Assessment of Leaf-Washing Techniques for Measuring Salt Secretion in Avicennia Marina (Forsk.) Vierh.

Abstract

Leaves of A. marina are unsuitable for measurements of salt secretion because the lower leaf surface, where most secretion occurs, is densely tomentose. Washing of leaves with distilled water, although effective in removing secreted salt, was followed by a period of apparently increased salt secretion. Washing of leaves with strong osmotica was not followed by such a large increase in the rate of salt secretion as was washing with distilled water, suggesting that this acceleration was due to an osmotic flow of water into the salt-gland complex from the washing liquid. The apparent overestimation of secretion rates over short periods was probably not due to incomplete removal of pre-secreted salt by the initial wash nor, on the basis of a comparison of leaf washings with salt contents of the leaf, to the leaching of salt from the leaf interior into the solution used to wash the leaf. Subtraction of the amount of salt secreted in the first 2 h from the total amount secreted over periods of up to 96 h resulted in roughly constant calculated rates of secretion, so that in this species the steady rate of salt secretion, not accelerated by washing with distilled water may be calculated by using a duplicate set of leaves to measure the salt secreted in the first 2 h, and subtracting this from the total secreted over a longer period. Rates of Cl- secretion, so corrected, were about 0.2 µmol m-² s-¹. Unless this allowance is made, secretion rates based on washing with distilled water are overestimates, although the degree of overestimation is reduced as the length of secretion period is increased.