Effect of NH⁺₄ and NO^-₃supply on plasma membrane H⁺-ATPase activity of sorghum roots

【Objectives】Ammonium and nitrate nutrition strongly affect the rhizosphere pH. The response of upland crop to the different pH in root with ammonium and nitrate supplies was studied. 【Methods】 Using a hydroponic experiment, sorghum(Sorghum bicolor)was supplied with NH+4-N or NO-3-N as sole nitrogen nutrition. After 3 weeks' cultivation, the plasma membrane vesicles of sorghum roots were isolated using a two-phase system with poly-glucose. The rhizosphere pH and the plasma membrane H+-ATPase hydrolytic activity were measured, and the enzyme concentration was tested by immune hybridizes.【Results】 The activity of H+-ATPase in the plasma membrane of sorghum roots supplied with NH+4-N(pH3)is 8.81 μmol Pi/(mg·min)and is markedly higher than that supplied with NO-3-N(pH 7)(3.82 μmol Pi/(mg·min). If the pH was adjusted to 7 in the medium with ammonium nutrition, the activity of H+-ATPase is decreased. In contrast, if the pH was adjusted to 3 in the medium with nitrate nutrition, the activity of H+-ATPase is increased, and the activity of H+-ATPase in sorghum roots treated by ammonium nutrition related to the protein concentration. These results indicate that the higher activity of plasma membrane H+-ATPase in sorghum roots is affected by the rhizosphere pH. However, ammonium has stronger influence on the activity of plasma membrane H+-ATPase, even at the same medium pH.【Conclusion】Plasma membrane H+-ATPase could adapt to the rhizosphere acidification by enhancing its activity, either under ammonium or nitrate application. In contrast to the H+ consumption after nitrate reduction, the H+ production after ammonium assimilation should be pumped outside the root cell with further enhancement of the plasma membrane H+-ATPase activity.This might be the mechanism of adaptation to ammonium nutrient by sorghum roots.