Effects of Salinity Stress on the Growth, Biomass Distribution, Chlorophyll Content, Mineral Uptake, Biochemical Constituents and Productivity of Roselle (Hibiscus sabdariffa L.)

Globally, soil salinisation is considered a major abiotic factor limiting crop growth and productivity, substantially reducing agricultural yield. In this context, the effects of salinity stress on the growth, biomass distribution, chlorophyll content, mineral uptake, biochemical constituents and productivity of roselle (Hibiscus sabdariffa L.) were evaluated. Using a completely randomised design, roselle seeds were raised in a nursery, transplanted into sand-filled polythene bags and supplied with nutrient solution for six weeks in a greenhouse. Plants were exposed to four concentrations of NaCl (0, 50, 100 and 200 mM). Agronomic parameters (flowering time, number of calyces per plant, yield and harvest index) were evaluated in the field over a five-month growth period at 0 and 50 mM NaCl concentrations. The greenhouse results showed that stem height, leaf area, number of leaves, dry biomass and chlorophyll content decreased significantly (P < 0.05) from 50 mM NaCl. Primary metabolites (proline, soluble carbohydrate, total free amino acids, soluble proteins) increased significantly (P < 0.01) with increasing salinity from 50 mM NaCl, while the other metabolites under study (flavonoids, total phenolic content, peroxidase, superoxide dismutase, tannins and saponins) increased significantly (P < 0.05) with increasing salinity from 50 mM NaCl. The distribution of K⁺, Ca²⁺, K⁺/Na⁺, P, N, Zn²⁺ and Mn²⁺ declined significantly (P < 0.05) from 50 mM NaCl onwards in both roselle varieties, whereas Na⁺ accumulation increased. The white variety showed better adaptation than the red variety in terms of morphological and physiological parameters associated with salt stress tolerance. Higher metabolite accumulation was indicative of tolerance to moderate salinity stress. However, the red variety produced more blooms and calyces than the white variety. These findings indicate that roselle productivity may be increased and its sustainable cultivation in saline-affected soils might be supported by variety selection based on physiological and biochemical tolerance traits.