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

ABSTRACT

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.