Physiological and Biochemical Responses of Acacia saligna to Salinity Stress and Its Tolerance Threshold

Salinity is one of the most impactful abiotic stresses limiting the growth and productivity of many plant species. This study aimed to evaluate the salt tolerance of Acacia saligna by exposing juvenile Acacia plants to increasing NaCl concentrations (control 0.35 g/L, 10, 20, 30, and 40 g/L) for 10 days following 9 months of greenhouse cultivation. We assessed the physiological and biochemical parameters using six replicates per treatment. Data were analysed using one-way ANOVA (p < 0.05) with Minitab 17 software, and group differences were determined via Tukey’s test. Normality of residuals was confirmed using the Shapiro–Wilk test.

Salt stress negatively affected growth, with the lowest number of leaves (7.8 leaves/plant) and the slowest growth rate (0.01 cm/day) observed at 40 g/L. Dry matter content increased with salinity, peaking at 30.38% under 40 g/L NaCl, while water content declined to 69.62%. Proline levels rose with salinity, reaching 0.173 µg/mg FW at 40 g/L, though a notable dip was observed at 20 g/L. Soluble sugar content peaked at 10 and 20 g/L (11.44 and 7.70 µg/mg FW, respectively) and then declined sharply at higher concentrations. Chlorophyll content increased progressively with salinity, reaching 12.28 mg/g FW at the highest stress level.

Our findings suggest that A. saligna exhibits flexible osmotic adjustment mechanisms favouring sugar accumulation under moderate salinity and proline under higher stress. However, optimal tolerance appears to be maintained up to 20 g/L NaCl, beyond which growth performance and sugar levels decline despite continued biochemical adaptation. Due to its resilience, A. saligna shows promise for use in reforestation and land rehabilitation on saline or degraded soils.