Diurnal Variations in Photochemical Energy Utilization and Osmotic Adjustments in Black Poplar Leaves Under Progressive Water Stress

Drought limits the productivity of fast‐growing woody crops, although the metabolic adjustments conferring water stress tolerance remain poorly understood. We investigated the responses of Populus nigra seedlings to water stress by integrating daily physiological measurements and NMR metabolomic analyses. Our aims were to: (i) determine key metabolic and biochemical responses in leaves subjected to moderate (WS1) and severe (WS2) water stress and (ii) identify the metabolites responsible for dissipating excess photochemical energy and maintaining cellular turgor. Despite the reduction in electron transport rate, photoprotective mechanisms were activated. A threefold increase in isoprene emission was observed at midday in WS1 plants, while ascorbic acid and other antioxidants were higher under WS2 conditions. Relative water content and osmotic potential decreased throughout the day, suggesting that passive osmotic adjustments were primarily driven by soluble sugar accumulation. Organic acid reduction in WS1 (−9%) and WS2 (−17%) plants suggested an inhibition of the tricarboxylic acid cycle. Additionally, amino acids were twofold higher in water‐stressed plants compared to controls, likely reflecting an increased demand for primary and secondary metabolite biosynthesis. Our results provide new insights into the daily response of P. nigra to water stress, highlighting a delicate balance between metabolic and physiological adjustments.