Salt Regulation in the Mangroves Rhizophora Mucronata Lam. And Aegialitis Annulata Rbr

Abstract

The mangrove Rhizophora mucronata grows in an intertidal region and excludes salt from its xylem (17meq. chloride per liter of sap) more efficiently than does the salt-secreting mangrove Aegialitis annulata (85-122 meq. chloride per liter of sap). From the transpiration stream each leaf of Rhizophora receives about 17 [mu]-equiv. chloride each day, but the chloride concentration of the growing leaf remains approximately constant (510-560 meq. chloride per liter of sap water). In Aegialitis input of chloride to a mature leaf is about 100 [mu]-equiv. per day and this input is balanced by secretion (mainly of sodium chloride) from the salt glands. Secretion collected under oil contains chloride, 450 [mu]-equiv/ml, sodium, 355 [mu]-equiv/ml, and potassium, 27 [mu]-equiv/ml. Secretion rates from leaves on the tree, based on leaf area, vary from 93 p-equiv. cm-2 sec-1 during the day to 3 p-equiv. cm-2 sec-1 in darkness; the secretion in light, based on an effective gland area, is about 25,000 p-equiv. cm"2 sec-1. The water potential of the secretion is close to that in the leaf suggesting that secretion involves active transport of salt and passive movement of water by local osmosis. Salt secretion is inhibited by carbonyl cyanide 3-chlorophenylhydrazone applied to the cut petiole or to the leaf surface. Cut leaves secrete salt in darkness at approximately the same rate as in light, in contrast to leaves on the tree. With infused radioactive chloride, the specific activity of chloride in the secretion reached a higher value than the mean value in the leaf, suggesting that some chloride passes freely from the leaf veins to the salt glands without equilibrating with the main chloride pool of the leaf. Light- and electron-microscope studies of the glands of Aegialitis are described.