Effect of Vanadate on Microsomal ATPase Activity, Acidification of the Medium and Auxin-stimulated Growth in Pea and Cucumber

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Abstract
The relationship between ATPase activity, medium acidification and auxin-stimulated growth in segments of pea stem (Pisum sativum L., cv. Alaska) and cucumber hypocotyl (Cucumis sativus L., cv. Long Green Ridge) was investigated using sodium orthovanadate, widely used as a selective inhibitor of plasma membrane-associated ATPase activity. ATPase activity of cucumber microsomal preparations was about seven times lower than similar preparations from pea (on a mg microsomal protein basis) and was much more effectively inhibited by vanadate. Similarly, acidification of the medium by abraded cucumber segments occurred to a lesser extent than with pea and showed a greater inhibition by vanadate. Both growth in controls and auxin-stimulated growth of cucumber segments were strongly inhibited by vanadate, whereas in pea auxin-stimulated growth was reduced by only half and controls showed little inhibition. Acidification of the medium by segments of both species was found to occur readily even in controls and showed little promotion in the presence of IAA, although growth in both species was rapidly and significantly promoted by IAA. These results indicate that acidification is brought about by a plasma membrane-associated ATPase, and suggest that while acidification is an essential factor for auxin-stimulated growth it may not be the mechanism by which the growth rate is controlled.