SummaryThe mangrove Rhizophora mucronata grows in an intertidal region and exchfdes salt from its xylem (17 m·equiv. chloride per litre of sap) more efficiently than does the salt· secreting mangrove AegialitiB annulata (85-122 m·equiv. chloride per litre of sap). From the transpiration stream each leaf of Rhizophora receives about 17 p.·equiv. chloride each day, but the chloride concentration of the growing leaf remains approximately constant (510-560 m·equiv. chloride per litre of sap water). In Aegialiti8 input of chloride to a mature leaf is about 100 p..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 p.-equiv/ml, sodium, 355 p.-equiv/ ml, and potassium, 27 p.-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-I. 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-chlorophenyThydrazone applied ·to the cut petiole or to the leaf surface. Out 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 AegialitiB are described.

No abstract

The cytoplasm of an Acetabularia cell is normally at a potential of about -170 my relative to the external solution; the vacuole is also at this potential. Although there is strict flux equilibrium for all ions, the potential is more negative than the Nernst potentials of any of the permeating ions. Darkness, CCCP, low temperature, and reducing [Cl-]o by a factor of 25 all rapidly depolarize the membrane and inhibit C1-influx. Some of these treatments do not inhibit the effluxes of K+ and Na+. Increasing [K+], also depolarizes the membrane both under normal conditions and at low temperature; in the latter case the membrane is partially depolarized in normal seawater (low [K+]o) and in high [K+]o positive potentials of up to + 15 my are attained. It is concluded that the membrane potential is controlled by the electrogenic influx of Cl-, and also, at least in some circumstances, by the diffusion of K+. In addition, it is suggested that electrogenic efflux of H+ may be important in transient nonequilibrium situations. An Appendix deals with the interpretation of simple nonsteady-state tracer kinetic data.

No abstract