Abstract
Lettuce (Lactuca sativa L.) is one of the most consumed fresh vegetables in the United States. However, lettuce production is heavily limited to California and Arizona, posing a high risk to the supply chain. Hydroponic production is a soilless cultivation method and provides a sustainable alternative to growing lettuce in the field. Light is a critical factor in plant development, and light quality highly affects plant morphogenesis. The goals of this study were 2-fold, with the first to investigate the growth of 26 lettuce cultivars under a hydroponic system supplemented with fluorescent light to determine adaptability. Subsequently, the second goal was to determine how light-emitting diodes (LEDs) affect lettuce plant morphology and photosynthesis compared with fluorescent light for four lettuce cultivars. Results showed that 23 of 26 lettuce cultivars were grown successfully using a hydroponic system. However, lettuce grown under fluorescent light experienced stem elongation—a morphological response to low-light conditions known as shade avoidance syndrome. Stem elongation decreased significantly under LED light, whereas other morphological characteristics remained relatively the same between the two light treatments. Although there were no differences in dry weight and leaf area, the carbon assimilation rate increased significantly in lettuce cultivars Coastal Star, Muir, Green Butter, and Rouge d’Hiver when treated with LED light. Correspondingly, intercellular carbon dioxide (CO2) decreased in these four lettuce cultivars under the LED light treatment. Our study results indicate that LED light increased photosynthetic activity and reduced stem elongation to enhanced lettuce quality.
Consumption of fruits and vegetables is known to lower the risk of cancer and cardiovascular diseases (Llorach et al. 2008; Nicolle et al. 2004). Lettuce (Lactuca sativa L.) belongs to the Asteraceae family and is one of the numerous vegetables used in different culinary aspects. Lettuce contains vitamins, dietary fiber, polyphenols, and phytochemicals, many of which are valuable bioactive components and antioxidants (Kim et al. 2016; Llorach et al. 2008; Mampholo et al. 2016; Yang et al. 2021). Lettuce is a cool-season crop and grows best in daytime temperatures between 18 and 25 °C, and in nighttime temperatures between 7 and 15 °C, thus limiting the time and regions where it can be grown in soil (Marklein et al. 2020). The restriction in planting area and the demand for lettuce promoted soilless cultivation systems such as hydroponics to expand production. Unlike absorption of nutrients from the soil, plants roots are suspended in nutrient-rich solutions to maximize nutrient uptake when grown in a hydroponic system (Lei and Engeseth 2021; Maucieri et al. 2019; Sambo et al. 2019). Typically, a hydroponic infrastructure is placed in a controlled environment (greenhouse) to help reduce soil-borne pests and diseases, and to protect against fluctuations in weather patterns (Lei and Engeseth 2021; Suo et al. 2021; Voutsinos et al. 2021). In addition, the proximity of lettuce production to the consumers saves time and labor in delivering high-quality fresh products. Hydroponic systems also contribute to sustainable agriculture practices by providing environmentally friendly food by reducing the carbon footprint, and pesticide and water use, while increasing production.
Irradiance is a critical environmental factor that affects plant growth and development. The variation in light intensity, photoperiod, and spectra can induce numerous physiological responses that shape plant morphogenesis (Hasan et al. 2017; Park and Runkle 2018; Son et al. 2016). For example, plants typically display shade avoidance responses, such as internode elongation and early flowering under low photosynthetic photon flux density (PPFD) (Hersch et al. 2014; Park and Runkle 2017; Ruberti et al. 2012). In contrast, a high PPFD can improve the net photosynthetic rate to promote plant growth (Kelly et al. 2020). In addition, specific wavelengths of light may enhance the production of secondary metabolites and other biologically active compounds (Hasan et al. 2017; Wollaeger and Runkle 2015). The rapid advancement in LED lighting technologies, such as the precise control of lighting spectra and lower power consumption, have placed it at the forefront of light choices in the controlled-environment agricultural sector. Many LED studies focused on the optimization of blue (400–500 nm) and red (600–700 nm) spectral fractions because they are related to the absorption peaks of chlorophylls a and b, which are critical to the photosynthetic performance of plants (An et al. 2020; Contreras et al. 2009; Huang et al. 2018; Kaiser et al. 2019; Wojciechowska et al. 2019). However, recent studies have indicated that green light (500–600 nm) may also contribute to positive effects on plant growth and development in addition to red and blue light (Folta and Maruhnich 2007; Li et al. 2021).
The first objective of our study was to evaluate 26 lettuce cultivars produced in a hydroponic system under fluorescent light. Subsequently, four lettuce cultivars were selected to determine how LED light programmed with variations in red, blue, and green light influenced hydroponically grown lettuce compared with fluorescent light. Our results will provide valuable information on the adaptability of lettuce cultivars for production in hydroponic systems, and how light quality may affect cultivar growth and quality. In addition, how different combinations of red, blue, and green light affects hydroponically grown lettuce may facilitate the advancement of the controlled-environment agricultural industry sector.
