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ASHS 2024 Annual Conference

 

Shade Nets Reduced Growth, Nutrition, and Sugars of Hydroponic Lettuce and Basil

Authors:
Harpreet Singh Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Bruce L. Dunn Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Charles Fontanier Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Hardeep Singh Department of Agronomy, University of Florida, 4253 Experiment Drive, Highway 182, Jay, FL 32565, USA

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Amandeep Kaur Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Lu Zhang Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Abstract

Colored shade nets are known to alter the light quality and quantity and thus can influence plant growth and nutritional quality of crops. Lettuce (‘Lollo Antonet’ and ‘Green Forest’) and basil (‘Aroma-2’ and ‘Genovese’) were grown in ebb-and-flow hydroponic tables for 4 weeks. Colored shade nets of aluminet, black, pearl, and red with 50% shading intensity along with a control (no-shade) were used in this experiment. Data for plant growth and leaf quality attributes were collected at harvest time. The no-shade treatment showed increased shoot fresh and dry weight, sugar, and relative chlorophyll content in both lettuce and basil cultivars, whereas plant height and net photosynthesis rates were increased under aluminet, pearl, and red nets. In basil, calcium and sulfur were greatest under no-shade, whereas zinc and copper were greatest under aluminet. Zinc, iron, calcium, magnesium, and manganese concentrations were greatest under no-shade in lettuce. The pearl-colored net increased leaf soluble solids content. No-shade produced the greatest starch values in basil, whereas pearl shade net produced the greatest starch in ‘Lollo Antonet’ in the fall. Light spectra varied with shade net resulting in 90%, 65%, 50%, 30%, and 70% of incident light occurring between 400 and 700 nm for no-shade, pearl, aluminet, black, and red shade nets, respectively. Overall, lettuce and basil plants under no-shade (daily light integral of 20 to 24 mol·m−2·d−1 and temperature of 26 to 30 °C) had increased plant growth and leaf quality in late spring and fall, compared with colored shade nets.

In areas having little arable land or poor water distribution systems, hydroponics is a means for production of fresh and healthy vegetables in greenhouses (Resh 1997). The advantages of hydroponics over soil production include the plants growing faster, plant fertility being very precise, and problems associated with poor soils can be avoided (Roberto 2014; Savvas and Passam 2002). There are numerous crops that can be grown using hydroponics in greenhouses that have short production cycles including lettuce (Lactuca sativa L.), basil (Ocimum basilicum L.), Swiss chard (Beta vulgaris L.), kale (Brassica oleracea L.), and various Brassica family crops (Singh 2017).

Lettuce is an herbaceous leafy vegetable and is grown worldwide for its importance in the daily human diet and nutrition (Mou 2009). Lettuce is mainly consumed as a salad and is ranked second in terms of vegetable consumption in the United States with per capita consumption of 13.5 kg (USDA 2020). Lettuce is a cool season crop with optimum temperatures ranging from 15.5 to 18.3 °C for growth (Masarirambi et al. 2018). Lettuce contains vitamin C, polyphenols, and fibers that help to improve health, prevent nutrient deficiencies, and reduce cardiovascular diseases (Shatilov et al. 2019).

Basil is a tender herbaceous warm-season plant that grows between 10 and 30 °C and prefers high light conditions (Currey et al. 2020). Basil is commonly grown in controlled environmental conditions and hydroponic systems (Sipos et al. 2021). Basil consumption is increasing rapidly due to its aromatic compounds, phenolic concentrations, and rich flavors (Dou et al. 2018). Basil is commonly used as an herb in various cooking operations such as flavoring and food preservation and provides some essential aromatic oils (Li and Chang 2015).

