Optimize your production parameters

Indoor lettuce production in vertical farms, warehouses and repurposed shipping containers offers an excellent opportunity to grow local, fresh, and consistent crops year-round, regardless of the season. With advancing environmental control technologies, the opportunity to hasten crop timing and increase quality parameters such as shelf-life, flavor, nutrients, color, and yield is possible by adjusting variables such as light intensity, quality, and duration; day and night temperatures; carbon dioxide concentrations; and the vapor pressure deficit. However, with all this control, decisions must be made about what to prioritize while considering both the impact on crops and the costs of inputs. In this first article of a five part-series on controlled environment agriculture (CEA) leafy green and herb production, we will highlight the impacts of light intensity and temperature on the growth of red oakleaf and green butterhead lettuce (Fig. 1). 

Temperature impacts on indoor lettuce

The average daily temperature (ADT) a crop is grown at primarily influences the rate of development, with additional impacts on quality. For lettuce, ADT greatly impacts size, color, texture, leaf number, and ultimately crop timing. Different cultivars have their own specific temperature ranges where developmental rates are reduced, optimal and supra-optimal, and even then, your set point temperature will depend on what crop quality parameters you want to prioritize. For instance, an ADT that leads to vibrant and deep red lettuce is often cooler than the ADT prioritizing rapid leaf development.  

Light intensity plays a pivotal role in plant growth and ultimately yield as it fuels the process of photosynthesis and light quality influences leaf size and color. For our purposes, we are primarily concerned with the cumulative amount of light received throughout the day, known as the daily light integral (DLI). Generally, raising the DLI up to a specific threshold enhances growth, quality, and yield. The advantages of additional light are often influenced by other environmental factors that can limit photosynthesis, such as the temperature and available carbon dioxide. 

Study design

Seeds of red oakleaf lettuce 'Rouxaï RZ' and green butterhead lettuce 'Rex' were sown in 200-cell rockwool plugs, placed in a growth chamber with an ADT of 72° F (22° C). 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 (LEDs) 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 (23° C)] [73° F (23° C)], or 82/70° F (28/21° C) [79° F (26° C)], and DLIs of 9 or 18 mol∙m−2∙d−1 with a 17-hour photoperiod. After 36 (Rouxaï RZ) and 37 (Rex) days, the lettuce was harvested and measurements were taken for shoot fresh and dry mass, leaf size, leaf number, plant size, color, and incidence of tipburn. A simplified economic model was used to estimate the economic viability of indoor lettuce production under the tested ADTs and DLIs. Results of the economic model will be presented in a future article. 

Results

The fresh mass of ‘Rex’ was impacted by the DLI, increasing by 29% (33 g; 1.2 oz.) across each ADT. While 'Rouxaï RZ' was impacted by both the DLI and ADT (Fig. 2). Increasing the ADT above 68° F resulted in an increase in fresh mass, but the increase was greater under a DLI of 18 mol∙m⁻²∙d⁻¹. Under the DLI of 9 mol∙m⁻²∙d⁻¹, raising the ADT resulted in a fresh mass increase of 30% (23 g; 0.81 oz.), while raising the ADT under a DLI of 18 mol∙m⁻²∙d⁻¹ further increased the fresh mass by 42% (45 g; 1.6 oz.). Surprisingly, increasing the ADT from 73 to 79° F did not impact the fresh mass of 'Rouxaï RZ'. In contrast, increasing the DLI from 9 to 18 mol∙m⁻²∙d⁻¹ led to a fresh mass increase of red lettuce 'Rouxaï RZ' of 41% at 68° F, and approximately 55% at both 73 and 79° F.  

The dry mass, a measure of biomass accumulation as a result of photosynthesis of both lettuce cultivars was greatest as the DLI and ADT were simultaneously increased (Fig. 3). The dry mass of red oak leaf lettuce and green butterhead increased by 25% and 20%, respectively, when the ADT was raised above 68° F. For both cultivars, raising the DLI from 9 to 18 mol∙m⁻²∙d⁻¹ increased dry mass by about 50% at 68° F and 55% at 73° F. While at 79° F, raising the DLI led to a 56% dry mass increase for 'Rouxaï RZ', and a 65% increase for ’Rex’.  

 ‘Rex’ leaf number was impacted by temperature, increasing from 22 to 28 leaves as the ADT increased from 68 to 73 or 79° F. However, for ‘Rouxaï RZ’ leaf number was impacted by both temperature and DLI. Three additional leaves unfolded when the DLI was raised from 9 to 18 mol∙m⁻²∙d⁻¹ at 68° F and 6 more leaves developed at 73 and 79° F. Raising the ADT from 68 to 73° F resulted in 5 additional leaves under 9 mol∙m⁻²∙d⁻¹ and 8 more under 18 mol∙m⁻²∙d⁻¹. This shows that raising either DLI or ADT increased leaf number, while raising both increased the leaf number the most for ‘Rouxaï RZ’.

For both cultivars, the number of plants with tipburn increased under a high DLI, but neither were influenced by the temperature. As DLI increased from 9 to 18 mol∙m⁻²∙d⁻¹, 25% of ‘Rouxaï RZ’ plants had tipburn, while 47% of ‘Rex’ developed some tipburn at the low DLI; while all plants developed tipburn at the high DLI (Fig. 4). We did not assess the severity of tipburn and did not have adequate airflow in the growth chambers, which can increase tipburn incidence. 

The color of the red lettuce cultivar ‘Rouxaï RZ’ was impacted by both the light intensity and temperature. Overall, we found that foliage was a richer and darker shade of red and yellow at the lower ADT and high light intensity, while at the higher temperatures and low light intensity brought out a brighter, light-green foliage. 

Key takeaways

Increasing the light intensity and temperature increased yield for both the red oakleaf 'Rouxaï RZ' and green butterhead 'Rex' lettuce cultivars, while reducing production time. However, under our conditions, a higher DLI led to increased tipburn occurrence for both cultivars. To maximize the economic return of increasing the PPFD, tip burn mitigation practices such as increased airflow and proper VPD management must be utilized in CEA production of lettuce. The decision between an ADT of 73-79° F will depend on temperature management costs. In a future article, we will share an economic analysis that will detail how to select the appropriate environmental parameter based on your input costs. 

Sean Tarr was an M.S. student at Michigan State University (MSU) 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.