Abstract
The effects of sole-source lighting on the growth and yield of hydroponically grown lettuce have been extensively studied, but research of postharvest performance is limited. We grew frill-leaf lettuce (Lactuca sativa) ‘Green Incised’ and ‘Hydroponic Green Sweet Crisp’ hydroponically in an indoor vertical research farm under daily light integrals (DLIs) of 12 or 18 mol⋅m−2⋅d−1 and the following three ratios of blue (B; 400–499 nm) and red (R; 600–699 nm) light from light-emitting diode fixtures: B5:R95, B20:R80, and B35:R65. We postulated that biomass accumulation would increase with the DLI and decrease with the B light fraction, and that postharvest longevity would increase with the DLI and the B light fraction. As expected, shoot fresh weight, leaf length and width, leaf number, and relative chlorophyll content (SPAD; ‘Green Incised’ only) decreased as the proportion of B light increased from 5% to 35%. Decreasing the DLI from 18 to 12 mol⋅m−2⋅d−1 reduced the shoot fresh weight and leaf number of both cultivars. Leaves of ‘Green Incised’ were up to 27% wider under B5:R95 and 60% longer under B5:R95 at 12 mol⋅m−2⋅d−1 than those under treatments with a higher DLI or more B light. The shoot fresh weight of ‘Hydroponic Green Sweet Crisp’ was greatest when grown under B5:R95 at 18 mol⋅m−2⋅d−1 and decreased as B light increased or DLI decreased. At the time of harvest, leaves of each cultivar and treatment were placed in clamshells and stored at 7 °C in darkness and evaluated for decay. ‘Green Incised’ that grew under B35:R65 and a DLI of 18 mol⋅m−2⋅d−1 had the shortest storage life, with 9.5 d and 11.4 d for replications 1 and 2, respectively, which were ∼2.5 to 4.0 d and 1.4 to 3.6 d earlier, respectively, than the storage life of lettuce grown under other treatments. In contrast, ‘Hydroponic Green Sweet Crisp’ was not influenced by light quality or DLI and had a storage life of 12.6 to 13.3 d and 13.5 to 14.3 d for replications 1 and 2, respectively. Therefore, a B light fraction between 5% and 20% and a DLI of 18 mol⋅m−2⋅d−1 produced high-yielding frill-leaf lettuce with a relatively long storage life.
Lettuce (Lactuca sativa) is a popular crop consumed worldwide and is available as different varieties, such as romaine, crisphead, butterhead, and leaf lettuce. In 2022, US$4.1 billion of lettuce was sold, and 30% of that amount was attributable to leaf lettuce (USDA NASS 2023). In the United States, lettuce is commonly produced outdoors in California and Arizona, but there is growing interest in producing it inside controlled environments, including greenhouses and indoor vertical farms, typically hydroponically. This is partly because leafy greens can be grown year-round, at higher densities, and in shorter cycles indoors compared with outdoor production (Mitchell and Sheibani 2020). For example, indoor vertical farms produced an annual average of 26 kg⋅m−2 of leafy greens compared with 3.9 kg⋅m−2 outdoors (Artemis 2021). The indoor vertical farming industry is rapidly growing, but it still seeks innovative ways to maximize growth and quality by manipulating light quantity and quality in concert with other environmental variables.
Red (R) light (600–699 nm) is typically considered the most efficient and effective waveband for driving photosynthesis (McCree 1971). The most common R light-emitting diodes (LEDs) used in horticulture emit a peak wavelength of approximately 660 nm. R light, in the absence of blue (B; 400–499 nm) light and/or green (G; 500–599 nm) light, yields taller plants (Cope et al. 2014) with greater fresh mass and dry mass (Pennisi et al. 2019; Son and Oh 2013; Spalholz and Hernández 2017) and leaf expansion (Lee et al. 2014; Spalholz and Hernández 2017). For example, ‘Red Oakleaf’ lettuce and ‘Green Oakleaf’ lettuce produced under 100% R light had 90% longer leaves and 45% greater fresh weight than those produced under 80% B light and 20% R light (Spalholz and Hernández 2017). Similarly, Lee et al. (2014) reported 51% and 40% increases in leaf length and width, respectively, and 93% greater shoot fresh weight under monochromatic R light compared with 30% B light and 70% R light for ‘Jeokchima’ lettuce. Additionally, ‘Waldmann’s Green’ lettuce and ‘Cherry Belle’ radish (Raphanus sativus) had taller stems under monochromatic R light at a photosynthetic photon flux density (PPFD; 400–700 nm) between 200 and 500 μmol⋅m−2⋅s−1 compared with plants grown under R+B light (Cope et al. 2014).
