Growing tomato in controlled environments under continuous light requires dynamic LEDs to entrain the circadian rhythm, adjust canopy architecture, and balance photostasis
Controlled environment agriculture (CEA), including greenhouses and indoor production, offers tremendous opportunities to meet food security across the world. The success of CEA depends on electricity, with artificial lighting consuming a large fraction. Previous work has introduced a novel alternating LED strategy that extends the photoperiod in greenhouse production, taking advantage of off-peak electrical pricing while maintaining a healthy crop. Tomato (Solanum lycopersicum L.) is a model species in terms of its sensitivity to extended photoperiods, experiencing photoperiodic injury under continuous light. The alternating LED strategy alleviated photoperiodic injury and it was hypothesized that circadian rhythm entrainment was responsible, as the injury itself has been associated with circadian asynchrony previously. The presented thesis continues to explore and optimize the strategy in different chapters. First, the principles of circadian entrainment are detailed. A perspective centers on the “latitudinal rule”, where predictable chronotypes have evolved to the longer photoperiods in northern latitudes that can guide selectable traits for breeders and tailor ideal entraining cues for any genotype. Second, diurnal transpiration patterns revealed a great way to assess and measure circadian entrainment on a whole-plant level as they integrate cellular-stomata responses with whole-plant hydraulic status. Practically, transpiration, and associated thermal indices, can be sensed remotely to bring circadian data into smart-agriculture. Finally, a modified dynamic LED strategy (dynamic 1) significantly outperformed control (16hr photoperiod with unchanging spectrum), and by-far surpassed constant light (24hr), in total biomass accumulation via morphological and photosynthetic adjustments. A second modification (dynamic 2) attempted to extend the daytime photoperiod by 4hrs and resulted in a subtle photosynthetic stress response. A deep phenotyping gas exchange plus chlorophyll fluorescence multi-curve dataset revealed photorespiration as both beneficial and hazardous in photoperiodic injury. Also, a simple high throughput protocol distinguished dynamic 2 as engaging a nonphotochemical quenching dissipative type response, whereas dynamic 1 engaged an ATP:NADPH balancing response that upregulated ATP synthase activity. Cyclic electron flow at nighttime was also found to likely contribute much needed ATP. Overall, a philosophy is proposed that optimizing plant growth should start with entraining circadian rhythms, and then fine-tune timing/ dosage to choreograph morphology and balance photostasis.