Leaf development regulates state transition capacity in trees.

State transitions (ST) balance excitation energy between photosystem I and II. This process has been extensively studied in Arabidopsis but the regulation and physiological significance of ST in other angiosperms remain largely unknown. Here, we investigate ST in hybrid aspen and other tree species using physiological, biochemical, ultrastructural, and genetic approaches. We discover a pronounced canopy gradient in greenhouse-grown aspens, with young, upper leaves exhibiting substantially higher fluorescence-derived state-transition capacity (qT) than lower, older leaves. Seasonal monitoring of field-grown trees reveals a conserved developmental decline in qT across species. Reduced qT correlates with increased grana stacking and lower LHCII/PSII ratios, but not with LHCII phosphorylation, suggesting developmental remodeling of thylakoid architecture may influence the functional reorganization of PSII antenna connectivity during state transitions. Using the serine/threonine protein kinase (STN7) knockout mutant characterized outside Arabidopsis, we show that aspens lacking ST exhibit altered PSI/PSII ratios, reduced PSII operating efficiency in young leaves, and slower growth under greenhouse conditions with naturally variable light conditions but not under constant-light climate room condition. These findings indicate that ST is important for performance under dynamic light conditions, particularly in young leaves, and reveal a previously unrecognized developmental control of photosynthetic regulation.