Growth and quality of plants grown indoors are typically more similar to those of plants grown under sunlight or a broad light spectrum when B light is added to monochromatic R light (Brown et al. 1995; Goins et al. 1997; Yorio et al. 2001). For example, the addition of B light to R light inhibits stem elongation and leaf expansion (Hernández and Kubota 2016) and increases chlorophyll content (Park and Runkle 2019; Son and Oh 2013) and phytonutrients (Craver et al. 2017; Kopsell et al. 2014; Vaštakaitė-Kairienė et al. 2022); however, it decreases fresh mass and dry mass (Izzo et al. 2021a). In general, plant compactness, biomass accumulation, and, therefore, yield, decrease as the R:B ratio decreases (Pennisi et al. 2019). For example, in ‘Cumlaude’ cucumber (Cucumis sativus) seedlings, height, leaf area, and shoot fresh weight decreased as the proportion of B light increased from 10% to 75% relative to R light, but the chlorophyll content increased (Hernández and Kubota 2016). Similarly, increasing B light from 7% to 66% (offset by decreases in R light) decreased leaf area and shoot fresh weight and increased the SPAD index of ‘Outredgeous’ lettuce (Izzo et al. 2021b).
Light quantity (or the photon flux density) also regulates plant growth and development. The daily light integral (DLI) controls fresh mass and dry mass (Kelly et al. 2020; Matysiak et al. 2022), stem elongation (Dou et al. 2018; Pramuk and Runkle 2005), and leaf size and area (Faust et al. 2005; Kelly et al. 2020) of horticultural crops. For example, as the DLI increased from 6.9 to 15.6 mol⋅m−2⋅d−1, the fresh mass and dry mass linearly increased by 95% and 86% for ‘Rex’ lettuce and by 89% and 68% for ‘Rouxai’ lettuce, respectively (Kelly et al. 2020). In another study, ‘Casual’ romaine lettuce (L. sativa var. longifolium) fresh mass and dry mass and leaf number increased as the DLI increased from 9.2 to 17.3 mol⋅m−2⋅d−1 (Matysiak et al. 2022). In addition, the plant height and diameter of ‘Elizium’ lettuce decreased, but its fresh mass and dry mass and leaf number increased as the DLI increased from 9.2 to 17.3 mol⋅m−2⋅d−1 (Matysiak et al. 2022).
Along with quality and yield, the effects of the growing environment on postharvest longevity (i.e., storage life) also should be considered. It is projected that the world population will reach 10 billion people by 2050. Therefore, we need to produce more food and waste less (Buzby et al. 2014). Postharvest storage research of fresh leafy greens is necessary to develop innovations so food can stay fresh longer. One product that is popular among consumers because of convenience is packaged salad mixes that are ready to eat. In the United States alone, 44% and 43% of survey respondents in 2021 purchased salad mix and lettuce, respectively (The Packer 2022). Sanitation at harvest (Gil et al. 2009) and storing lettuce in light after harvest can increase storage life (Ma et al. 2014). However, despite considerable published research of the indoor production of lettuce, very few studies have investigated how light quantity and quality during production influence postharvest performance.
Min et al. (2021) reported that storing plants for 1 week at the end of production at 10 °C with a PPFD of 110 or 270 μmol⋅m−2⋅s−1 improved the storage life of ‘Expertise RZ Salanova’ leaf lettuce by ∼3 d and ∼4 d, respectively, compared with lettuce exposed to 1 week of darkness. In addition, spinach grown under a DLI of 17 mol⋅m−2⋅d−1 from R+B LEDs (R:B ratio of 2) before storage had a storage life that was 5 d longer at 4 °C than that of plants grown under R+white (W) LEDs (R:B ratio of 7) (Nicole et al. 2017). Similarly, wild arugula (Diplotaxis tenuifolia) plants grown under a DLI of 15 mol⋅m−2⋅d−1 from R+B LEDs had a storage life 2 d longer than that from R+W LEDs when stored in darkness at 4 °C (Nicole et al. 2017). These findings suggest that a higher light intensity, B light fraction, or both during production can extend the storage life of leafy greens. We assessed how DLI and the B light fraction during production influenced leaf characteristics and biomass accumulation as well as the postharvest performance of two frill-leaf lettuce cultivars. We postulated that biomass accumulation would increase with the DLI and decrease with the B light fraction, and that postharvest longevity would increase with the DLI and the B light fraction.
