Leaf secretion composition of the mangrove species Avicennia germinans (L.) in relation to salinity: a case study by using total-reflection X-ray fluorescence analysis.

The aim here was to use total-reflection X-ray fluorescence (TXRF) to determine the elemental composition (besides Na), and the relative contribution of each element in leaf secretion of the mangrove species Avicennia germinans, grown under contrasting salinities (0-865 mol m(-3) NaCl). Na was determined by conventional atomic absorption spectrometry (AAS). Total secretion was 2.00+/-0.28 mmol m(-2) per day in control plants (at 0 NaCl) and increased progressively up to 46.87+/-7.14 mmol m(-2) per day at 865 mol m(-3) NaCl. Na and Cl accounted for 85.91+/-1.81% of the total secretion in control plants and about 96.32+/-0.30% in salt-treated plants. The excretion of Na exceeded that of Cl when salinity increased and this led to a progressive increase in Na/Cl ratios from 0.46+/-0.02 in control plants up to 2.75+/-0.42 (at 865 mol m(-3) NaCl). Other elements were also secreted in sizeable amounts such as K, S, Ca, Br and Zn. However, the relative importance of these elements, in terms of total secretion, was considerable in control plants (15% of the total secretion) and declined significantly under salinity treatments (<5% of the total secretion). In conclusion, TXRF has been shown to be a powerful tool allowing quantitative determination of Cl (1.2-13 mmol m(-2) per day), secreted in relatively large quantities, as well as other elements secreted in intermediate (S, Ca and K; 0.07-1.00 mmol m(-2) per day) and in trace quantities (Br and Zn; 0.6-4 µmol m(-2) per day).