Materials and Methods
Plant materials and light treatments.
Seeds of 26 different lettuce cultivars were purchased from Fedco Seeds (Clinton, ME, USA) and Johnny’s Selected Seeds (Fairfield, ME, USA) to determine suitability for growth in a hydroponic system. The name of the cultivar, lettuce type, and corresponding breeder can be found in Supplemental Table S1. Seeds of these cultivars were germinated in Grodan rock wool cubes (Milton, Ontario, Canada) under a Sun Blaze T5 HO46 (Vancouver, WA, USA) fluorescent lamp with a photoperiod of 16 h of light and a constant light intensity of 120 μmol⋅m–2⋅s–1. Five-day old seedlings were transplanted randomly to net pots in the hydroponic system under the same photoperiod and growth light conditions. Three seedlings of each variety were used for the experiment, and the experiment was repeated twice. The hydroponic system was set up using 6-inch-diameter polyvinylchloride pipes connected to a pump and a water reservoir, with added Peters fertilizer 5–11–26 (Summerville, SC, USA). The added concentrations of nitrogen, phosphorus, and magnesium were 150, 50, and 50 mg/L respectively. The planting hole diameter was 3 inches, and holes were spaced 6 inches evenly apart. Two hydroponic systems were placed in an iPower Hydroponic Mylar Grow Tent (Irwindale, CA, USA) at 25 °C and 60% relative humidity. The mature lettuce heads and roots were collected after 45 d, and the fresh and dry weights were recorded.
Four hydroponically grown lettuce cultivars with the heaviest heads for romaine (L. sativa L. var. longifolia) and butterhead (L. sativa var. capitata) lettuce were selected for the LED light treatment study. Cultivars Coastal Star and Rouge d’Hiver were representatives of romaine lettuce, and cultivars Muir and Green Butter represented butterhead lettuce. The seed germination procedure was the same as described previously. Four plants from each cultivar were transplanted randomly to the hydroponic system as one replicate under two light treatments. Either fluorescent light or the Heliospectra Rx30 LED light was used, and three replicates were carried out for each light treatment. Light fixtures were suspended at a height of 1.52 m above the hydroponic growing system. For fluorescent light, PPFD was 120 μmol⋅m–2⋅s–1 at the top of the polyvinylchloride pipe. For LED light, the intensity at 450-, 660-, and 570-nm wavelengths was optimized to mimic the natural spectrum of light intensity from dawn to dusk, with a total of 16 h of light. The specific light intensity throughout 24 h is presented in Supplemental Fig. S1. The photoperiod, temperature, and relative humidity conditions remained the same, as mentioned earlier, for both light treatments. The weight measurements were recorded after 35 d under the light treatment to minimize leaf senescence.
Morphological and photosynthesis measurements.
The response of net photosynthesis to incident PPFD was measured using the third leaf from the top of the plant of six randomly selected 21-day-old lettuce plants from each cultivar under different light treatments. A portable photosynthesis instrument (LI-6800; Li-COR, Lincoln, NE, USA) with a 2- × 3-cm light source chamber was used to conduct leaf-level gas exchange measurements. Light-saturated net CO2 assimilation rates were measured at 25 °C with a relative humidity of 40% to 50% under 1000 μmol⋅m–2⋅s–1 PPFD, 400 ppm reference CO2, and a flow rate of 500 μmol⋅s–1. All photosynthetic measurements were recorded ∼3 to 5 min or when CO2 concentrations in the sample infrared gas analyzer had stabilized between 0930 and 1430 HR.
Lettuce head fresh weight (HFW) was recorded immediately after the root was detached at harvest, and root fresh weight (RFW) was measured after dry-patting with a paper towel to remove excess water. Stem length was measured from the base of the lettuce to the tip with a standard ruler. Afterward, all the leaves were pulled from the stem and passed through a LICOR-3100 area meter to determine the total leaf area. The leaves, stem, and root were then dried in a conventional oven at 65 °C for 3 days, and dry weight was recorded subsequently. The root-to-shoot (R/S) ratio was determined by normalizing the dry weight using the square root method before dividing the dry root weight by the dry shoot weight (Agren and Franklin 2003).
Experimental design and statistical analysis.
A one-way analysis of variance followed by Tukey’s honestly significant difference test were used to compare significant differences at P < 0.05 among means for cultivar head weight under the hydroponic system. The data comparing fluorescent light and LED light were analyzed using Student’s t test. All statistical analyses and graphs were produced using RStudio Version 3.6.1 (Boston, MA, USA).