Materials and methods
Plant material.
‘Green Incised’ and ‘Hydroponic Green Sweet Crisp’ Salanova lettuce were selected for study based on commercial grower input indicating a short storage life. On 5 Nov 2021 (Rep. 1) and 8 Nov 2022 (Rep. 2), seeds (Johnny’s Seeds, Winslow, ME, USA) were sown in 200-cell rockwool sheets (AO 25/40 Starter Plugs; Grodan, Milton, ON, Canada) that were presoaked in deionized water at a pH of 4.5, which was adjusted using diluted (1:31) 95% to 98% sulfuric acid (J.Y. Baker, Inc., Phillipsburg, NJ, USA). Rockwool sheets were placed in plastic trays, covered with a clear plastic humidity dome, and placed in a growth canopy in the Controlled Environment Lighting Laboratory at Michigan State University. The air temperature was controlled at 23 °C, the CO2 concentration and relative humidity (RH) were ambient, and warm-white LEDs (peak = 639 nm; correlated color temperature = 2700 K; OSRAM, Munich, Germany) delivered a total photon flux density (400–750 nm) of 180 μmol⋅m−2⋅s−1 for 24 h⋅d−1 (Table 1). After 2 d, the photoperiod was shortened to 20 h⋅d−1 (0200–2200 hr), and humidity domes were removed on day 4. Seedlings were manually sub-irrigated as necessary with a nutrient solution comprising deionized water and water-soluble fertilizer (12N–1.7P–13.3K RO Hydro FeED; JR Peters, Inc., Allentown, PA, USA) and magnesium sulfate (Epsom salt; Pennington, Madison, GA, USA) to provide the following nutrients (in mg⋅L−1): 125 nitrogen (N), 18 phosphorus (P), 138 potassium (K), 73 calcium (Ca), 47 magnesium (Mg), 0.2 boron (B), 0.2 copper (Cu), 1.6 iron (Fe), 0.5 manganese (Mn), 0.4 zinc (Zn), and 35 sulfur (S). The pH and electrical conductivity (EC) setpoints of the nutrient solution were 5.6 and 1.6 mS⋅cm−1, respectively, as measured by a pH and EC meter (HI9814; Hanna Instruments Inc., Woonsocket, RI, USA). The experiment was performed twice. The mean ± SD for air temperature, CO2 concentration, and RH during the seedling phase (measured as described later) were 22.8 ± 0.4 °C, 408 ± 26 μmol⋅mol−1, and 33% ± 4% for Rep. 1, and 21.8 ± 0.3 °C, 438 ± 26 μmol⋅mol−1, and 27% ± 5% for Rep. 2, respectively.
The actual daily light integral (DLI; mol⋅m−2⋅d−1) and light quality characteristics for lettuce grown as seedlings and after transplanting under blue (B; 400–499 nm), green (500–599 nm), red (R; 600–699 nm), and far-red (700–750 nm) light-emitting diodes. Subscripted numbers represent the percentage of each waveband.
Growing culture and environment.
Seedlings were transplanted on 15 Nov 2021 (Rep. 1) and 18 Nov 2022 (Rep. 2) to two vertical hydroponic growing racks, each with three canopies, to receive six various lighting treatments. Each treatment included 15 lettuce plants of each cultivar that were grown in 36-cell polystyrene rafts (61 × 122 × 2.5 cm; Beaver Plastics, Ltd., Acheson, AB, Canada) with 20-cm horizontal and 15-cm diagonal centers that floated in the deep-culture hydroponic system (Active Aqua AAHR24W; Hydrofarm, Petaluma, CA, USA). Each rack was supplied with a 75-L reservoir to supply the hydroponic nutrient solution, which consisted of deionized water supplemented with the same water-soluble fertilizer and magnesium sulfate as mentioned previously to provide (in mg⋅L−1): 150 N, 22 P, 166 K, 87 Ca, 58 Mg, 0.2 B, 0.2 Cu, 1.9 Fe, 0.6 Mn, 0.01 molybdenum (Mo), 0.4 Zn, and 43 S. The nutrient solution was continuously recirculated and oxygenated with an air stone (20.3 × 2.5 cm; Active Aqua AS8RD; Hydrofarm) and a 60-W air pump (Active Aqua AAPA70L; Hydrofarm); the pH was adjusted with diluted (1:31) 95% to 98% sulfuric acid or potassium bicarbonate to decrease or increase the pH, respectively, using the same pH and EC meter as previously described. The pH and EC were measured daily, and the means ± SD were 5.7 ± 0.3 and 1.9 ± 0.2 mS⋅cm−1 (Rep. 1) and 5.8 ± 0.3 and 1.9 ± 0.1 mS⋅cm−1 (Rep. 2), respectively.
