Systemic reorganization of the photosynthetic apparatus underlies acclimation to nitrogen starvation across leaf ages in barley

Nitrogen (N) deficiency severely constrains global crop productivity, yet acclimation strategies in cereals remain incompletely understood. Hydroponically grown barley ( Hordeum vulgare ) plants were subjected to N starvation for 14 days. Photosynthetic properties were monitored across leaf ages and starvation durations to dissect acclimation adjustments. Growth was markedly inhibited, with shoot biomass decreasing by ~ 55% whereas root biomass increased, elevating the root/shoot ratio. N was actively remobilized from senescing older leaves to support developing younger ones. Consequently, older leaves exhibited severe depletion of photosynthetic pigments and photosystem II (PSII) efficiency, whereas younger leaves retained substantial electron transport and carbon fixation capacities. This reorganization entailed selective protein degradation: reduction in the content of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), Cytochrome b 6 f (Cyt f ) and core proteins of photosystems, with PSI components decreased more strongly than PSII components. Light-harvesting complexes and the non-photochemical quenching (NPQ) regulator PsbS were preferentially retained, resulting in enlarged relative functional antenna size and sustained NPQ capacity. Notably, carbon assimilation was more strongly impaired than electron transport, indicating sustained photochemistry despite reduced carbon fixation. Strikingly, this acclimation pattern was conserved across leaf ages despite vastly different stress severities, indicating an active regulatory program rather than progressive starvation damage. These findings reveal a conserved strategy that prioritizes photoprotection over carbon gain, offering targets to improve nitrogen use efficiency in cereal canopies.