Results
Fresh and dry weight, and R/S of 26 hydroponically grown lettuce cultivars.
Most romaine lettuce thrived under the hydroponic system, in which the average HFW was between 22 and 46 g when harvested after 45 d. The heaviest HFW belongs to ‘Coastal Star’ at 80.6 g—a significant difference from the poorest performing romaine lettuce, which was ‘Jericho’ at 8.5 g. Following romaine, looseleaf lettuce HFWs ranged from 24 to 45 g. ‘Green Salad Bowl’ was heaviest among the looseleaf at 50.9 g, whereas ‘Lollo Rossa’ was not able to grow past the seedling stage and died soon afterward (Fig. 1B). Most butterhead cultivar average HFW was lighter when compared with romaine and looseleaf. The heaviest butterhead was ‘Red Iceberg’ at 34.9 g, whereas both ‘All Year Round’ and ‘Little Gem’ were less than 4 g. Even when the cultivars were a cross between butterhead and looseleaf, the cultivars only weighed between 11 and 34 g. However, the cultivar Blushed Butter, which is a cross between butterhead and romaine achieved a relatively heavy HFW of 71.4 g under the hydroponic system (Fig. 1B). ‘Coastal Star’ not only had the heaviest HFW, but also produced the heaviest head dry weight of 5.8 g, followed by ‘Blushed Butter COS’ and ‘Green Salad Bowl’, which were 4.8 and 3.8 g, respectively (Supplemental Fig. S3A). The three heaviest HFW cultivars—Coastal Star, Blushed Butter, and Green Salad Bowl—had the heaviest average RFWs of 24.0, 23.0, and 20.5 g, respectively (Fig. 1C). However, more than half the lettuce cultivar RFWs were less than 10 g (Fig. 1C). Interestingly, the three heaviest HFW cultivars had an R/S ratio less than 0.3, whereas ‘Truchas’, ‘Red Butter’, and ‘Red Oakleaf’ had the highest ratio, which was more than 1.0 (Fig. 1A).
Mean weight measurements of (A) the root-to-shoot ratio, (B) the fresh head weight, and (C) the fresh root weight of each lettuce cultivar. Legend denotes color represented by different lettuce types.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Mean weight measurements of (A) the root-to-shoot ratio, (B) the fresh head weight, and (C) the fresh root weight of each lettuce cultivar. Legend denotes color represented by different lettuce types.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Mean weight measurements of (A) the root-to-shoot ratio, (B) the fresh head weight, and (C) the fresh root weight of each lettuce cultivar. Legend denotes color represented by different lettuce types.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
‘Coastal Star’, ‘Muir’, ‘Green Butter’, and ‘Rouge d’Hiver’ growth parameters under fluorescent light vs. LED light.
‘Coastal Star’, ‘Muir’, ‘Green Butter’, and ‘Rouge d’Hiver’ grown under the LED light treatment had average head HFWs of 52, 31, 30, and 50 g when harvested after 35 d, respectively. Of the four cultivars, only ‘Green Butter’ HFW was significantly less under the LED light treatment compared with the fluorescent light treatment (Fig. 2A). In addition, there was no significant difference in dry head weight between the fluorescent and LED light treatments for all four cultivars (Fig. 2C). The average RFWs of ‘Coastal Star’, ‘Muir’, ‘Green Butter’, and ‘Rouge d’Hiver’ were 17.5, 11.7, 12.0, and 17.0 g under the LED light, respectively. Although the fresh and dry root weights were slightly greater under the LED light treatment for ‘Coastal Star’, ‘Muir’, and ‘Rouge d’Hiver’, they were not significantly different from the fluorescent light treatment (Fig. 2B and D). The stem length was visually shorter under the LED light treatment in all four cultivars when compared with the fluorescent light treatment (Fig. 3).
