Salt allocation during leaf development and leaf fall in mangroves
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By taking samples along individual branches and measuring leaf size, thickness and Na+ and K+ concentrations, we have shown in Bruguiera cylindrica, Avicennia rumphiana and Avicennia marina that there are two phases of salt accumulation by leaves. This is confirmed by re-analysis of published data for other species. The first phase is a rapid increase in leaf content as it grows from bud to maturity; the second is a slower but continuous change in quantity in the leaf, via changes in ion concentration and/or in leaf thickening. Leaf thickening must not be overlooked in estimating changes in leaf contents with age. Generally, leaf Na+ content increases significantly, and K+ content falls slightly. Mangrove leaves thus continue as sinks for Na+ throughout their lifetime. At the end of a leaf's life, just before abscission, no burst of salt accumulation has been found. Yellow, senescent leaves do not have higher Na+ than old green ones. We point out that leaf drop involves losing both salt and biomass, not just salt, and hence does not reduce the salt concentration in the plant. We conclude that leaf drop is not a salt excretion mechanism, but is simply the point in time at which the leaf ceases to accumulate salt. Using a simple model, the contribution to salt accumulation of slowly accumulated Na+ by mature leaves has been calculated. For B. cylindrica, the most extensively studied species, 60% of the salt in the leaf is accumulated slowly in the mature phase, thus more than during the initial phase of rapid expansion growth. The limited data suggest that gland-bearing species show smaller changes in mature leaf Na+ content than do gland-less species.
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