When cultivating kale in a controlled environment, optimizing the photoperiod (day length) and light intensity is essential for maximizing yield. However, different kale varieties respond uniquely to light conditions. This study compares the growth responses of two kale varieties, ‘Half Tall’ and ‘Scarlet,’ under two different photoperiod and intensity treatments while maintaining the same daily light integral (DLI).
Even though both treatments provided the same total amount of light per day, yield differences were observed based on photoperiod and intensity variations.
Understanding DLI, Photoperiod, and Light Intensity
What is DLI?
Daily Light Integral (DLI) refers to the total amount of photosynthetically active radiation (PAR) a plant receives over a 24-hour period, measured in mol/m²/day. DLI integrates both light intensity and duration to provide a cumulative measure of light exposure.
Each plant species has an optimal DLI range:
- Too low → Slow growth, weak stems, poor flowering
- Too high → Damage to the plant’s photosynthetic machinery, reducing efficiency
How Photoperiod Affects Growth
Photoperiod plays a significant role in regulating flowering and growth phases, depending on whether a plant is classified as a long-day or short-day species.
In this experiment, we compared two photoperiod and intensity combinations while keeping DLI constant. Treatment 1. Short photoperiod → 10 hours at 300 µmol/m²/s, treatment 2. Long photoperiod → 20 hours at 150 µmol/m²/s
Light Spectrum Used in the Experiment
For this study, we used Valoya’s Lumi-VF spectrum with LL-series fixtures, a white grow LED light optimized for biomass production and plant development in vertical farming environments.
Results: How Kale Responds to Photoperiod and Light Intensity
Longer Photoperiod Led to Increased Plant Height. A 20-hour photoperiod with lower intensity (150 µmol/m²/s) resulted in taller plants across both kale varieties. ‘Scarlet’ grew 15% taller and ‘Half Tall’ grew 14% taller
Longer Photoperiod Increased Fresh Weight in ‘Half Tall’. ‘Half Tall’ produced 28% more fresh weight in leaves and 39% more in stems under the long photoperiod compared to the short photoperiod. ‘Scarlet’ showed no significant difference in fresh weight between treatments. Dry Weight Was Higher Under Longer Photoperiod. ‘Scarlet’ had a 43% increase in stem dry weight under the longer photoperiod. ‘Half Tall’ had a 31% increase in total dry weight under the longer photoperiod.
Discussion: Why Does a Longer Photoperiod Improve Growth?
A longer photoperiod with lower intensity can be beneficial for biomass accumulation in kale due to several physiological factors:
- Prolonged carbon assimilation → More hours of photosynthesis allow for gradual biomass accumulation.
- Increased cell expansion and leaf development → Longer light exposure supports structural growth.
- Reduced risk of photoinhibition → High-intensity light can stress plants, while lower intensity for a longer duration maintains efficiency.
Energy Efficiency Considerations
Lowering the intensity of LED fixtures improves energy efficiency because LED efficacy increases when dimmed. However, there is a trade-off with the longer photoperiod, which extends the overall lighting duration. Finding an optimal balance between photoperiod and intensity can lead to both energy savings and higher plant biomass.
Conclusion
A longer photoperiod with lower intensity resulted in:
Taller plants in both varieties, increased fresh weight in ‘Half Tall’ and higher dry weight in both varieties
These findings suggest that adjusting photoperiod and intensity rather than just total DLI can enhance kale production. Additionally, longer photoperiods with lower intensity may be an energy-efficient strategy for maximizing kale yield in controlled environments and vertical farming.