Mean weight measurements of (A) fresh head weight, (B) fresh root weight, (C) dry head weight, and (D) dry root weight for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment from a Sun Blaze T5 HO46 fluorescent lamp (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 μmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment from a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. *Significant difference from lettuce grown under fluorescent light at P < 0.05 (t test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Mean weight measurements of (A) fresh head weight, (B) fresh root weight, (C) dry head weight, and (D) dry root weight for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment from a Sun Blaze T5 HO46 fluorescent lamp (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 μmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment from a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. *Significant difference from lettuce grown under fluorescent light at P < 0.05 (t test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Mean weight measurements of (A) fresh head weight, (B) fresh root weight, (C) dry head weight, and (D) dry root weight for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment from a Sun Blaze T5 HO46 fluorescent lamp (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 μmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment from a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. *Significant difference from lettuce grown under fluorescent light at P < 0.05 (t test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Representative images of lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent (left) vs. light-emitting diode (LED) (right) light treatments.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Representative images of lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent (left) vs. light-emitting diode (LED) (right) light treatments.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Representative images of lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent (left) vs. light-emitting diode (LED) (right) light treatments.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
On average, the LED light treatment reduced stem length by 50% in ‘Costal Star’, 54% in ‘Green Butter’, 57% in ‘Muir’, and—most notably—by 77% in ‘Rouge d’Hiver’, when compared with the same cultivars under the fluorescent light treatment (Fig. 4B). ‘Green Butter’ had the greatest number of leaves, with ∼45 leaves under both light conditions, whereas the average number of leaves for the remaining three cultivars was between 30 and 35 (Fig. 4C). ‘Coastal Star’ and ‘Rouge d’Hiver’ had the greatest total leaf area, averaging 1000 cm2, whereas the total leaf area for ‘Muir’ and ‘Green Butter’ ranged from 700 to 900 cm2 (Fig. 4A). However, there was no difference in both the number of leaves and the total leaf area between the LED light and fluorescent light treatments. When treated with LED light, the R/S ratio in ‘Coastal Star’, ‘Muir’, ‘Green Butter’, and ‘Rouge d’Hiver’ increased slightly, but was not significantly different from the fluorescent light treatment (Supplemental Fig. S2).
Mean morphological measurements of (A) total leaf area, (B) stem length, and (C) number of leaves for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown a in hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment was provided by a Sun Blaze T5 HO46 (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 μmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment was provided using a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. Significant difference from lettuce grown under fluorescent light at *P < 0.05 and **P < 0.01 (t-test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Mean morphological measurements of (A) total leaf area, (B) stem length, and (C) number of leaves for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown a in hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment was provided by a Sun Blaze T5 HO46 (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 μmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment was provided using a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. Significant difference from lettuce grown under fluorescent light at *P < 0.05 and **P < 0.01 (t-test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Mean morphological measurements of (A) total leaf area, (B) stem length, and (C) number of leaves for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown a in hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment was provided by a Sun Blaze T5 HO46 (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 μmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment was provided using a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. Significant difference from lettuce grown under fluorescent light at *P < 0.05 and **P < 0.01 (t-test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
‘Coastal Star’, ‘Muir’, ‘Green Butter’, and ‘Rouge d’Hiver’ photosynthesis under fluorescent light vs. LED light.
Assimilation rate increased significantly for all four lettuce cultivars under the LED light treatment compared with the fluorescent light treatment. In ‘Coastal Star’ and ‘Green Butter’, the assimilation rate was about three times greater under LED light and about two times greater in ‘Muir’ and ‘Rouge d’Hiver’ (Fig. 5A). Intercellular CO2 correlated negatively with assimilation rate under LED light, where it was significantly decreased in all lettuce cultivars except for ‘Green Butter’ (Fig. 5B). In comparison with fluorescent light, intercellular CO2 of lettuce grow under LED light decreased by 33%, 35%, and 30% in ‘Coastal Star’, ‘Green Butter’, and ‘Rouge d’Hiver’, respectively (Fig. 5B). The stomatal conductance (gs) and transpiration rate shared a similar trend in which only ‘Coastal Star’ showed a significant increase in both parameters with the LED light treatment (Fig. 5C and D). In contrast, both transpiration rate and gs remained relatively the same in ‘Muir’ and ‘Rouge d’Hiver’, whereas there was no significant difference in ‘Green Butter’ under LED light compared with fluorescent light (Fig. 5C and D).
Mean photosynthetic parameters of (A) assimilation rate, (B) intercellular carbon dioxide (CO2), (C) transpiration rate, and (D) and stomatal conductance for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment was provided by a Sun Blaze T5 HO46 (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 µmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment was provided using a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. *Significant difference from lettuce grown under fluorescent light at P < 0.05 (t test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Mean photosynthetic parameters of (A) assimilation rate, (B) intercellular carbon dioxide (CO2), (C) transpiration rate, and (D) and stomatal conductance for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment was provided by a Sun Blaze T5 HO46 (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 µmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment was provided using a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. *Significant difference from lettuce grown under fluorescent light at P < 0.05 (t test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Mean photosynthetic parameters of (A) assimilation rate, (B) intercellular carbon dioxide (CO2), (C) transpiration rate, and (D) and stomatal conductance for lettuce cultivars Costal Star, Muir, Green Butter, and Rouge d’Hiver grown in a hydroponic system under fluorescent light vs. light-emitting diode (LED) light treatments. All cultivars were harvested 35 d after initiation of light treatments. Fluorescent light treatment was provided by a Sun Blaze T5 HO46 (Vancouver, WA, USA; blue bars) at a photosynthetic photon flux density of 120 µmol⋅m–2⋅s–1 and a 16-h photoperiod. LED light treatment was provided using a Heliospectra Rx30 (Chicago, IL, USA; red bars) at intensities at 450-, 660-, and 570-nm wavelengths (Supplemental Fig. S1) optimized to the natural spectrum from dawn to dusk, with a total of 16 h of light. *Significant difference from lettuce grown under fluorescent light at P < 0.05 (t test); n = 12 plants/treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Discussion
Plant morphology of hydroponically grown lettuce.
