Effect of NaCl on photosynthesis,salt accumulation and ion compartmentation in two mangrove species, Kandelia candel and Bruguiera gymnorhiza

In a 4-week study, we investigated the effects of increasing soil NaCl (100–400 mM) on photosynthesis, salt uptake and transport, and intracellular compartmentation of Na⁺ and Cl⁻ in 1-year-old seedlings of Kandelia candel (L.) Druce and Bruguiera gymnorhiza (L.) Savigny. Increasing NaCl stress significantly elevated Na⁺ and Cl⁻ in root and shoot tissues (stem + leaf) of both species, but B. gymnorhiza showed a rapid Na⁺ accumulation upon the initiation of salt stress and leaves contained 90% more Na⁺ and 40% more Cl⁻ than K. candel at the end of experiment. Net photosynthetic rate (Pn) declined with increasing salinity, and the most marked reduction occurred after exposure of mangrove seedlings to a severe salinity, 400 mM NaCl. However, the inhibitory effects of severe stress varied with species: Pn decreased by 80% in K. candel whereas in B. gymnorhiza the decline was 60%. The quantum yield (AQY) and carboxylation efficiency (CE) response to severe salinity showed a trend similar to Pn, in which a lesser reduction of AQY and CE was observed in B. gymnorhiza (33–35%), as compared to K. candel (43–52%). X-ray microanalysis of leaf mesophyll cells showed evidence of distinct vacuolar compartmentation of Na⁺ in K. candel but Cl⁻ in B. gymnorhiza after seedlings were subjected to 100 mM NaCl for 7 d. Moreover, Na⁺ within cell wall, cytoplasm, vacuole and chloroplast remained 23–72% lower in stressed B. gymnorhiza as compared to K. candel. In conclusion, B. gymnorhiza exhibited effective salt exclusion from chloroplasts although increasing salt stress caused a rapid and higher build up of Na⁺ and Cl⁻ in the leaves. We suggest that the salt-induced Pn reduction in the two mangrove species is correlated with the ability to exclude Na⁺ and Cl⁻ from the chloroplast, rather than with the bulk leaf salt concentration.