A wireless thermostat controller (Honeywell International, Inc., Morris Plains, NJ, USA) operated the ventilation and air conditioning units (HBH030A3C20CRS; Heat Controller, LLC., Jackson, MI, USA) to control airflow and maintain the air temperature setpoint. Thermocouples (0.13-mm type E; Omega Engineering, Inc., Norwalk, CT, USA), infrared temperature sensors (OS36–01-K-80 F; Omega Engineering, Inc.), light quantum sensors (LI-190R; LI-COR, Inc., Lincoln, NE, USA), CO2 sensors (GMD20; Vaisala, Inc., Louisville, CO, USA), and RH and temperature probes (HMP110; Vaisala, Inc.) were positioned in representative areas in the growth room to measure and record the environmental variables. Data were collected by a datalogger (CR1000; Campbell Scientific, Inc., Logan, UT, USA) coupled with a multiplexer (AM16/32B; Campbell Scientific, Inc.) and logged every 10 s; averages were recorded hourly. The mean ± SD air temperature, CO2 concentration, and RH were 22.5 ± 0.2 °C, 402 ± 17 μmol⋅mol−1, and 38% ± 6% for Rep. 1, and 22.8 ± 0.2 °C, 418 ± 28 μmol⋅mol−1, and 41% ± 6% for Rep. 2, respectively.
Lighting treatments.
For 15 d, plants were grown under six lighting treatments delivered by B (peak = 447 nm) and R (peak = 660 nm) LEDs that were housed together in adjustable LED fixtures (PHYTOFY RL; OSRAM SYLVANIA Inc., Munich, Germany) controlled in increments of 1 μmol⋅m−2⋅s−1 via manufacturer software (PHYTOFY RL; OSRAM SYLVANIA Inc.). Each canopy was equipped with three LED fixtures positioned 43 cm above each treatment with 41-cm center spacing for light uniformity. Lighting treatments delivered a PPFD of either 167 or 250 μmol⋅m−2⋅s−1 for 20 h⋅d−1 (DLI of 12 or 18 mol⋅m−2⋅d−1, respectively) in B and R light ratios (subscripts in %) of B5:R95, B20:R80, or B35:R65 (Table 1; Fig. 1).
The photon flux density and distribution of six lighting treatments that delivered daily light integrals of 12 or 18 mol⋅m−2⋅d−1 using blue (B; 400–499 nm) and red (R; 600–699 nm) light-emitting diodes. The subscript following each waveband indicates its percentage.
Citation: HortScience 59, 7; 10.21273/HORTSCI17843-24
Data collection.
Twelve plants of each cultivar and treatment were harvested 15 d after transplanting on 30 Nov 2021 (Rep. 1) and 3 Dec 2022 (Rep. 2) to assess plant and leaf morphology, relative chlorophyll content, and shoot fresh weight. Lettuce plants were excised at the base of the stem from the rockwool cube, and height was measured with a ruler from the base to the tallest point of the plant. The plant diameter was taken from the tip of one leaf to the opposite tip of another leaf across the plant with a ruler. Plants were weighed using an analytical scale to obtain fresh weight. The sixth fully expanded leaf (>5 cm) was excised, and the leaf length and width were measured from the tallest and widest points, respectively. From that same leaf, the relative chlorophyll concentration (SPAD) was measured at three different locations on the leaf surface using a chlorophyll meter (MC-100; Apogee, Logan, UT, USA), and the averages were recorded.
Postharvest storage.