Lettuce can be grown successfully using hydroponic systems (Ezziddine et al. 2021; Lei and Engeseth 2021; Suo et al. 2021; Voutsinos et al. 2021). As anticipated, 23 of 26 lettuce cultivars tested in our study were able to be grown hydroponically in a controlled environment at 25 °C and under fluorescent light. The fresh and dry weight results indicate that romaine lettuce cultivars were the heaviest, followed by the looseleaf and butterhead cultivars. Because of limited indoor space, the hydroponic system used in our study was scaled down to manage plant growth. As a result, most of the lettuce HFWs were between 30 and 60 g. However, a greater HFW would be expected when a wider planting area is used in commercial production systems. In addition, the lettuce was harvested at the same time for measurement purposes, but each cultivar has its own mature date and the weight may change accordingly. Thus, a maximum HFW can be achieved by allowing the lettuce to mature before harvesting, and a similar HFW may be possible between different types of lettuces (Afton et al. 2020). The competition for space and light in a tightly controlled environment may also contribute to the poor development of lettuce cultivars Little Gem, Jericho, All Year Round, and Lollo Rossa. In other words, the growth habit and vigorousness of these lettuce cultivars were inferior to their competitors. This is expected because romaine and looseleaf lettuce are known to be robust and have big, broad leaves, whereas butterhead lettuce such as ‘Little Gem’ are typically more compact. Thus, proper plant spacing may be more critical for ‘Little Gem’, ‘Jericho’, ‘All Year Round’, and ‘Lollo Rossa’ in comparison with the other 23 lettuce cultivars (De Freitas Furtado et al. 2017; Maboko and Du Plooy 2009, 2013).
In field production, external conditions such as soil fertility and water availability highly influence crop shoot and root growth. For example, plants experiencing water stress typically decrease shoot growth to expend their energy on root growth in search of water, causing an increased R/S ratio (Agren and Franklin 2003; Barton and Montagu 2006; Benjamin et al. 2014; Toscano and Romano 2021). Similarly, the ratio can also be affected by nitrogen or phosphorus in the soil, depending on the specific requirement of the plant species (Bonifas et al. 2005; de Souza Campos et al. 2019). Under the hydroponic system, where nutrients and water are readily accessible, it was interesting to find that the R/S ratio varied greatly in some lettuce cultivars. The R/S was greatest in cultivars Truchas, Red Butter, and Red Oakleaf, in which the average R/S was more than 1, whereas the lowest R/S (which was less than 0.25) was found in lettuce cultivars Muir, Parris, Coastal Star, and Butter Crunch (Fig. 1A). Different studies have indicated that the addition of blue and red light can increase the biomass of lettuce and tomatoes (Solanum lycopersicum) under hydroponic systems (Kaiser et al. 2019; Nguyen et al. 2021). Thus, we speculate that light might be a factor influencing R/S. To test our hypothesis, we compared two lettuce cultivars (Muir and Coastal Star) with a lower R/S and two cultivars (Green Butter and Rouge d’Hiver) with a greater R/S under either fluorescent or LED light. Despite the R/S value being slightly greater with the LED treatment, no significant difference was detected between the light treatments for all four lettuce cultivars (Supplemental Fig. S1). Although our LED setting included a range of intensities for blue and red light, it is possible that the addition of green light minimized the change in R/S value when compared with red and blue alone.
Shade avoidance syndrome under fluorescent light.
