Electron probe X-ray microanalysis was used to analyse the effects of sub-zero temperatures on K+ distribution in compartments within non-acclimated and cold acclimated rye (Secale cereale L. cv Voima) leaf cells and to evaluate membrane leakage of ions caused by freezing-injury. The specimens were rapidly frozen from growing temperatures and from two different sub-zero temperatures (LT50 and LT100) to which the leaves had already been slowly cooled. Measurements were made in the cytoplasm, vacuole and cell walls in freeze-substituted mesophyll cells. At ambient temperatures, the mean K+ concentration in the cytoplasm (100 mol m-3) differed significantly from that of the vacuole (49 mol m-3) in the non-acclimated (NA) cells, while in cold acclimated (A) cells, the concentrations were similar (109 vs 93 mol m-3, respectively). At LT50 temperatures, the K+ concentration in NA-cells decreased significantly in the cytoplasm (59 mol m-3) but increased in the cell walls. In the A-cells, on the other hand, the mean K+ concentration increased significantly (about three-fold) in all major compartments. At LT100 temperatures, K+ concentrations in the cytoplasm and cell walls decreased when compared with corresponding LT50 values in the A-cells but increased in the NA-cells. The increased potassium concentration in the cytoplasm of A-cells at LT50 temperature is compatible with the observed cell shrinkage and an absence of plasma membrane damage. The decreased potassium concentration in the cytoplasm of NA-cells at LT50 temperature is compatible with the slight cell shrinkage and suggests that the plasma membrane in these cells shows increased permeability due to freeze injury.
