Carbon input is not carbon throughput: flux control in plant responses to elevated CO2

Elevated atmospheric CO 2 consistently boosts photosynthetic carbon fixation, but these increases rarely lead to proportional gains in biomass, yield, or long-term productivity. This ongoing mismatch is often related to acclimation, downregulation, or sink limitation, yet the exact mechanisms remain only partially understood. Here, I argue that this paradox mainly stems from confusion between carbon input and metabolic flux. Photosynthesis shows how quickly carbon enters the system, while growth depends on continuous carbon flow through downstream metabolic and developmental sinks. Elevated CO 2 primarily increases carbon pressure within plant metabolic networks without necessarily boosting flux, because flux is limited primarily by sink development rather than by enzyme activity.