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Annals of Botany 89: 213-220, 2002
© 2002 Annals of Botany Company

Regulation of Rhizosphere Acidification by Photosynthetic Activity in Cowpea (Vigna unguiculata L. Walp.) Seedlings

THEERTHAM P. RAO*,1, KATSUYA YANO1, MORIO IIJIMA1, AKIRA YAMAUCHI1 and JIRO TATSUMI1

1Laboratory of Crop Science, Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464–8601, Japan

 * For correspondence. Fax: +81 (0) 52 789 5558, e-mail tprao{at}agr.nagoya-u.ac.jp

Received 1 August 2001; Returned for revision 4 October 2001; Accepted 5 November 2001.

In contrast to cereals or other crops, legumes are known to acidify the rhizosphere even when supplied with nitrates. This phenomenon has been attributed to N2 fixation allowing excess uptake of cations over anions; however, as we have found previously, the exposure of the shoot to illumination can cause rhizosphere acidification in the absence of N2 fixation in cowpea (Vigna unguiculata L. Walp). In this study, we examined whether the light-induced acidification can relate to photosynthetic activity and corresponding alterations in cation–anion uptake ratios. The changes of rhizosphere pH along the root axis were visualized using a pH indicator agar gel. The intensity of pH changes (alkalization/acidification) in the rhizosphere was expressed in proton fluxes, which were obtained by processing the images of the pH indicator agar gel. The uptake of cations and anions was measured in nutrient solution. The rhizosphere was alkalinized in the dark but acidified with exposure of the shoots to light. The extent of light-induced acidification was increased with leaf size and intensity of illumination on the shoot, and completely stopped with the application of photosynthesis inhibitor. Although the uptake of cations was significantly lower than that of anions, the rhizosphere was acidified by light exposure. Proton pump inhibitors N,N'-dicyclohexyl carbodimide and vanadate could not stop the light-induced acidification. The results indicate that light-induced acidification in cowpea seedlings is regulated by photosynthetic activity, but is not due to excess uptake of cations.

Key words: Acidification, agar gel, bromocresol purple, cation–anion uptake, Vigna unguiculata L. Walp, pH, photosynthesis, proton flux, proton pump inhibitors, rhizosphere.


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