Steady-state 14CO2 tracing reveals early disruption of assimilation–export coupling in tomato under static continuous light but not under dynamic 24-h LED schedules

Artificial lighting is a major cost in controlled-environment agriculture (CEA), motivating longer photoperiods at lower photosynthetic photon flux density (PPFD) to meet a target daily light integral (DLI). In tomato, static 24-h lighting can induce photoperiodic injury and reduced photosynthetic performance, often discussed as an end-product (triose-phosphate) limitation that constrains inorganic phosphate (Pi) recycling. Dynamic 24-h LED schedules that impose day–night spectral and intensity cues have been proposed to extend photoperiod without injury, but their effects on assimilation–export coupling remain unclear. We investigated whether early dysfunction under static 24-h light reflects a downstream sink-side export bottleneck that limits carbon export regardless of source (newly assimilated and mobilization of stored carbon), or a source-leaf limitation that selectively reduces concurrent export during assimilation while preserving export supported by mobilizing stored carbon, and whether dynamic schedules avoid these constraints. We used steady-state 14 CO 2 labeling with open-flow gas exchange to quantify net CO 2 exchange rate (NCER), concurrent export, ¹ 4 C retention, chase-derived reserve-supported export (remobilization), and transpiration in intact source leaves of tomato ( Solanum lycopersicum “Money Maker”) grown under a 16-h/8-h control, a static 24-h regime, or two DLI-matched dynamic 24-h schedules. After 4 days of static 24-h light (pre-injury), export declined more than NCER, lowering relative export by ~12% and increasing the retained labeled pool. Whole-night remobilization efficiency scaled with retained pool size was conserved across treatments (~30%), including the Day 4 static regime, arguing against an early shared sink/transport bottleneck. After 3 weeks, static continuous light caused severe injury with suppressed NCER, export and reduced water-use efficiency (WUE), whereas dynamic regimes remained injury-free and maintained daytime assimilation–export coupling and WUE comparable to the control. Together, these data indicate that pre-injury static continuous light first disrupts assimilation–concurrent export coupling, a symptom not observed under dynamic 24-h LED schedules.