Fig. 1.

Rhizosphere pH regulated by uptake of NH₄⁺ and NO₃^– in rice roots. (A) The rhizosphere pH of rice roots shown with a colour pH indicator. (B) Agar profile showing rhizosphere pH after removing the roots. Rice seedlings (Oryza sativa L ssp. japonica, Nipponbare) were grown in full nutrient solution containing 1.25 mM NH₄NO₃ for 4 weeks and then transferred to 2.5 mM NH₄⁺ or 2.5 mM NO₃^– for 72 h. After 72 h N treatment, the plant root was washed by dipping in 0.2 mM CaSO₄ for 1 min before placement on the agar. An intact plant was placed on agar (0.9 g l^–1, containing the pH indicator (0.03 g l^–1 bromocresol purple). The initial pH was 5.2–5.3 from 11.00 h to 11.30 h, roots were kept in darkness covered with a moist paper tissue and under a 0.5×12×12 cm³ Plexiglas plate, and the picture was taken after 2–4 h in contact with the pH indicator agar. (C) pH of the hydroponic growth medium during 2.5 mM NH₄⁺ or 2.5 mM NO₃^– solution after 24, 48, and 72 h. The initial pH was 5.2–5.3.