Distinct Foliar Uptake Pathways for Phosphorus and Nano-Hydroxyapatite in Potato Revealed By Synchrotron μCT and ³³P Imaging.

Potato (Solanum tuberosum L.) has a high phosphorus (P) requirement, yet its shallow root system and the strong P-fixing capacity of many soils limit the effectiveness of soil fertilisation. Foliar application of nano-hydroxyapatite particles (nHAPs) has emerged as a promising alternative P fertilisation practice. In this study, we compared foliar uptake mechanisms of nHAPs and orthophosphate ions (Pi) in potato using advanced bioimaging techniques and quantified their P uptake efficiency (PUE) with ³³P radioisotopes in controlled-environment and field experiments. Imaging revealed distinct uptake pathways: Pi penetrated directly through the cuticle, whereas nHAPs primarily entered through hydraulically activated stomata. Foliar application of nHAPs achieved a PUE of approximately 50% after 7 days - far exceeding global averages for soil-applied P fertilisers - while avoiding the leaf scorching commonly associated with Pi sprays. Pi uptake was insensitive to leaf surface polarity but increased when the surface tension of the foliar spray was low. In contrast, nHAP uptake was greater on adaxial leaf surfaces and favoured by higher surface tension. These findings demonstrate that aligning foliar nano particle formulations with their respective uptake pathways, spray properties, and leaf traits can substantially enhance nano-particle uptake efficacy. This work provides new mechanistic insight to support more efficient and sustainable P management practices in potato production and broader agricultural systems.