After harvesting, the leaves of each plant were removed from the stem, placed in a 19-L container, and gently mixed into a homogenous sample for each treatment. Then, 20 g of lettuce leaves were weighed and placed in an air-tight clear plastic clamshell container (volume, 177 mL; Clear Hinged Deli Container; Genpak, Charlotte, NC, USA) for the postharvest analysis, with six containers in each treatment. Then, the containers were placed in a cardboard box to limit light exposure and remained in a walk-in cooler at an air temperature of 7 °C for 13 d (Rep. 1) or 17 d (Rep. 2). Postharvest storage at 7 °C was used to accelerate product decay. On days 5, 7, 9, 11, and 13 (and on days 15 and 17 for Rep. 2), each container was opened and the visual quality was assessed by two evaluators using the decay and wilting rating scales of Kader et al. (1973), with scores ranging from 1 to 9 (1 = excellent quality; 4 = threshold for storage life/salability; and 9 = extremely poor quality).
Experimental design and data analysis.
The experiment was set as a completely random design in a factorial arrangement with six treatment combinations [three light quality treatments (B:R ratios) and two DLI treatments]. The experiment was performed twice over time. Data were pooled when the interaction of replication × treatments was not significant, or when data followed similar trends. Cultivars were analyzed separately, and PROC GLIMMIX was used to perform a two-way analysis of variance and means separation with Tukey’s honestly significant difference test (P ≤ 0.05). Regression equations were fitted using PROC REG with SAS (version 9.4; SAS Institute Inc., Cary, NC, USA).
Results
Growth.
The DLI and light quality influenced almost all measured variables of ‘Green Incised’; DLI did not influence the leaf width (Table 2; Fig. 2A). The DLI and light quality interacted to influence the plant height and diameter, leaf length, and SPAD value for ‘Green Incised’, but not the fresh weight, leaf width, or leaf number. Plants were up to 42% taller and 48% wider under B5:R95 and a DLI of 12 mol⋅m−2⋅d−1 (low DLI) than they were under the high DLI (18 mol⋅m−2⋅d−1) and B light fractions (Table 3). The shoot fresh weight of ‘Green Incised’ was 8% greater and 48% greater under B5:R95 compared with that under B20:R80 and B35:R65, respectively, and it was 23% greater under the higher DLI than it was under the lower DLI; it linearly decreased as the B light fraction increased (Fig. 3A). Leaves were 12% to 60% longer under B5:R95 at a DLI of 12 mol⋅m−2⋅d−1 than they were under all other treatments and decreased linearly (low DLI) or quadratically (high DLI) with an increasing B light fraction (Fig. 3C). Leaf width also decreased linearly by 21% as the B light fraction increased (Fig. 3E). Although plants under B5:R95 and B20:R80 developed 10% more leaves than those under B35:R65 or developed 4% more leaves under the high DLI than under the low DLI, this difference was minimal for production outcomes. Regardless of the DLI, the SPAD value was greatest under the highest B light fraction.
Analysis of variance of the effects of the daily light integral (DLI), light quality (LQ), and their interaction on the morphological traits of ‘Green Incised’ lettuce and ‘Hydroponic Green Sweet Crisp’ lettuce.
‘Green Incised’ (A) and ‘Hydroponic Green Sweet Crisp’ (B) frill-leaf lettuce after 14 d under six sole-source lighting treatments that provided a daily light integral of 12 or 18 mol⋅m−2⋅d−1 and blue (B) and red (R) light in proportions (%) of B5:R95, B20:R80, or B35:R65.
Citation: HortScience 59, 7; 10.21273/HORTSCI17843-24
The means of morphological traits for ‘Green Incised’ lettuce and ‘Hydroponic Green Sweet Crisp’ lettuce grown under a daily light integral (DLI) of 12 or 18 mol⋅m−2⋅d−1 provided by blue (B; 400–499 nm) and red (R; 600–699 nm) light, with subscripted numbers representing their percentages. Capital and lowercase letters that are different from each other are statistically significant between rows and columns, respectively, for each measured variable according to Tukey’s honestly significant difference test (P ≤ 0.05).
The fresh weight (A and B), leaf length (C and D), and leaf width (E and F) of ‘Green Incised’ and ‘Hydroponic Green Sweet Crisp’ frill-leaf lettuce after 14 d under a 5%, 20%, or 35% blue light fraction and two daily light integrals. Each symbol represents the mean [n = 24 or 48 (pooled only)], and error bars are the SEM. Linear and quadratic regression equations and the corresponding coefficient of determination (r2) values are provided for each daily light integral (or pooled daily light integral) per measurement and cultivar. ***Significant at P ≤ 0.001.
