Aims Sonneratia apetala, a native mangrove species in India, Bengal and Sri-Lanka, was introduced in 1985 to Dongzhaigang Mangrove Nature Reserve in Hainan Island, China from Sundarban, southwest of Bangladesh. Laguncularia racemosa, another mangrove species from Mexico was introduced to the same reserve in 1999. Our objective was to investigate the salinity tolerance mechanism of S. apetala and L. racemosa in order to elucidate adaptive strategies of the halophytes in stressful saline habitat.
Methods We investigated the effects of increasing soil NaCl (100–400 mmol·L–1) on gas exchange, O2–· production rate, activity of antioxidant enzymes, and the relevance to salt tolerance over four weeks in 1-year-old seedlings of S. apetala and L. racemosa.
Important findings Seedlings of the two mangrove species acclimated to different salinity levels through changing physiological and morphological traits. However, there were significant differences between S. apetala and L. racemosa in photosynthesis and anti-oxidant defense under salt stress. Increasing NaCl stress significantly elevated net photosynthetic rate (Pn), and stomatal conductance (Gs), intercellular CO2 concentration (Ci) and transpiration rate (Tr) decreased. As a result, water use efficiency (WUE) increased in leaves of S. apetala seedlings. But L. racemosa showed a rapid increase of Pn in the initiation of salt stress, and Pn remained lower than control levels at the end of the experiment. As a result, Ci and Gs increase with the decrease of Pn, Tr and WUE. The reduction occurred after exposure of L. racemosa seedlings to severe salinity, 400 mmol·L–1 NaCl (28 d). These results indicated that the inhibitory effects of severe salinity were more pronounced in L. racemosa under the same salinity. Moreover, the magnitude of variation of S. apetala maximum photochemical efficiency of PSII (Fv/Fm) was significantly less than that of L. racemosa leaves. We speculated that photosynthetic capacity of S. apetala was higher than L. racemosa. O2–· production rate markedly increased after the two seedlings were subjected to 100 mmol·L–1 NaCl and 200 mmol·L–1 NaCl for 7 days and 14 days, respectively. However, O2–· production rate in S. apetala leaves markedly increased upon increasing salinity and reached the highest level after seedlings were subjected to 400 mmol·L–1 NaCl for 28 days, which was 5.3 fold of that in controls. In contrast to S. apetala, O2–· production rate in L. racemosa leaves remained lower than control levels at the end of the experiment. Activity of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD) and glutathione reductase (GR) was elevated corresponding to the increased O2–· production in the salt-stressed two mangrove plants; however, the magnitude of increase of L. racemosa antioxidant enzyme activities was significantly greater than that of S. apetala, during the period of salt stress. We suggest that L. racemosa plants were able to sense salt stress and up-regulated the antioxidant enzymes to reduce salt-induced lipid peroxidation and membrane permeability, which contributed to maintenance of membrane integrity and salt tolerance in L. racemosa. Sonneratia apetala seedlings might adapt resistance to severe salinity through improving photosynthesis by higher WUE and maximum photochemical efficiency of PSII (Fv/Fm).