LED Interlighting for Tomato and Cucumber: Canopy Effects and Yield
In commercial greenhouse vegetable production, when cultivating plants with a developed multi-tiered canopy, top lighting inevitably creates a pronounced vertical radiation gradient. The upper leaf canopy intercepts the majority of the light flux, while only a minor fraction of energy reaches the middle and lower tiers. A deficit of photosynthetically active radiation deep within the canopy leads to a decrease in leaf chlorophyll content and accelerates premature leaf senescence and abscission. In a scientific review by the Federal Scientific Agroengineering Center VIM, researchers D. A. Burynin and co-authors systematized the results of domestic and international studies dedicated to interlighting for tomato and cucumber in protected cultivation.
Impact of Intra-Canopy Lighting on Tomato Productivity
The review authors analyzed a series of trials where LED interlighting modules were placed directly inside the crop canopy at heights from 0.65 to 1.5 m above the substrate surface or mounted vertically along the stems. Localized delivery of light quanta to shaded zones activates the photosynthetic apparatus of lower-tier leaves, maintaining stable chlorophyll levels and extending the active period of carbon dioxide assimilation.
When summarizing commercial operational data, it was established that under combined (interlighting and top lighting) illumination, tomato yield increased by 12 to 23% [18–20]. This gain is achieved through growth stimulation and increased average fruit weight. In one reviewed trial, even with a high installed top lighting capacity of 161 W/m², adding 60 W/m² of interlighting increased yield by 23% during the winter period [20]. Researchers emphasize that lighting efficacy is closely linked to planting density: fully realizing the lighting potential requires adjustments to crop management and stem density.
Results of Interlighting Module Application on Cucumber
Cucumber is characterized by rapid vegetative mass formation and high sensitivity to illumination in the middle canopy tier, where fruits set and develop. The rapid closure of upper canopy leaves reduces PAR delivery to fruiting nodes, so redistributing part of the light capacity to the inter-row space delivers a tangible agronomic outcome.
According to data presented in the review, experimental implementation of interlighting at intensities of 49 and 99 μmol/(m²·s) in cucumber cultivation demonstrated high efficiency and increased productivity by 22.3% and 30.8%, respectively [23]. In parallel, researchers evaluated energy performance: the study recorded a 16% increase in lighting energy efficiency when using interlighting at an intensity of 75 μmol/(m²·s) compared to top lighting [26]. Basic requirements for light regimes and photon irradiance levels for this crop are detailed in the guide on supplemental lighting for cucumber in protected cultivation.
At the same time, the review authors point to the critical role of spectral composition. Intra-canopy irradiation of cucumber with a spectrum containing 20% blue light caused marginal leaf burn, deformation, and curling of leaf blades, which limited overall plant productivity. This demonstrates the necessity of careful selection of spectral parameters and spatial light distribution at the lighting system design stage.
Engineering Aspects and Microclimate
Installing LED luminaires inside a crop canopy imposes stricter requirements on luminaire housing thermal management. Unlike high-pressure sodium (HPS) lamps, LED emitters do not generate a powerful infrared flux toward plants. Nevertheless, dissipating convective heat from diodes and integrated drivers remains a key objective.
The review emphasizes that the operating temperature of module external surfaces must not cause thermal damage to contacting vegetative organs. With proper heat dissipation, the thermal energy released by the housing serves as a localized heating source for the fruiting zone, reducing the load on lower greenhouse heating circuits. To monitor photon flux density and correctly correlate electrical and photometric parameters, engineers are referred to the methodology for converting watts to micromoles.
Significance for Greenhouse Lighting System Modernization
Combining top lighting and interlighting modules allows growers to overcome seasonal insolation constraints during winter and early spring cycles. However, implementing this technology in commercial greenhouse facilities requires addressing practical operational factors:
- Mechanical accessibility assessment: luminaire suspension in inter-rows must not obstruct crop maintenance operations, tomato vine lowering, or harvesting;
- Spectrum balancing: a high red fraction (75–80% or more) provides high photon efficacy, but requires precise blue spectrum dosing to prevent leaf blade disorders;
- Electrical and structural load: connecting additional cable lines and suspension structures to trellises requires dedicated structural calculations for frame strength and switchboard capacity.
Before full-scale installation of interlighting equipment, greenhouse agronomic and engineering teams should perform test measurements of PPFD distribution across the canopy height and compare electricity costs against expected yield gains.