Citation: HortScience 59, 7; 10.21273/HORTSCI17843-24
The DLI only influenced the fresh weight and leaf number of ‘Hydroponic Green Sweet Crisp’, whereas light quality influenced all measured variables except for SPAD (Table 2; Fig. 2B). There was only an interactive effect of DLI and light quality on shoot fresh weight. Light quality had the only influence on height and diameter; plants were 16% to 29% taller and 17% to 29% wider under B5:R95 compared with those under B20:R80 and B35:R65 (Table 3). Shoot fresh weight was 24% to 78% greater under B5:R95 and a DLI of 18 mol⋅m−2⋅d−1 than it was under the other treatments, and it linearly decreased as the proportion of B light increased and DLI decreased (Fig. 3B). Under B5:R95, leaves were 23% and 35% longer and 16% and 21% wider compared with those under B20:R80 and B35:R65, respectively (Fig. 3D and F). ‘Hydroponic Green Sweet Crisp’ had 8% or 9% more leaves when grown under B5:R95 than when grown under higher B light fractions, and it had 5% more leaves when grown under the high DLI than when grown under the lower DLI. The SPAD value was not influenced by DLI, light quality, or their interaction (Table 3).
Storage life.
‘Green Incised’ grown under the higher DLI and highest B light fraction (18 mol⋅m−2⋅d−1 and B35:R65) had a storage life (threshold score of 4) of 9.5 d at 7 °C for Rep. 1 (∼2.5–4.0 d shorter than that under all other treatments); for Rep. 2, the storage life was 11.4 d at 7 °C (1.4–3.6 d shorter than that under all other treatments) (Fig. 4A and B). Compared with all other treatments, as time passed, the B35:R65 and high DLI treatment showed faster decay on days 9, 11, and 13 (Rep. 1) and on day 15 (Rep. 2). In addition, ‘Green Incised’ grown under B5:R95 at either DLI or B20:R80 at the high DLI had less visual deterioration on day 13 compared with that grown under B35:R65 at either DLI (Rep. 1) while also remaining below the threshold for storage life.
The storage life of ‘Green Incised’ and ‘Hydroponic Green Sweet Crisp’ frill-leaf lettuce for replication 1 (A and C) and replication 2 (B and D). The score scale increased from 1 to 9 (1 = excellent quality; 9 = extremely poor quality). A score of 4 was considered the threshold for salability. Each symbol represents the mean (n = 6 or 12 when pooled), and error bars are the SEM for each treatment. Different letters represent a significant difference according to Tukey’s honestly significant difference test (P ≤ 0.05) between days.
Citation: HortScience 59, 7; 10.21273/HORTSCI17843-24
For ‘Hydroponic Green Sweet Crisp’, the storage life for all lighting treatments was 12.6 to 13.3 d for Rep. 1; for Rep. 2, the storage life was 13.5 to 14.3 d (Fig. 4C and D). During Rep. 1, ‘Hydroponic Green Sweet Crisp’ grown under the B5:R95 low DLI treatment had faster visual decay than that grown under all other treatments on day 9, and that grown under B5:R95 (high DLI) or B35:R65 (low DLI) on day 11. In contrast, for Rep. 2, the rate of decay for plants grown under the B35:R65 and high DLI treatment increased over time and was greater on day 17 compared to that under all other treatments. However, all treatments had surpassed the threshold of salability before day 17.
Discussion
Growth characteristics.
The height and diameter of both frill-leaf lettuce cultivars in this study decreased as the B light fraction increased from 5% to 35%. Similarly, the semi-head lettuce ‘Azart’ was 41% more compact as the B light fraction increased from 7% to 34% (Yudina et al. 2022). In addition, the height of ‘Buttercrunch’ bibb-type lettuce and ‘Vates Blue Curled’ kale (Brassica oleracea var. acephala) decreased as B light increased from 5% to 17% (Naznin et al. 2019). The plant diameter decreased as the DLI independently increased in ‘Green Incised’, but not in ‘Hydroponic Green Sweet Crisp’. We attribute this discrepancy to the differences in the cultivar response, which are common in lettuce. For example, the DLI did not influence the diameter of ‘Casual’ lettuce, but ‘Elizium’ was more compact under a DLI of 17.3 mol⋅m−2⋅d−1 than it was under a DLI ≤13.8 mol⋅m−2⋅d−1 (Matysiak et al. 2022).
