The Mineral Ion Relations of Mangroves I. Root Cell Compartments in a Salt Excluder and a Salt Secreter Species at Low Salinities

The efflux kinetics of ²²Na and ³⁶Cl from 48 h pre-loaded Rhizophoramangle and Avicennia germinans mangrove seedling roots grownin low salinity conditions have been documented. Estimates ofthe percent isotope distributed throughout the whole plant,the uptake rates, the amounts calculated to be present in theroot cell compartments, and the rate constants and half-timesof efflux were made. Little to no ²²Na or ³⁶Cl was noted asbeing distributed to leaf tissue in the red mangrove, Rhizophoramangle, substantiating its characterization as a non-secreting,salt excluding species. Significantly greater net uptake ratesfor Na and Cl were observed for the black mangrove, Avicenniagerminans, a non-excluding, salt-secreting species. The typicalintracellular compartmentation found in higher plant root tissueswas noted to be present in these mangroves and the efflux kineticsclearly indicated that Na and Cl were handled differentiallyby these compartments and their membranes. Rhizophora manglerevealed greater apparent contents of Na than Cl in both thevacuolar and cytoplasmic phases (5.4 and 7.7 fold greater, respectively).The inverse was found for Avicennia germinans, with the apparentvacuolar compartment amount for Cl being 4.6 times greater thanthe Na amount and the cytoplasmic Cl amount was 1.5 times morethan Na contents. The rate constants of efflux showed a patternof significantly greater Na than Cl efflux from each respectivered mangrove compartment, while the black mangroves had a patternof greater efflux rates of Cl than Na in each compartment. Thehalf-times of efflux for Na from the black mangrove cytoplasmiccompartment were 3.5 times longer, while vacuolar half-timesof efflux were near unity for Na and Cl. The results are discussedin terms of Scholander's model of salt secreting and salt excludingspecies. (Received March 26, 1984; Accepted June 26, 1984)