Comparative physiological and photosynthetic responses of three mangrove species to salinity stress and exogenous...

Extreme salinity is a major environmental constraint affecting plant growth and physiological performance in coastal ecosystems. Although mangroves are naturally adapted to saline environments, their tolerance and physiological responses vary among species, and the effects of exogenous melatonin (MT) under different salinity levels remain insufficiently understood. This study compared the physiological and photosynthetic responses of Aegiceras corniculatum, Kandelia obovata, and Rhizophora stylosa under NaCl-only and corresponding NaCl + MT treatments. Seedlings were subjected for 84 days to seven treatments: control (T1), 100 mM NaCl (T2), 200 mM NaCl (T3), 400 mM NaCl (T4), 100 mM NaCl + 100 μM MT (T5), 200 mM NaCl + 100 μM MT (T6), and 400 mM NaCl + 100 μM MT (T7). Each species × treatment combination comprised one container-reservoir treatment experimental unit containing five seedling subsamples. At 200 mM NaCl, predicted maximum leaf area (K) was 14.6% and 43.0% higher in A. corniculatum and K. obovata, respectively, under the corresponding NaCl + MT treatment than under NaCl alone. Severe salinity (T4) net photosynthetic rate (P_N) was 70.18% and 64.17% lower in A. corniculatum and K. obovata, respectively compared with the control. Whereas R. stylosa maintained comparatively higher photosynthetic activity across treatments. Furthermore, reductions were observed in other photosynthesis parameters, with A. corniculatum showing the greatest sensitivity, whereas R. stylosa maintained higher photosynthetic activity under saline conditions. Maximum PSII quantum efficiency (Fv/Fm) remained within a relatively narrow range across treatments, whereas photochemical and non-photochemical responses differed among species. Pigment responses were also species specific, at 200 mM NaCl, total chlorophyll in K. obovata was 58.0% higher under T6 than under the corresponding NaCl-only treatment (T3). These findings indicate that MT-associated responses varied with mangrove species and salinity level, with the clearest changes in specific growth, gas-exchange, photoprotective, and pigment traits under NaCl treatments. Because no MT-only treatment was included, the independent effect of MT under non-saline conditions could not be evaluated; moreover, the shared container-reservoir design limited independent treatment-level replication.