Apoplastic pH in roots of Lupinus angustifolius L. in response to pH>6

Root growth of lupin (Lupinus angustifolius L.) is particularly sensitive to pH>6. Inefficient cell wall acidification and impaired wall synthesis were proposed to cause the poor root growth at pH>6. We used confocal laser scanning microscopy and centrifugation methods to measure root apoplastic pH change in response to external pH. The apoplastic pH determined by CLSM was 5.2 at pH 5.4, and increased to 5.5 at pH 7.4. The apoplastic pH measured using the centrifugation method was 5.9 at pH 5.5, and increased by 0.1–0.3 units when external pH increased to 7.4. It is suggested that the arrested root lupin growth at pH>6 does not result from an impaired cell wall acidification.