Abstract: The characteristics of root plasma membrane ATPase (PM-ATPase) of "Weiyou 49", a K+ -deficit tolerant rice (Oryza sativa L. ) variety and of "Yuanyou 1", a K+ -deficit non-tolerant rice variety, had some similarities:Their optimum pH value were both about 6.0; Their activities reached the maximum at ATP concentration of 3 mmol/L; Km was 0.85 mmol/L and external K+ stimulated their activities. However, when [K+ ] was less than or equal to 50 mmol/L in the medium, the increasing of K + stimulated the activity of the PM-ATPase of "Weiyou 49" much more than that of "Yuanyou 1". When [K+ ] was between 100 to 200 mmol/L, the difference of the PM-AT- Pase activities decreased between the two rice varieties caused by K + stimulation. The basic H + extrusion of the two varieties had no apparent difference, but the H + extrusion stimulated by K + was different. The H+ extrusion of "Weiyou 49" was relatively more sensitive to external K+ . The experiment using inhibitors showed that there were close relationship between the PM-ATPase activi- ties stimulated by K+ and K+ uptake in the two varieties. The inhibition of PM-ATPase activity and H+ -extrusion stimulated by K+ reduced the K+ uptake of the root segments in both varieties. So the possible reason for "Weiyou 49" growing well in the low external K+ was that its PM-ATPase and H+ extrusion was more sensitive to external K+ , especially when [K+ ] was low.

Key words: Plasma membrane ATPase, H+ extrusion, K+ uptake, K+ -deficit tolerant rice variety