Shorter and more compact plants produced under high fractions of B light or a lower DLI were associated with less shoot fresh weight during the current study. Similarly, Izzo et al. (2021a) reported a 47% reduction in the shoot fresh weight of ‘Waldmann’s Green’ lettuce as B light increased from 7% to 66%. Likewise, the shoot fresh weights of ‘Sunmang’ lettuce and ‘Grand Rapids TBR’ lettuce were greater under 100% R light than they were when treatment included 13% to 100% B light (Son and Oh 2013). The fresh weights of ‘Casual’ lettuce and ‘Elizium’ lettuce increased when the DLI increased from 9.2 to 17.3 mol⋅m−2⋅d−1 and from 9.2 to 13.8 mol⋅m−2⋅d−1, respectively (Matysiak et al. 2022). In addition, the fresh weight of ‘All Star’ lettuce mix increased by 241% as the DLI increased from 8 to 14 mol⋅m−2⋅d−1 (Baumbauer et al. 2019).
The decrease in lettuce growth we observed under higher B light fractions can be attributed to fewer and smaller leaves compared with plants grown with less B light. Smaller leaves capture fewer photons than larger ones, which can limit overall plant growth (Bugbee 2017). These responses are consistent with ‘Outredgeous’ lettuce and ‘Waldmann’s Green’ lettuce, for which the leaf area decreased by 46% and leaf number decreased by 20%, respectively, when grown under 66% B light compared with 7% B light (Izzo et al. 2021a, 2021b). Cope and Bugbee (2013) found comparable results for leaf area, which decreased in ‘Hoyt’ soybean (Glycine max) and ‘Cherry Belle’ radish as B light increased from 10% to 28%. When accounting for the effects of the DLI on leaf growth in our study, only the leaf length of ‘Green Incised’ lettuce and leaf number of ‘Green Incised’ lettuce and ‘Hydroponic Green Sweet Crisp’ lettuce were influenced. The leaves of ‘Green Incised’ were shorter under the higher DLI, which was consistent with the results of some previous reports, but not those of others. Similar to our study, ‘Ziwei’ lettuce had 19% shorter leaves as the DLI increased from 5 to 15 mol⋅m−2⋅d−1 when grown under W+R LEDs that provided a R:B ratio of 2.7 (Yan et al. 2019). Similarly, the leaf length of ‘Rex’ lettuce decreased as the DLI increased from 6.9 to 15.6 mol⋅m−2⋅d−1 with a 16-h photoperiod (Kelly et al. 2020). However, although leaves were shorter as the DLI increased, there was no statistical difference in leaf length between DLIs of 10.4 to 15.6 mol⋅m−2⋅d−1 when the photoperiod was 24 h⋅d−1 (Kelly et al. 2020). Additionally, there was no consistent effect of the DLI on the leaf length of ‘Rouxai’ lettuce (Kelly et al. 2020). For both ‘Green Incised’ and ‘Hydroponic Green Sweet Crisp’, the leaf number slightly increased as the DLI increased. Similarly, the leaf number of ‘Ziwei’ lettuce increased by 9% as the DLI increased from 5 to 15 mol⋅m−2⋅d−1, and ‘Casual’ lettuce and ‘Elizium’ lettuce had 36% and 27% more leaves, respectively, as the DLI increased from 9.2 to 17.3 mol⋅m−2⋅d−1 (Matysiak et al. 2022; Yan et al. 2019).
Based on previous studies, we predicted an increase in the chlorophyll concentration (or SPAD) with increasing fractions of B light, DLI, or both, and this occurred with ‘Green Incised’, but not with ‘Hydroponic Green Sweet Crisp’. Similar to ‘Green Incised’, the relative chlorophyll concentrations of ‘Cherokee’ lettuce and ‘Waldmann’s Green’ lettuce increased as B light increased from 0% to 66% (Clavijo-Herrera et al. 2018). Furthermore, ‘Waldmann’s Green’ lettuce, ‘Cherry Belle’ radish, and ‘California Wonder’ pepper (Capsicum annuum) grown under a DLI of 28.8 mol⋅m−2⋅d−1 had greater chlorophyll concentrations than those grown under a DLI of 11.5 mol⋅m−2⋅d−1 (Cope et al. 2014). It is possible that the B light fractions (5%–35%) and DLIs (12 and 18 mol⋅m−2⋅d−1) in our study were too similar to elicit differences in SPAD in ‘Hydroponic Green Sweet Crisp’.
