compared, using a modified Hoagland nutrient solution. Leaf area, plant height, branch number, as well as leaf, stem, root and total dry biomass were measured at the beginning and at the end of the experiment. At the same time, a growth analysis was performed, and steviol glycosides (SVglys) content and profile were evaluated. The results obtained underlined that in stevia, salinity at 12.5 mM onwards negatively affected the main biometric and productive parameters, lowering plant height, root biomass and leaf and stem dry yields. Similarly, stressed plants showed decreased relative growth rate (RGR) and leaf area expansion. The RGR was reduced by 10.5 %, 19.0 % and 26.7 % in the 12.5, 25 and 50 mM treatments, respectively. Net assimilation ratio (NAR) remained stable until 25 mM NaCl treatment, and significantly decreased at 50 mM. At the same time, due to a limited leaf expansion, significant reductions of leaf area ratio (LAR) and specific leaf area (SLA) in stressed plants were observed, with the lowest values for 50 mM-treated plants. On the other hand, no significant differences in leaf weight ratio (LWR) were detected. Regarding the SVglys content and profile, significant differences were observed for stevioside (Stev), rebaudioside A (RebA), RebA/Stev ratio and Stev + RebA content. Stev and Stev + RebA content significantly decreased in 50 mM-treated plants, while both the RebA and RebA/Stev ratio significantly increased at 25 mM and then decreased in 50 mM-treated plants. These results confirm that in stevia, similar to several other plant species, the higher salinity level resulted in lower crop yield and growth, suggesting a moderate NaCl tolerance.