Phosphatase activity and acidification in lupine and maize rhizosphere depend on phosphorus availability and root properties: Coupling zymography with planar optodes

Increase of phosphatase activity and rhizosphere acidification are two main physiological mechanisms of phosphorus mobilization by plants. To understand plant P-acquisition strategies, the spatial distribution of phosphatase activity and pH in the rhizosphere in response to P availability between legumes and cereals and along individual roots were investigated. Zymography of acid and alkaline phosphatase activities visualizing P hydrolysis potential was combined with planar optodes to localize pH changes in maize and lupine rhizosphere. Lupine was more adapted to P-deficiency than maize by increasing phosphatase activity (up to 30%), decreasing pH for 0.8 units in the rhizosphere and having larger hotspot areas compared to P addition. The P deficiency-induced increases of phosphatase activities and acidification were position-specific along taproot of lupine: acid phosphatase was highest while alkaline phosphatase was least at apical parts; pH was 1.7 units lower at the apical than at the basal part. The phosphatase activities and pH along lateral roots of lupine were independent on P availability but had specific distribution patterns: acid phosphatase activity declined from the root tips toward basal part, while the alkaline phosphatase activity was slightly increased from root tips; the pH was highest at the root tips (first 5 mm) and declined thereafter until stabilized after 20 mm. We conclude that 1) lupine is better adapted to P deficiency than maize by a stronger increase of phosphatase activities and facilitation of rhizosphere acidification; 2) The organic P mobilization by phosphatases and mineral P solubilization by acidification along individual roots were dependent on the root zone.