Photosynthetic gas exchange and accumulation of phytotoxins in mangrove seedlings in response to soil physico-chemical characteristics associated with waterlogging

We evaluated photosynthetic gas exchange and the accumulation of iron, manganese and sulfur in seedlings of five mangrove species (Aegiceras corniculatum (L.) Blanco, Avicennia marina (Forsk.) Vierh., Bruguiera gymnorrhiza (L.) Lamk., Hibiscus tiliaceus L. and Rhizophora stylosa Griff.) growing under anoxic soil conditions at low irradiance. Seedlings of the viviparous mangroves showed no significant responses to root anoxia. The presence of ferrous sulfate or manganous sulfate had a smaller effect on CO₂ assimilation, transpiration rate and stomatal behavior than the presence of sodium sulfide. Sodium sulfide inhibited photosynthetic gas exchange and caused complete stomatal closure in all species. Stomatal closure was probably the result of the damaging effect of sulfide ions on root cell membranes. Some leaf epinasty and wilting were also observed in response to the Na₂S treatment in all species. A combination of FeSO₄, MnSO₄ and Na₂S had a smaller effect on photosynthesis than Na₂S alone, especially for Avicennia marina and Rhizophora stylosa seedlings, which maintained appreciable rates of CO₂ assimilation (2.49 and 3.84 μmol m^-2 s^-1, respectively) in the presence of all three phytotoxins. Roots of phytotoxin-treated seedlings of all species accumulated significant amounts of the corresponding toxin compared with roots of the control plants. The FeSO₄ or MnSO₄ treatments had no effect on foliar concentrations of iron or manganese, whereas the Na₂S treatment resulted in an accumulation of S in the leaves of all species. Interactions between Fe²⁺ and Mn²⁺ and sulfide in the culture medium appeared to reduce their uptake by the seedlings. We conclude that high concentrations of sulfides in mangrove sediments may adversely affect both growth and survival of mangrove seedlings at low irradiances.