Bioimaging Techniques Reveal Foliar Phosphate Uptake Pathways and Leaf Phosphorus Status.

Maja Arsic,Casey L Doolette,Helle Juel Martens,Stine Le Tougaard,Daniel Pergament Persson,Enzo Lombi,Jan Kofod Schjoerring,Søren Husted

doi:10.1104/pp.20.00484

Maja Arsic, Casey L Doolette + Show 6 more

Open Access

https://doi.org/10.1104/pp.20.00484

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Abstract

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. First, a whole-leaf P status assay was developed using an IMAGING PAM system; nonphotochemical quenching was a proxy for P status, as P-deficient barley developed nonphotochemical quenching 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 with untreated P-deficient plants, indicating Pi remobilization. Second, vanadate was tested as a possible foliar Pi tracer using high-resolution laser ablation-inductively coupled plasma-mass spectrometry elemental mapping. The strong colocalization of vanadium and P signal intensities demonstrated that vanadate was a sensitive and useful Pi tracer. Vanadate 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.

Highlights

By 2050, food production may need to increase by 70% to feed a projected 9.7 billion people (Hunter et al.2017)
The P status of P-deficient plants treated by foliar P fertilization was moderately restored after 1 day and P functionality was maintained for 4 days before declining back to P-deficient status by day 6 and 7 (Fig. 1)
Day 6 showed a slight anomaly for both controls, where there was a slight increase in P status in P-deficient plants and a decrease in P status for P-sufficient youngest fully expanded leaf (YFEL), both values remained within 10% of the ‘P status’ range cut-offs

Summary

Introduction

By 2050, food production may need to increase by 70% to feed a projected 9.7 billion people (Hunter et al.2017). Phosphate rock, the main source of P fertilizers, is a finite natural resource and the known rock phosphate reserves are estimated to last as little as years in the gloomiest forecasts (Gilbert 2009; Edixhoven et al 2013) to another 3-400 years in more optimistic forecasts (Syers et al 2011). This makes P a potential strategic natural resource similar to oil, as very few countries control the vast majority of the known reserves (Gilbert 2009; Elser and Bennett 2011; Edixhoven et al 2013). Over 80% of mined P is used in fertilizer manufacturing

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