Comparison of Reductions in Adenosine Triphosphate Content, Plasma Membrane-associated Adenosine Triphosphatase Activity, and Potassium Absorption in Oat Roots by Diethylstilbestrol

The possibility was investigated that diethylstilbestrol (DES) inhibits potassium absorption in oat (Avena sativa L. cv. Goodfield) roots by inhibiting mitochondrial functions in addition to inhibiting the plasma membrane ATPase. DES at 10(-6) molar stimulated the mitochondrial ATPase slightly, but higher concentrations had no effect. Oxidative phosphorylation by isolated mitochondria was inhibited 50% by 2.6 x 10(-5) molar DES; concentrations of 10(-4) molar or greater were completely inhibitory. After a lag of about 2 minutes, 10(-4) molar DES produced a linear decrease in ATP content of excised roots. After 20 minutes, the ATP content of the tissue was about 50% of the control and remained at that level after 30 minutes in DES.Comparison of changes in ATP content, plasma membrane ATPase activity, and K(+) absorption rate with time in the presence of DES showed that the rapid decrease in K(+) absorption rate corresponded more closely with the decrease in ATPase activity than the decrease in ATP content. Total inhibition of the ATPase was calculated by multiplying together the percentage decreases in ATPase activity and ATP content. At times greater than 10 minutes this "net" ATPase activity corresponded very closely with the K(+) absorption rate.These results show that DES can inhibit potassium absorption by reducing mitochondrial ATP production in addition to inhibiting the plasma membrane ATPase. However, the rapid (less than 5 minutes) inhibition of absorption is caused by direct inhibition of the ATPase rather than a reduced ATP supply because the ATP content is lowered only slightly whereas the ATPase is inhibited dramatically in that time. The relationship between plasma membrane ATPase activity and K(+) absorption rate as inhibited by DES supports the hypothesis that the ATPase is involved in cation absorption by plant roots.