Aluminum-Enhanced Proton Release Associated with Plasma Membrane H+-Adenosine Triphosphatase Activity and Excess Cation Uptake in Tea (Camellia sinensis) Plant Roots

Cultivated tea (Camellia sinensis) plants acidify the rhizosphere, and Aluminum (Al) toxicity is recognized as a major limiting factor for plant growth in acidic soils. However, the mechanisms responsible for rhizosphere acidification associated with Al have not been fully elucidated. The present study examined the effect of Al on root-induced rhizosphere acidification, plasma membrane H+-adenosine triphosphatase (H+-ATPase) activity, and cation-anion balance in tea plant roots. The exudation of H+ from tea plant roots with or without Al treatment was visualized using an agar sheet with bromocresol purple. The H+-ATPase activity of plasma membranes isolated from the roots was measured after hydrolysis using the two-phase partition system. The Al treatment strongly enhanced the exudation of H+, and the acidification of tea plant roots by Al was closely associated with plasma membrane H+-ATPase activity. The root plasma membrane H+-ATPase activity increased with Al concentration. The Al content, amount of protons released, and H+-ATPase activity were significantly higher in roots treated with Al than in those untreated. The results of the cation-anion balance in roots showed an excess of cations relative to anions, with the amount of excess cation uptake increasing with increasing Al concentrations. These suggest that Al-enhanced proton release is associated with plasma membrane H+-ATPase activity and excess cation uptake. Findings of this study would provide insights into the contributing factors of soil acidification in tea plantations.