Similar to the R/S results, other morphological measurements including RFW, dry head weight, dry root weight, total leaf area, and total number of leaves did not change significantly between the two light treatments for lettuce cultivars Coastal Star, Muir, Green Butter, and Rouge d’Hiver (Figs. 2B–D, 5A, and 5C). However, ‘Green Butter’ showed a 37% decrease in HFW when treated with LED light, whereas the other three lettuce cultivars showed no differences in their weight (Fig. 2A). The lower HFW in ‘Green Butter’ was a result, in part, of its compactness induced by LED light, resulting in leaf shrinkage and reduced stem elongation (Figs. 3 and 5B). We observed that many of the hydroponically grown lettuce cultivars displayed stem elongation under fluorescent light, which is a symptom of shade avoidance syndrome resulting from low PPFD (Supplemental Fig. S4). Compared with the broad spectrum presented in fluorescent light, LED light can be tailored to increase light intensity in specific wavelengths, especially blue light, which is known to affect phytochrome activity in stem elongation (Matysiak and Kowalski 2021). As a result of both a high PPFD and the extension of blue light, stem length was reduced significantly in all lettuce cultivars under LED light (Fig. 4B). Photosynthesis was also expected to be more efficient under LED light because of the chlorophyll absorption of blue and red light (Kaiser et al. 2019; Nguyen et al. 2021; Park and Runkle 2018; Randall and Lopez 2014). Indeed, the assimilation rate was significantly greater in all lettuce cultivars, and the intercellular CO2 was less accordingly. Besides plant morphogenesis, light exposure is also a key factor in the flavonoid biosynthesis pathway (Hasan et al. 2017; Nguyen et al. 2021). Various studies have indicated that blue light can increase flavonoid biosynthesis in lettuce, tomato, and basil (Ocimum basilicum L.), but this effect is species dependent (Kaiser et al. 2019; Matysiak and Kowalski 2021; Nguyen et al. 2021). Although no significant difference was observed in many of the lettuce plant morphological characteristics, such as leaf number and leaf area, between fluorescent and LED light, their flavonoid content may have been altered. Considering the nutritious benefits of lettuce, how LED light may affect the biosynthesis of different phytochemicals in hydroponically grown lettuce should be elucidated in the future.
References
Afton, W., Fontenot, K., Kuehny, J. & Motsenbocker, C. 2020 Evaluation of yield, marketability, and nitrate levels of lettuce cultivars produced in southern Louisiana HortTechnology 30 632 637 https://doi.org/10.21273/HORTTECH04642-20
Agren, G.I. & Franklin, O. 2003 Root:shoot ratios, optimization and nitrogen productivity Ann Bot. 92 795 800 https://doi.org/10.1093/aob/mcg203
An, S., Arakawa, O., Tanaka, N., Zhang, S. & Kobayashi, M. 2020 Effects of blue and red light irradiations on flower colouration in cherry blossom (Prunus × yedoensis ‘Somei-yoshino’) Sci Hortic. 263 109093 https://doi.org/10.1016/j.scienta.2019.109093
Barton, C.V.M. & Montagu, K.D. 2006 Effect of spacing and water availability on root:shoot ratio in Eucalyptus camaldulensis For Ecol Manage. 221 52 62 https://doi.org/10.1016/j.foreco.2005.09.007
Benjamin, J.G., Nielsen, D.C., Vigil, M.F., Mikha, M.M. & Calderon, F. 2014 Water deficit stress effects on corn (Zea mays, L.) root:shoot ratio Open J Soil Sci 4 4 151 160 https://doi.org/10.4236/ojss.2014.44018
Bonifas, K.D., Walters, D.T., Cassman, K.G. & Lindquist, J.L. 2005 Nitrogen supply affects root:shoot ratio in corn and velvetleaf (Abutilon theophrasti) Weed Sci. 53 670 675
Contreras, S., Bennett, M.A., Metzger, J.D., Tay, D. & Nerson, H. 2009 Red to far-red ratio during seed development affects lettuce seed germinability and longevity HortScience 44 130 134 https://doi.org/10.21273/hortsci.44.1.130
De Freitas Furtado, G., Cavalcante, A.R., Chaves, L.H.G., Santos, J.A. Jr & Gheyi, H.R. 2017 Growth and production of hydroponic pepper under salt stress and plant density Am J Plant Sci. 8 2255 2267 https://doi.org/10.4236/ajps.2017.89151
de Souza Campos, P.M., Cornejo, P., Rial, C., Borie, F., Varela, R.M., Seguel, A. & López-Ráez, J.A. 2019 Phosphate acquisition efficiency in wheat is related to root:shoot ratio, strigolactone levels, and PHO2 regulation J Expt Bot. 70 5631 5642 https://doi.org/10.1093/jxb/erz349
Ezziddine, M., Liltved, H. & Seljåsen, R. 2021 Hydroponic lettuce cultivation using organic nutrient solution from aerobic digested aquacultural sludge Agronomy (Basel) 11 1484 https://doi.org/10.3390/agronomy11081484
