Physiological responses of the mangrove Rhizophora mangle grown in the absence and presence of NaCl

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Abstract
Propagules of the mangrove, Rhizophora mangle L., were precultivated for 9 months in a greenhouse. The young plants were transferred into unaerated nutrient solutions without and with 200 mol m 3 NaCl and subsequently their growth, their water relations and the photosynthetic properties of their leaves were studied. Growth of the salttreated plants was significantly increased, while the control plants gradually died off after finishing the experiments. The shoot water potential and the stomatal resistance of the leaves were lowered while the chlorophyll contents and the chlorophyll a/b ratio in the leaves of salt‐treated plants were increased by NaCl, the net result being an enhanced rate of CO2 assimilation. The leaves of both sets of plants showed diurnal fluctuations in malic acid concentration which were more pronounced in the leaves of salt treated plants which, additionally, were more succulent. However, the plants showed no net CO2 fixation at night, indicating that Rhizophora mangle is a CAM‐cycling plant. After 200 d of cultivation without or with NaCl, the Na+, Cl and K+ concentrations in tissues and vacuoles were measured. Energy‐dispersive X‐ray microprobe analyses on root vacuoles of control plants reveal Na+ preference, on those of salt treated plants a strong K+ preference. Vacuolar K+ concentrations are neither affected by NaCl nor do they vary across the root radius. High vacuolar Na+ and Cl concentrations are found in the hypodermis followed by a stepwise decrease towards the inner root cortex cells. Ion concentrations of the photosynthetically active leaf tissues seem to be regulated by (1) radial filtration across the root cortex: (2) ion exchange of the xlem parenchyma cells: and (3) sequestration of Na+ and Cl in the hypodermal water storage tissue of the leaves.