Physiology and ecologic relevance of salt secretion by the salt gland of Glaux maritima L.

Abstract

The effects of salinity in the root medium, time, and relative humidity on the salt secretion of Glaux maritima were investigated. Both in the greenhouse and in the field increasing salinity stimulated sodium and chloride secretion, whereas the essential elements potassium, calcium, and magnesium remained at low secretion levels, which might be interpreted as efficient mineral economy. The low secretion level of potassium is remarkable, because growing on a nutrient solution containing 6 mM potassium, the concentration of the plant sap increases to 150 mM K⁺ and the secreted quantity amounts to only 2 m mol l^-1 plant sap 24 h^-1. Attempts were made to establish the secretion rate. The maximum secretion rate calculated may be 80 pEq NaCl cm^-2 s^-1, but for long periods (days) the secretion rate will be lower. Measurement of salt secretion unavoidably leads to removal of secreted salt. Salt was removed by rinsing with distilled water, which artificially accelerates the secretion process or parts of it by diffusion of salt from the cuticle cavity or secretory cells. At increasing salinities the amount of secreted ions showed a fivefold increase, whereas the osmotic potential of the plant sap was raised only twofold, indicating the importance of secretion as a rapid regulation mechanism with regard to the salt economy.