Folta, K.M. & Maruhnich, S.A. 2007 Green light: A signal to slow down or stop J Expt Bot. 58 3099 3111 https://doi.org/10.1093/jxb/erm130
Hasan, M.M., Bashir, T., Ghosh, R., Lee, S.K. & Bae, H. 2017 An overview of LEDs’ effects on the production of bioactive compounds and crop quality Molecules 22 9 1420 https://doi.org/10.3390/molecules22091420
Hersch, M., Lorrain, S., de Wit, M., Trevisan, M., Ljung, K., Bergmann, S. & Fankhauser, C. 2014 Light intensity modulates the regulatory network of the shade avoidance response in Arabidopsis Proc Natl Acad Sci USA. 111 6515 6520 https://doi.org/10.1073/pnas.1320355111
Huang, J.Y., Xu, F. & Zhou, W. 2018 Effect of LED irradiation on the ripening and nutritional quality of postharvest banana fruit J Sci Food Agric. 98 5486 5493 https://doi.org/10.1002/jsfa.9093
Kaiser, E., Weerheim, K., Schipper, R. & Dieleman, J.A. 2019 Partial replacement of red and blue by green light increases biomass and yield in tomato Sci Hortic. 249 271 279 https://doi.org/10.1016/j.scienta.2019.02.005
Kelly, N., Choe, D., Meng, Q. & Runkle, E.S. 2020 Promotion of lettuce growth under an increasing daily light integral depends on the combination of the photosynthetic photon flux density and photoperiod Sci Hortic. 272 109565 https://doi.org/10.1016/j.scienta.2020.109565
Kim, M.J., Moon, Y., Tou, J.C., Mou, B. & Waterland, N.L. 2016 Nutritional value, bioactive compounds and health benefits of lettuce (Lactuca sativa L.) J Food Compos Anal. 49 19 34 https://doi.org/10.1016/j.jfca.2016.03.004
Lei, C. & Engeseth, N.J. 2021 Comparison of growth characteristics, functional qualities, and texture of hydroponically grown and soil-grown lettuce Lebensm Wiss Technol. 150 111931 https://doi.org/10.1016/j.lwt.2021.111931
Li, F., Li, Y., Li, S., Wu, G., Niu, X. & Shen, A. 2021 Green light promotes healing and root regeneration in double-root-cutting grafted tomato seedlings Sci Hortic. 289 110503 https://doi.org/10.1016/j.scienta.2021.110503
Llorach, R., Martínez-Sánchez, A., Tomás-Barberán, F.A., Gil, M.I. & Ferreres, F. 2008 Characterisation of polyphenols and antioxidant properties of five lettuce varieties and escarole Food Chem. 108 1028 1038 https://doi.org/10.1016/j.foodchem.2007.11.032
Maboko, M.M. & Du Plooy, C.P. 2009 Effect of plant spacing on growth and yield of lettuce (Lactuca sativa L.) in a soilless production system S Afr J Plant Soil 26 195 198 https://doi.org/10.10520/EJC87938
Maboko, M.M. & Du Plooy, C.P. 2013 High-plant density planting of basil (Ocimum basilicum) during summer/fall growth season improves yield in a closed hydroponic system Acta Agric Scand B Soil Plant Sci. 63 748 752 https://doi.org/10.1080/09064710.2013.861921
Mampholo, B.M., Maboko, M.M., Soundy, P. & Sivakumar, D. 2016 Phytochemicals and overall quality of leafy lettuce (Lactuca sativa L.) varieties grown in closed hydroponic system J Food Qual. 39 805 815 https://doi.org/10.1111/jfq.12234
Marklein, A., Elias, E., Nico, P. & Steenwerth, K. 2020 Projected temperature increases may require shifts in the growing season of cool-season crops and the growing locations of warm-season crops Sci Total Environ. 746 140918 https://doi.org/10.1016/j.scitotenv.2020.140918
Matysiak, B. & Kowalski, A. 2021 The growth, photosynthetic parameters and nitrogen status of basil, coriander and oregano grown under different led light spectra Acta Sci Pol Hortorum Cultus 20 13 22 https://doi.org/10.24326/asphc.2021.2.2
Maucieri, C., Nicoletto, C., Os, E.V., Anseeuw, D., Havermaet, R.V. & Junge, R. 2019 Hydroponic technologies 77 110 Goddek, S., Joyce, A., Kotzen, B. & Burnell, G.M. Aquaponics food production systems: Combined aquaculture and hydroponic production technologies for the future. Springer Cham, Switzerland https://doi.org/10.1007/978-3-030-15943-6_4
Nguyen, T.K., Cho, K.M., Lee, H.Y., Cho, D.Y., Lee, G.O., Jang, S.N., Lee, Y., Kim, D. & Son, K.-H. 2021 Effects of white led lighting with specific shorter blue and/or green wavelength on the growth and quality of two lettuce cultivars in a vertical farming system Agronomy (Basel) 11 2111 https://doi.org/10.3390/agronomy11112111
Nicolle, C., Cardinault, N., Gueux, E., Jaffrelo, L., Rock, E., Mazur, A., Amouroux, P. & Rémésy, C. 2004 Health effect of vegetable-based diet: Lettuce consumption improves cholesterol metabolism and antioxidant status in the rat Clin Nutr. 23 605 614 https://doi.org/10.1016/j.clnu.2003.10.009
