Advancing phosphorus foliar fertilization with nano-hydroxyapatite: mechanisms and agricultural relevance

Phosphorus (P) use in agriculture remains inefficient and poses long-term environmental concerns. We investigate the uptake, redistribution, and efficacy of foliar-applied nano-hydroxyapatite (nHAp) in P-deficient barley (Hordeum vulgare), aiming to clarify its mode of action and evaluate its potential as a practical alternative to conventional phosphate salts. Using a simple, low-cost wet synthesis, we produced chemically-labelled nHAp with uniform elongated morphology (median length and width of 33.6 and 5.3 nm, respectively), excellent colloidal stability, pH-dependent solubility, and full redispersibility, suitable for highly concentrated foliar formulations. Infiltration experiments reveal that nHAp dissolves gradually in situ, with peak phosphate release between 1-3 days post-treatment, enabling local and systemic P recovery in deficient plants without inducing leaf scorching, unlike conventional P fertilizers. Bioimaging reveals that nHAp, applied as droplets to the leaf surface, penetrates via stomata and diffuses through the apoplast, where it dissolves and releases P without entering mesophyll cells. Uptake efficiency was strongly influenced by formulation surface tension and air humidity, and varied markedly between crop species. When comparing barley with potato, it was shown that potato exhibited high uptake at single applications, whereas barley required repeated treatments due to lower uptake efficiency. These findings position nHAp as a promising nanobiotechnological approach for sustainable and slow-release foliar P fertilization.