Bio-imaging techniques reveal foliar phosphate uptake pathways and leaf phosphorus status.

Global demand for phosphorus (P) requires new agronomic practices to address sustainability challenges while increasing food production. Foliar P fertilization could increase P use efficiency; however, leaf entry pathways for inorganic phosphate ion (Pi) uptake remain unknown and it is unclear whether foliar P applications can meet plant nutrient demands. We developed two techniques to trace foliar P uptake in P-deficient spring barley (Hordeum vulgare) and to monitor the effectiveness of the treatment on restoring P functionality. Firstly, a whole-leaf P status assay was developed using an Image PAM system;non-photochemical quenching (NPQ) was a proxy for P status, asP-deficient barley developed NPQ at a faster rate than P-sufficient barley.. The assay showed restoration of P functionality in P-deficient plants 24 h after foliar P application. Treated leaves reverted to P-deficiency after 7 d, while newly emerging leaves exhibited partial restoration compared to untreated P-deficient plants, indicating Pi remobilization. Secondly, vanadate (V) was tested as a possible foliar Pi analogue using high resolution laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)elemental mapping. The strong co-localization of 51V and 31P signal intensities demonstrated that V was a sensitive and useful Pi tracer. V and Pi uptake predominantly occurred via fiber cells located above leaf veins, with pathways to the vascular tissue possibly facilitated by the bundle sheath extension. Minor indications of stomatal and cuticular Pi uptake were also observed. These techniques provided an approach to understand how Pi crosses the leaf surface and assimilates to meet plant nutrient demands.