Park, Y. & Runkle, E.S. 2017 Far-red radiation promotes growth of seedlings by increasing leaf expansion and whole-plant net assimilation Environ Exp Bot. 136 41 49 https://doi.org/10.1016/j.envexpbot.2016.12.013
Park, Y. & Runkle, E.S. 2018 Spectral effects of light-emitting diodes on plant growth, visual color quality, and photosynthetic photon efficacy: White versus blue plus red radiation PLoS One 13 e0202386 https://doi.org/10.1371/journal.pone.0202386
Randall, W.C. & Lopez, R.G. 2014 Comparison of supplemental lighting from high-pressure sodium lamps and light-emitting diodes during bedding plant seedling production HortScience 49 589 595 https://doi.org/10.21273/HORTSCI.49.5.589
Ruberti, I., Sessa, G., Ciolfi, A., Possenti, M., Carabelli, M. & Morelli, G. 2012 Plant adaptation to dynamically changing environment: The shade avoidance response Biotechnol Adv. 30 1047 1058 https://doi.org/10.1016/j.biotechadv.2011.08.014
Sambo, P., Nicoletto, C., Giro, A., Pii, Y., Valentinuzzi, F., Mimmo, T., Lugli, P., Orzes, G., Mazzetto, F., Astolfi, S., Terzano, R. & Cesco, S. 2019 Hydroponic solutions for soilless production systems: Issues and opportunities in a smart agriculture perspective Front Plant Sci. 10 923 https://doi.org/10.3389/fpls.2019.00923
Son, K.H., Jeon, Y.M. & Oh, M.M. 2016 Application of supplementary white and pulsed light-emitting diodes to lettuce grown in a plant factory with artificial lighting Hortic Environ Biotechnol. 57 560 572 https://doi.org/10.1007/s13580-016-0068-y
Suo, R., Wang, W., Ma, Y., Fu, L. & Cui, Y. 2021 Effect of different root lengths for retaining freshness of hydroponic lettuce J Agric Food Res. 4 100151 https://doi.org/10.1016/j.jafr.2021.100151
Toscano, S. & Romano, D. 2021 Morphological, physiological, and biochemical responses of zinnia to drought stress Horticulturae 7 10 362 https://doi.org/10.3390/horticulturae7100362
Voutsinos, O., Mastoraki, M., Ntatsi, G., Liakopoulos, G. & Savvas, D. 2021 Comparative assessment of hydroponic lettuce production either under artificial lighting, or in a Mediterranean greenhouse during wintertime Agriculture 11 503 https://doi.org/10.3390/agriculture11060503
Wojciechowska, R., Hanus-Fajerska, E., Kamińska, I., Koźmińska, A., Długosz-Grochowska, O. & Kapczyńska, A. 2019 High ratio of red-to-blue LED light improves the quality of Lachenalia ‘Rupert’ inflorescence Folia Hortic. 31 93 100 https://doi.org/10.2478/fhort-2019-0006
Wollaeger, H.M. & Runkle, E.S. 2015 Growth and acclimation of impatiens, salvia, petunia, and tomato seedlings to blue and red light HortScience 50 522 529 https://doi.org/10.21273/HORTSCI.50.4.522
Yang, X., Gil, M.I., Yang, Q. & Tomás-Barberán, F.A. 2021 Bioactive compounds in lettuce: Highlighting the benefits to human health and impacts of preharvest and postharvest practices Compr Rev Food Sci Food Saf. 21 1 4 45 https://doi.org/10.1111/1541-4337.12877
Lettuce cultivar name, type, and associated breeder used in the study.
Light intensities setting for LED light treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Light intensities setting for LED light treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Light intensities setting for LED light treatment.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Root: Shoot ratio of four lettuce varieties under fluorescent vs. LED light.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Root: Shoot ratio of four lettuce varieties under fluorescent vs. LED light.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Root: Shoot ratio of four lettuce varieties under fluorescent vs. LED light.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
(A) Dry head weight and (B) dry root of each lettuce cultivar and their respective type.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
(A) Dry head weight and (B) dry root of each lettuce cultivar and their respective type.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
(A) Dry head weight and (B) dry root of each lettuce cultivar and their respective type.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Representative image of hydroponically grown lettuce under fluorescent light. Orange arrows indicate stem elongation.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Representative image of hydroponically grown lettuce under fluorescent light. Orange arrows indicate stem elongation.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
Representative image of hydroponically grown lettuce under fluorescent light. Orange arrows indicate stem elongation.
Citation: HortScience 57, 11; 10.21273/HORTSCI16780-22
