Potassium mediates photosynthetic efficiency in tomato through genetic regulation rather than anatomical variation.

Potassium (K) is classically viewed as a guard-cell osmoticum, yet its influence on the photosynthetic efficiency under steady-state and fluctuating light remains unresolved. We combined gas exchange, cellular anatomy, and transcriptomic profiling to explore how K deficiency limits CO2 assimilation (AN) in tomato (Solanum lycopersicum). Contrasting our expectations, the decline of AN and mesophyll conductance (gm) under K deficiency in tomato was not attributed to alternation of leaf and cellular anatomy but was linked to downregulation of carbonic anhydrases and plasma-membrane aquaporins that facilitate CO2 diffusion. The maximum carboxylation rate of Rubisco (Vcmax) declined in parallel, coinciding with the repression of Rubisco small-subunit genes and Rubisco activase. Under fluctuating light, K deficiency significantly slowed stomatal opening and accelerated stomatal closure, increasing potential loss of CO2 fixation after transition from low to high light. Transcript data implicated K-transport, anion-channel, and sugar-transporter genes as the molecular brakes. Therefore, leaf K content influences photosynthesis by regulating diffusional and biochemical capacities. These findings highlight the importance of K in photosynthesis under both stable and variable light environments, offering new targets for improving crop photosynthetic resilience.