FLUXES OF CA-2- ACROSS THE SURFACES OF DETACHED NEEDLES FROM SITKA SPRUCE TREES, PATHWAYS AND COMPARTMENTATION(, K+ AND CL)

Uptake and efflux of Cl-36-, Ca-45(2+), and K-42+ were measured in wat er-infiltrated detached needles from Sitka spruce (Picea sitchensis (B ong.) Carr.) trees incubated in 1 mol m-3 KCl or CaCl2 or 2 mol m-3 Na Cl solutions or in artificial rain water containing mmol m-3 amounts o f these ions. Surface efflux was measured separately from leakage from the cut ends of the needles. Needles loaded with Cl-36- and killed in liquid N2 before elution displayed a rapid and extensive loss of radi oisotope, indicating that mesophyll cell membranes were the limiting f actor for Cl-36- exchange. Data for live needles revealed a novel phas e of Cl-36- efflux, with an exchange halftime of about 20 min, which w as faster than that for either the vacuole or the cytoplasm, but much slower than that for the free space. The novel phase was interpreted a s representing diffusion of Cl- through the predominantly negatively c harged cuticle. Killing needles loaded with Ca-45(2+) or K-42+ also in creased efflux relative to that from live needles, but only to a limit ed degree, indicating that the main factor limiting cation efflux was the cuticle. During the first hours of Ca-45(2+) uptake, the isotherms displayed a shoulder, indicating that there was a significant Donnan free space phase in the cuticle for Ca2+. A shoulder was absent from K -42+ uptake isotherms because of the preferential adsorption of divale nt cations on the exchange sites.