Soil and water salinity are among the most critical global environmental constraints, threatening agricultural productivity and aquatic ecosystem stability. Azolla pinnata, a free-floating aquatic fern widely used as a biofertilizer in rice cultivation, plays an important role in sustainable agriculture but remains highly sensitive to saline environments. The present study investigated the physiological and photosynthetic responses of A. pinnata to different NaCl concentrations (0.0, 0.5, 1.0, 1.5, and 2.0 g/L) or (0, 9, 17, 26, 34 mM) a 9-day exposure period. Healthy fronds were cultured under controlled conditions, and responses were assessed through antioxidant enzyme activity, lipid peroxidation, and chlorophyll a fluorescence (OJIP) transient. The results demonstrated a time- and dose-dependent effect of salinity. Superoxide dismutase (SOD) activity increased under moderate stress but declined sharply under prolonged high salinity, coinciding with elevated malondialdehyde (MDA) levels, indicating oxidative damage and loss of membrane integrity. Chlorophyll fluorescence analysis revealed progressive impairment of photosystem II (PSII), with significant reductions in Fv/F0, F0/Fm, and electron transport rate (ET0/RC) at 1.5–2.0 g/L. Biophysical parameters such as RC/CSm (density of active PSII reaction centers per excited cross-section) and ET0/CSm (electron transport rate beyond QA– per cross-section) also declined, highlighting widespread PSII inactivation. The performance indices, PIabs (overall photosynthetic performance based on energy fluxes per absorbed photon) and PIcs (performance per excited cross-section), showed transient improvement under moderate stress but collapsed at 2.0 g/L, marking the threshold of adaptive capacity. Principal component analysis (PCA) and heatmaps clearly differentiated treatments, with high salinity strongly associated with stress markers. Overall, the findings reveal that A. pinnata can tolerate NaCl concentrations up to approximately 1.0 g/L (≈17 mM) through short-term protective adjustments, but suffers severe oxidative and photochemical damage at higher levels (≥1.5 g/L or ≈26 mM). These insights underscore its limitations as a biofertilizer in salt-affected agroecosystems and highlight chlorophyll a fluorescence as an effective tool for early stress detection in aquatic plants.
Bhatt et al. (Sun,) studied this question.