Steady-state ¹⁴CO₂ tracing reveals early disruption of assimilation-export coupling in tomato under static continuous...

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 ¹⁴CO₂ labeling with open-flow gas exchange to quantify net CO₂ exchange rate (NCER), concurrent export, ¹⁴C 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.