Long Photoperiod and Moderate PPFD: Optimizing Lettuce Lighting

Optimizing daily supplemental lighting schedules in protected cultivation directly impacts the balance between electricity costs and canopy leaf development. In a study published in Frontiers in Plant Science (2025), researchers evaluated the interactive effects of light intensity, photoperiod length, spectral composition, and temperature regimes on the morphology and biomass accumulation of salad crops.

Experimental Methodology and Parameters

The experiment was conducted on Rex lettuce in climate chambers. Researchers evaluated 18 combinations of microclimate and lighting parameters. During the trials, Rex lettuce was tested under three temperatures (20, 24, and 28 °C), two far-red light fractions (0 and 20% FR in the 400–800 nm range), and three light intensity levels (150, 200, and 300 μmol/(m²·s)).

To eliminate the influence of differing cumulative radiation sums, a fixed daily light integral was maintained across all treatments. While maintaining a constant DLI of 13 mol/(m²·d), light intensities of 150, 200, and 300 μmol/(m²·s) were paired with photoperiods of 24, 18, and 12 hours, respectively.

Interaction of Spectrum, Photoperiod, and Temperature

The study authors found that the response of the canopy and stem fundamentally depends on the presence of far-red radiation in the spectrum. Under 0% FR light, the combination of lower radiation intensity, an extended photoperiod, and warmer temperatures synergistically increased leaf area and photon capture. However, when 20% FR was introduced, the synergistic effect on leaf expansion disappeared: the plant redirected resources into stem elongation, which is undesirable for commercial head lettuce.

The physiological conclusion of the study clarifies the mechanism of crop mass accumulation. Plant biomass accumulation depended primarily on photon capture (r² = 0.93), rather than single-leaf photosynthetic efficiency. Increasing leaf area in the early and middle growth stages via a longer day and moderate photon flux allowed the crop to achieve canopy closure faster and intercept available radiation more efficiently under an unchanged daily light integral.

Implications for Power Distribution and Tariffs

For the chief agronomist and chief electrical engineer of a greenhouse facility, these results provide physiological justification for revising connected electrical loads and daily luminaire runtimes. In standard design scenarios, artificial lighting systems are frequently sized for short photoperiods with high photosynthetic photon flux density (PPFD), demanding higher installed grid capacity.

If the cultivation schedule and cultivar traits allow extending the photoperiod (up to 18–24 hours under controlled microclimates), growers can decrease the nominal PPFD level without sacrificing DLI. This reduces peak electrical loads on transformer substations and enables distributing supplemental lighting throughout the 24-hour cycle to maximize the use of night and off-peak tariff zones. Practical deployment of such regimes is discussed in the review of lettuce supplemental lighting projects.

Production Limitations

When adapting these experimental findings to commercial greenhouses, several production realities must be considered:

• The trial was conducted in enclosed climate chambers without natural solar radiation, where background microclimate and DLI were strictly controlled and constant.
• In commercial glass greenhouses, natural PAR varies by season and time of day, requiring photoperiod regulation to be combined with dynamic dimming based on solar light sensors.
• Extended photoperiods at elevated temperatures (especially continuous 24-hour lighting) on susceptible cultivars can trigger physiological disorders, such as tipburn, if air movement and transpiration are not properly managed.
• The presence or absence of far-red spectra in installed luminaires changes crop behavior: under elevated FR fractions, reducing PPFD in warm temperatures causes unwanted stem elongation instead of compact rosette formation.

Practical Takeaways for Specialists

Before implementing modifications to lighting strategies or crop management protocols, the following steps are recommended:

• Head Agronomist: verify the spectral composition of installed luminaires regarding far-red radiation. For spectra without excess far-red light, run small-scale trials with an extended photoperiod and moderate PPFD to promote leaf area expansion.
• Chief Engineer: recalculate the electrical power profile when transitioning from a short lighting cycle with high PPFD to an extended schedule with lower intensity. Compare the resulting 24-hour load profile with current electricity tariff tiers.
• Climate Control Specialist: synchronize temperature control with the lighting schedule to prevent excessive heat buildup in the crop canopy during nighttime luminaire operation.