Wild Barley Exhibits Higher Phosphorus-Use Efficiency and Greater Rhizosheath Carboxylates Than Cultivated Barley Under Low-Phosphorus Conditions.

Phosphorus (P) deficiency constrains cereal production, yet improving low-P performance remains challenging because P efficiency depends on rhizosphere mobilisation and internal utilisation. Wild relatives may provide insights into traits weakened during domestication. Here, we compared cultivated barley (Hordeum vulgare; CB) and wild barley (H. spontaneum; WB) to identify traits underpinning low-P tolerance. Twenty barley genotypes (10 CB and 10 WB) were grown under low (P5) and moderate (P20) P supply, and twelve low-P response physiological traits were integrated into a composite tolerance index (D-value) to assess the multidimensional basis of low-P tolerance. Under both P5 and P20, WB showed consistently higher physiological P-use efficiency (PPUE) than CB, while root morphological traits were broadly similar between CB and WB. This decoupling of biomass production from tissue P status indicates more efficient internal utilisation of absorbed P in WB. Under P5, the amount of rhizosheath citrate per plant was strongly stimulated and showed a pronounced species effect with WB exhibiting significantly higher rhizosheath citrate than CB. By contrast, increasing P supply shifted carboxylate composition towards malate dominance (85% at P20). The D value showed strong and consistent associations with key growth, PPUE, root morphology and rhizosphere functional traits in both CB and WB, validating its effectiveness as an integrative metric of low-P tolerance. Overall, WB's superior low-P performance is primarily driven by higher PPUE and elevated citrate exudation under acute P limitation, highlighting the value of targeting internal P utilisation and rhizosphere mobilisation efficiency to improve P-use efficiency in barley.