Frosthardiness development in Pinus sylvestris: The involvement of a K+-stimulated Mg2+-dependent ATPase from purified plasma membranes of pine

Purified plasma membranes were prepared from needles of Scots pine (Pinus sylvestris L., Provenance Sodra Ydre), and the behavior of a K+ stimulated Mg2+-dependent ATPase was studied. The microsomal fraction (1000-30 000 g) was used for partition in a dextranpolyethylene glycol two-phase system. Both upper and lower phases were washed twice with new upper and lower phase, respectively, yielding the fractions U3 and L3. After staining with STA (silicotungstic acid) and examining with an electron microscope, an enrichment of the plasma membranes in the U, phase was seen. This fraction was also enriched in K+ stimulated Mg2+-dependent ATPase. The ATPase was rather specific towards ATP compared to other nucleosides, and had a pH optimum between 6 and 7. The activity of this ATPase was followed during a frost hardening-dehardening cycle and showed an increased activity towards the end of the hardening period, whereas the activity decreased to normal during the dehardening period.