Adaptation of the mangrove Laguncularia racemosa to high NaCl salinity

The aim of this study was to investigate the physiological mechanisms and morphological as well as anatomical changes of Laguncularia racemosa to survive high NaCl salinity (500 mol * m(-3) NaCl). Combined structural and physiological investigations were made at tissue and at single cell level to identify salt induced structural and physiological characteristics and their ecological advantages. To survey the mechanisms enabling Laguncularia to survive the extreme conditions we studied the influence of high salinity on several leaf parameters (water potential, Chlorophyll a and b, succulence, number of stomata and glands per surface area) and the solute composition (osmotic potential, the ions K+, Na+, Ca2+, Mg2+, Cl-, and soluble proteins). As a first reaction of salinity to adapt water relations, the leaf water potential decreased below the soil conditions mainly by uptake of osmotically active substances. Our results point to an K and Ca-buffer in the vacuoles of the leaf tissues and in form of Ca-oxalate crystals enabling the plant to alleviate the inhibitory effects of NaCl on growth, maintaining plasma-membrane selectivity and other essential functions of these elements. It was shown that an additional mechanism of Laguncularia to avoid excessive element concentrations inside the plant tissues is the excretion of the surplus via salt glands. Even the extrafloral nectaries were able to excrete Na, Cl and K under these conditions. The mangrove took advantage of the extreme habitat by using the antibactericidal impact of NaCl in the cavity above the headregion. The salinity-induced multiplicity of structural and functional changes constitutes a group of indicators for the salt tolerance of Laguncularia. The results are discussed in relation to the ecological sustainment and the economic feasibility of Laguncularia ia utilization.