Storage life.
In general, the storage life of ‘Green Incised’ was shorter when grown under B35:R65 and a DLI of 18 mol⋅m−2⋅d−1 compared with the other lighting treatments. Typically, the lettuce in this treatment presented earlier wilting, browning, water-soaked decay, and odor compared with the other treatments. This could be attributed to a decline in the carbohydrate concentration, which would decrease the scavenging of radical oxygen species. Although carbohydrates were not measured during the current study, usually they deteriorate in darkness and allow reactive oxygen species (e.g., hydrogen peroxide) to accumulate and cause tissue browning, senescence, and a shorter storage life (Iakimova and Woltering 2015; Woltering and Seifu 2014; Woltering and Witkowska 2016). To our knowledge, the interaction of the B light fraction × DLI on storage life has not been previously studied. However, there is some evidence that preharvest B light or DLI independently influences the storage life of fresh foods. For example, the storage life of sweet basil (Ocimum basilicum) was the shortest when grown under B60:R40 light, and it was extended by 2 d at 20 °C under G20:R80 light (Jensen et al. 2018). In contrast, the storage life of arugula and lettuce at 4 °C was longer under 35% B light from R+B LEDs than it was with R and W LEDs or treatments with far-R light (Nicole et al. 2017). It is possible that the decrease in storage life under the highest B light fraction was attributable to the greater dark respiration; dark respiration in ‘Azart’ lettuce increased as B light increased from 7% to 34% (Yudina et al. 2022). When fresh foods are stored, excessive respiration typically decreases the quality and storage life (Fonseca et al. 2002). However, we did not measure respiration. More research of this topic is necessary.
In contrast to our findings, higher DLIs extended the storage life of basil, cucumber, and lettuce (Larsen et al. 2022; Lin and Jolliffe 1996; Woltering and Witkowska 2016). For example, butterhead and iceberg lettuce grown under a PPFD of 250 μmol⋅m−2⋅s−1 from W fluorescent light with a 12-h photoperiod (DLI of 10.8 mol⋅m−2⋅d−1) had a longer storage life when placed in darkness at 12 °C compared with that of plants grown under approximately half as much light (Woltering and Witkowska 2016). When initially harvested, the butterhead lettuce grown under the higher DLI had more chlorophyll than plants grown under the low DLI. However, for two types of fresh-cut butterhead and iceberg lettuce, plants that were grown under the low DLI and subsequently stored in darkness exhibited a greater decrease in chlorophyll fluorescence and, thus, damage to photosystem II and chlorophyll degradation than plants grown under the higher DLI (Woltering and Witkowska 2016). In our study, ‘Green Incised’ grown under the higher DLI (18 mol⋅m−2⋅d−1) had a higher SPAD value at harvest than that grown under the lower DLI, but it had a slightly shorter storage life. Conversely, there was no treatment effect on the SPAD value of ‘Hydroponic Green Sweet Crisp’, which could be why there was little to no visible difference in degradation among treatments during postharvest storage. However, we did not measure chlorophyll fluorescence or SPAD during the postharvest stage to confirm the correlation of chlorophyll and tissue degradation. Such measurements would be useful during future experiments.
To conclude, lighting regimens used to grow fresh leafy green vegetables indoors impact growth and, to some extent, storage life. Different cultivars of frill-leaf lettuce responded somewhat differently to postharvest storage. Further studies should be performed to investigate how the light environment influences the subsequent respiration rate, chlorophyll fluorescence and concentration, and carbohydrate status before and during postharvest storage to gain a better understanding of why the high B with high DLI treatment led to quicker decomposition in ‘Green Incised’ but not in ‘Hydroponic Green Sweet Crisp’. Lettuce grown indoors under a low to moderate B light fraction (B5:R95 or B20:R80) and a relatively high DLI of 18 mol⋅m−2⋅d−1 had the greatest biomass among the treatments studied, and this treatment did not negatively affect subsequent storage life.
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