Optimizing light conditions in controlled environments can significantly improve kale yield. Photoperiod and the ratio of far-red (FR) light play crucial roles in plant development, but different varieties may respond differently. This study examines how two kale varieties, ‘Half Tall’ and ‘Scarlet,’ react to far-red pulses applied either in the middle or at the end of the photoperiod. Interestingly, only ‘Scarlet’ exhibited measurable changes in response to the treatments.
The Role of Far-Red Light in Plant Growth
Far-red light (700-750 nm) was traditionally considered weakly photosynthetic, but research has shown that it enhances photosynthesis when combined with red and blue light. The Emerson effect demonstrates that red and far-red together boost photosynthetic efficiency more than red or far-red light alone. Additionally, far-red penetrates deeper into the canopy, improving light distribution and influencing physiological processes such as flowering and shade avoidance. The right balance of red to far-red (R:FR) LED grow lights can increase biomass and yield in many crops.
Experimental Design: Far-Red Pulses and Photoperiod
One treatment used our Lumi-VF white grow light, designed for vertical farming, which naturally includes 8% far-red with an R:FR ratio of 6.5. In other treatments, we simulated Lumi-VF without far-red and instead provided a 20-minute pulse of 100 µmol/m²/s far-red light either in the middle of the day or at the end of the photoperiod. The total photoperiod was set to 15 hours.
Results: Impact of Far-Red Timing on Kale Growth
Neither plant height, leaf number, nor plant diameter was significantly affected by far-red treatments in either variety. However, fresh weight increased in ‘Scarlet’ when far-red was applied at the end of the photoperiod. This treatment resulted in greater fresh weight in both stems and leaves, as well as a higher dry weight in stems. ‘Half Tall’ showed no significant response to any far-red treatment.
Discussion: Why End-of-Day Far-Red Boosts Biomass in ‘Scarlet’
Applying far-red at the end of the photoperiod mimics natural sunset conditions, where far-red light is more abundant. This deeper canopy penetration can excite Photosystem I and II (PSI, PSII), enhancing electron transport and CO₂ assimilation. The extended photosynthetic activity can lead to increased carbon fixation and cell expansion, boosting biomass without triggering excessive stem elongation.
A possible explanation for ‘Scarlet’s’ stronger response lies in its higher anthocyanin content, which gives it a red pigmentation and may allow greater absorption of far-red light. This could lead to enhanced phytochrome deactivation, reducing growth suppression and promoting biomass accumulation.
Conclusion
End-of-day far-red treatment can enhance kale biomass, but its effectiveness varies by variety. ‘Scarlet’ responded positively, likely due to its anthocyanin-driven light absorption properties, while ‘Half Tall’ remained unaffected. These findings suggest that far-red optimization should be tailored to specific kale varieties for maximum yield in controlled environments.
