EFFECTS OF PH AND MINERAL-NUTRITION SUPPLY ON K+(RB-86(+)) INFLUX AND PLASMA-MEMBRANE ATPASE ACTIVITY IN ROOTS OF SUGAR-BEETS

Sugar beet seedlings (Beta vulgaris L. cv. Primahill) were cultivated at varying pH and nutrient levels addition rates. Hydrolyzing ATPase activity of root plasma membranes and K+(Rb-86(+)) flux into roots Of intact plants were determined after the different treatments. Relatively high pH (6.5), high concentration of nutrients and a high sodium: potassium (Na:K) ratio in the root medium increased the specific Na+- and (Na+ + K+)-activation of the ATPase. In contrast, a low relative addition rate of nutrients and low pH (5.3) enhanced the specific K+-activation. The 2,4-dinitrophenol (DNP)-sensitive (metabolic) influx of K+(Rb-86(+)) increased > 3 times when the seedlings were raised at pH 5.0, as compared with pH 6.0, and was higher after a relatively low addition rate of nutrients than after a higher rate, or high concentration of nutrients during the cultivation. Thus the specific K+-activation of the ATPase and the K+(Rb-86(+)) influx were increasing under the same conditions. If K+ influx depends on the activity of the H+-ATPase, these results suggest that the activity of the H+-pump increases at low pH and limited supply of nutrients. At high pH and high Na+:K+ ratio in the nutrient solution, the increased activation of the ATPase by Na+, and less activation by K+, indicates a larger capacity to extrude Na+ than to extrude protons and take up K+.