Determining the net flux of charge released by maize roots by directly measuring variations of the alkalinity in the nutrient solution

The net flux of charge released by maize, i.e. the strong ion exchange bala nce between the roots and their environment, was determined in acidic and a lkaline solutions, i.e. solutions with a low and a high pH buffering capaci ty, respectively. The work was based on direct measurement of total alkalin ity in culture solutions over a period of several days. The results show there was little difference in the net flux of charge rele ased by maize in acidic and alkaline solutions: In both cases, approximatel y -1 mu mol(c) (kg DM)(-1) s(-1). As the maize was grown in a non-limiting nitrate solution, the charge flux was negative, corresponding to a net rele ase of hydroxyls into the rhizosphere. In contrast, the change in the amoun ts of free protons in the solution was approximately 1 nmol (kg DM)(-1) s(- 1), i.e. 3 orders of magnitude lower than the net charge flux. Moreover, it was negative in acidic media , i.e. the solution pH increased, and positiv e in alkaline media, i.e. the solution pH decreased. This decrease probably resulted from the release of inorganic carbon by the roots. The effect on the change in solution pH was only slight in acidic conditions but consider able in alkaline conditions, where it reduced the pH even though the cultur e solution was alkalinised by the roots. The difference in the way that acidic and alkaline solutions function demon strates the importance of the pH buffering capacity of the solution in dete rmining the net flux of charge released by the plants. It underlines the di fficulty of estimating the net charge flux from pH change measurements in t he rhizosphere.