Salinity and light effects on the salt balances of the mangroves Rhizophora mangle and Avicennia germinans.
SHAFTEL, R.*, L.HOFFMAN and N.ANTEN
Stanford University 1
We studied how salinity and light availability influence the salt balance of mangrove seedlings. We used Avicennia germinans and Rhizophora mangle seedlings grown at two salinity levels (7 and 55 ppt NaCl) and two light levels (5 and 25% photosynthetically active radiation, PAR). We measured substrate, xylem sap, and leaf Na+ and Cl- concentrations as well as whole-plant transpiration rates. In addition, we measured salt secretion rates for A. germinans. Transpiration per unit leaf area and leaf ion concentrations increased with increasing light in R. mangle. In A. germinans increased light availability did not affect transpiration, leaf ion concentration or secretion rates. Leaf salinity changed very little with salinity treatment compared to large differences in substrate salinity for both species. Mechanisms that have been proposed to control salinity levels are exclusion, secretion and reduced salt uptake through reduced transpiration. In our data we observed a dramatic reduction in transpiration per unit leaf area with increased salinity and estimated that this reduction may reduce salt uptake by as much as 50%. The reduction in transpiration per unit leaf area was greater at low than at high light. Reducing leaf transpiration is costly in terms of carbon gain. We argue however, that this cost is not as great at low than at high light. At low light, carbon gain is limited by light capture rather than stomatal conductance. This may explain why in a previous study we found a stronger reduction in growth with salinity at high than at low light.
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