Water use efficiency in cereal crops: stomatal, hydraulic, biochemical mechanisms, and translational strategies for improvement

Water use efficiency in cereal crops is governed by molecular processes operating across biological scales that are connected imperfectly and non-linearly, from guard cell ion channel gating to canopy evapotranspiration. Despite the global urgency of improving yield under water limitation, no existing synthesis has integrated the molecular machinery of stomatal regulation, mesophyll CO₂ conductance, root hydraulic conductance, isohydric and anisohydric stomatal strategies, carbon isotope discrimination as a proxy tool, and specific genetic targets for improvement within a single cereal-focused mechanistic framework. This review advances that synthesis, examining WUE primarily through the lens of non-stomatal carbon fixation rather than stomatal regulation alone. The ABA-dependent stomatal closure cascade and the opposing blue-light opening pathway are conserved in core architecture across cereals, but their calcium-dependent and H⁺-ATPase branches remain characterised mainly by sequence homology rather than direct cereal evidence. Mesophyll conductance and Rubisco carboxylation efficiency represent underexploited, genetically undefined targets for improving carbon gain. Root hydraulic conductance connects soil water to canopy transpiration on timescales of minutes to days, while whole-plant isohydric to anisohydric strategy is best understood as an emergent, environment-dependent outcome of these faster processes rather than a fixed trait. Carbon isotope discrimination remains a practical relative-ranking proxy rather than a direct measurement. The EPFL-ERECTA stomatal density pathway and the H⁺-ATPase aperture system show the strongest cereal-specific experimental support among engineering targets, though compensatory anatomical and physiological responses mean anatomical change alone is insufficient evidence of durable benefit. Nitrogen-water signalling through CEP-CEPR1 peptide signalling likely shapes WUE as a downstream emergent property, though direct experimental validation is lacking. Five specific research priorities are identified to advance WUE improvement from mechanistic understanding toward field-level translation.