Unlocking the potential of indoor lettuce production

The rate of crop development is largely determined by the average daily temperature (ADT). Species and cultivars have specific ADT ranges, where going below or above the optimum can reduce their developmental rates. In the case of lettuce, ADT can also influence quality parameters such as the size, color, texture and number of leaves, and, most importantly, the time to harvest. With all the parameters influenced by temperature, it can be difficult to identify an ADT that will promote one quality parameter and not negatively impact another. Like most other plants, lettuce growth and yield are responsive to the carbon dioxide (CO₂) concentration in its growing environment. The efficiency of photosynthesis is dependent and limited by environmental parameters, such as CO₂, temperature, light and the availability of water and nutrients. In a tightly sealed controlled environment, CO₂ levels can quickly drop below the ambient concentration of 400 ppm when a crop is actively growing. Raising the CO₂ concentration above ambient can increase yield, but there are diminishing returns as more CO₂ is provided. Additionally, the rate of photosynthesis increases until a species- or cultivar-specific saturation point is reached. At greater CO₂ concentrations, the optimal temperature for photosynthesis typically increases. However, going beyond that optimal temperature will cause the photosynthetic efficiency to decline.

Study design

Green butterhead lettuce ‘Rex’ and red oakleaf lettuce ‘Rouxaï RZ’ were sown in 200-cell rockwool plugs and placed in a walk-in indoor growth chamber with an ADT of 72 F (22 C) and CO2 concentration of 500 ppm (Fig. 1). A photosynthetic photon flux density (PPFD) of 180 µmol∙m⁻²∙s⁻¹ was maintained for 24 hours for three days with sole-source light-emitting diodes providing white light, then reduced to 20 hours until transplant on day 11. The seedlings were transplanted into deep-flow hydroponic systems inside three walk-in growth chambers, each having different day/night and [ADT] set points of 72/59 F (22/15 C) [68 F (20 C)], 77/64 F (25/18 C) [73 F (23 C)], or 82/70 F (28/21 C) [79 F (26 C)], and CO2 concentrations of 500, 800 or 1200 ppm under a daily light integral of 18 mol∙m−2∙d−1. Lettuce ‘Rouxaï RZ’ and ‘Rex’ were harvested 36 or 37 days after transplant, respectively.

Results

The fresh mass of ‘Rex’ increased alongside the temperature, with an increase of 18% as ADT was raised from 68 to 79 F (Figs. 2 and 3). However, the fresh mass of red lettuce ‘Rouxaï RZ’ increased by 32% as ADT increased from 68 to 73 F, then only 8% as ADT further increased from 73 to 79 F. For both cultivars, increasing CO2 concentration above 500 ppm resulted in an increase in fresh mass, but increasing above 800 to 1200 ppm did not. For example, as CO2 increased from 500 to 800 ppm the fresh mass of ‘Rouxaï RZ’ increased by 33% and 16% for ‘Rex’ (Figs. 1 and 3).

The dry mass of both lettuce cultivars was greatest at a CO2 concentration of 800 ppm, regardless of the ADT. On the other hand, the lowest dry mass recorded for both cultivars were at an ADT of 68 F and CO2 concentration of 500 ppm. Both cultivars increased in size alongside ADT.

Leaf number was influenced by ADT and CO2 concentration for both cultivars. ‘Rex’ leaf number was mainly impacted by ADT — six leaves unfolded as the ADT increased from 68 to 73 F and eight more leaves unfolded as the temperature increased from 73 to 79 F. ‘Rex’ unfolded the fewest number of leaves (22 leaves) at an ADT of 68 F and a CO2 concentration of 500 or 1200 ppm, while the most leaves (43) at 79 F and 1200 ppm. ‘Rouxaï RZ’ unfolded 21 leaves at 68 F and eight more leaves at 73 F, regardless of CO2 concentration. Only four additional leaves unfolded as the ADT increased from 73 to 79 F at a CO2 concentration of 1200 ppm. At 68 F, the leaves were darker and redder. At 79 F, the plants were a lighter and more vibrant green (Fig. 2).

For ‘Rouxaï RZ’, the presence of tip burn was influenced by the CO2 concentration but not temperature. At 500 ppm CO2, 25% of the crop had some severity of tip burn, while at 1200 ppm CO2, 67% of the crop exhibited tip burn. Interestingly, there was no tip burn present at a CO2 concentration of 800 ppm. ‘Rex’, however, had tip burn present regardless of the ADT or CO₂ concentration.

Key takeaways 

For both the red oakleaf ‘Rouxaï RZ’ and green butterhead ‘Rex’ lettuce cultivars, we recommend a CO₂ concentration of 800 ppm and ADT of 73 F. These setpoints provided the greatest biomass and leaf number while keeping input costs down. Additionally, these setpoints induced redder foliage than at 79 F for ‘Rouxaï RZ’ and reduced the incidence of tip burn for ‘Rex’.

Sean Tarr was an M.S. student at Michigan State University and Roberto Lopez is an Associate Professor and Controlled Environment/Floriculture Extension Specialist in the Department of Horticulture at MSU. The authors gratefully acknowledge Fluence Bioengineering for LEDs, JR Peters for fertilizer, Rijk Zwaan for seeds, Grodan for rockwool, Hydrofarm for hydroponic tanks and the USDA National Institute of Food and Agriculture Hatch project nos. MICL02472 and USDA-NIFA Specialty Crop Research Initiative award no. 2019-51181-30017.