Co-occurring increased phosphatase activity and labile P depletion in the rhizosphere of Lupinus angustifolius assessed with a novel, combined 2D-imaging approach

Innovative non-destructive 2D imaging methods such as zymography and diffusive gradients in thin films (DGT) have been developed recently to assess the distribution of phosphatase activity and labile solutes at the root-soil interface. We report on the first combination of these techniques for the spatial distribution and potential interaction of labile phosphorus (P) and associated acid phosphatase activity in the rhizosphere of blue lupin (Lupinus angustifolius) in two contrasting soils Sand' and 'Loam'). Zymography, based on 4-methylumbelliferylphosphate (MU-P) for acid phosphatase activity mapping, and DGT gels capable of binding labile P were deployed to individual root axes to visualise P mobilisation and depletion in the rhizosphere of blue lupin grown in rhizotrons in glasshouse conditions for 45 days. Acid phosphatase activity was evidently higher in the rhizosphere and co-occurred with P-depletion zones around the roots in both soils. Lateral root profiles showed that elevated acid phosphatase activity as well as P-depletion extended up to 2 mm from the root centre into the rhizosphere. Despite larger total and organic P pools in the Loam, P was less plant available (DGT labile P) than in the Sand. However, phosphatase activities in the rhizosphere and plant P contents were similar in both soils. These results indicate that enzyme-catalysed hydrolysis of organic P was limited in the Loam, due to low P availability, hence explaining similar P contents in lupins from both soils. P did not accumulate in labile pools in the rhizosphere indicating that P supply via phosphatase could not compensate the plants demand for P resulting in P depletion zones. While our work demonstrates the applicability of combined zymography and DGT deployments to study rhizosphere processes at microscale, we also discuss limitations and perspectives of this approach.

Automated summary

Innovative non-destructive 2D imaging methods such as zymography and diffusive gradients in thin films (DGT) have been combined to assess the distribution of labile phosphorus and acid phosphatase activity in the rhizosphere of blue lupin. Higher acid phosphatase activity and P-depletion zones were observed in the rhizosphere of blue lupin, extending up to 2 mm from the root centre. Despite differences in total and organic P pools between soils, plant-available P was lower in the Loam soil. The enzyme-catalysed hydrolysis of organic P was limited in the Loam soil, explaining similar P contents in lupins from both soil types.