From germination to maturity, plants respond physiologically and morphologically to environmental factors such as light, temperature, nutrient application, and humidity. Light is the major factor that attributes to growth and development in plants and controls various mechanisms such as photosynthesis and photomorphogenesis (Teixeira 2020). According to McCree (1972), sunlight reaching the earth’s surface has a vast spectral range (250 to 2500 nm), but only light between 400 and 700 nm is considered photosynthetically active radiation (PAR). Plants have developed various adaptation molecules to efficiently detect or absorb light, such as phytochromes, chlorophylls, carotenoids, and cryptochromes (Belkov et al. 2019). Every crop has an optimal requirement of light, as low light can reduce the crop quality and too high light intensity will decrease crop productivity and can cause heat stress (Torres and Lopez 2012).

Plant-based diets have been used by people having various degenerative diseases (Nicolle et al. 2004). Fruits and vegetables are important sources of micronutrients and vitamins critical to cellular function (Martin et al. 2002). Environmental factors such as temperature, light, and relative humidity are major concerns that can affect optimal productivity and nutritional quality of crops grown in both field and greenhouse conditions (Ntsoane et al. 2016). Light is an unstable and hard to control environmental factor (Belkov et al. 2019), but changes in the light quality spectrum could modify crop physiological and biochemical processes (Ilic et al. 2017). These alterations in turn affect quality and quantity of phytochemicals and nutrients in plant leaves. After light, the temperature is the next major factor that controls growth and development in plants. High temperatures due to intense solar radiation can cause various abiotic stresses in plants that deteriorate the produce quality (Ilic and Fallik 2018). Application of colored shade nets has been increasing in greenhouse production to alter micro environmental conditions, which helps to increase production and quality of horticultural crops, but there is a lack of knowledge about which colored shade nets is best suited for which crops.

Quality and production of both lettuce and basil can be reduced due to increased temperature and light intensity during peak summer months and colored shade nets can be a better alternative to provide plants with conducive environmental conditions for better growth. Colored nets are made from photo-selective materials and help to change the spectral composition of incident light (Ganelevin 2008; Shahak et al. 2008). Colored shade nets are available as red, black, pearl, yellow, blue, aluminet, and green, with shading factor ranging from 5% to 90%. These shade nets can protect plants from wind, birds, hail damage, and light intensity, and disperse light radiation up to 50% that is reaching the plant canopy (Díaz-Pérez et al. 2020; Stamps 2009). Colored shade nets are made to screen different portions of light and transform incoming light radiation specifically by absorbing, transmitting, or reflecting targeted bands of light (Shahak et al. 2008). This scattered light radiation has better light use efficiency in plants because of the diffused component of light that can penetrate deeper in the plant canopy (Shahak et al. 2008). The objective of this study was to determine if the use of selected colored shade nets improve the nutrition, quality, and growth of two cultivars of a cool season (lettuce) and a warm-season crop (basil) grown hydroponically in late spring and fall consistent with smaller growers that would grow two different crops under the same conditions.

Materials and Methods

Location and greenhouse conditions.

The research was conducted at the research greenhouse facility at Oklahoma State University, Stillwater campus (36°08′0″.9″N, 97°05′1″.9″W). Illuminance, temperature, and humidity were recorded with an Illuminance ultraviolet recorder TR-74Ui (T&D, Matsumoto, Japan). No supplemental light was used in the greenhouse, and the daily light integral (DLI) averaged 15.7 ± 2.9 mol·m−2·d−1 PAR. The controller was set to 21/18 °C in the greenhouse resulting in a daily average of 27.8 ± 1.6 °C.

Plant material and treatments.

Lettuce (‘Lollo Antonet’ and ‘Green Forest’) and basil (‘Aroma-2’ and ‘Genovese’) seed were obtained from Johnny’s Selected Seeds (Winslow, ME, USA) on 15 Apr 2021. Seeds were placed in Horticube foam cubes (Oasis Grower Solutions, Kent, OH, USA) with one seed placed in each 1.90 cm × 2.22 cm × 3.81 cm size cube on late spring (April–May) and fall (August–September). Trays were kept under mist for 3 weeks. Treatments included red, black, aluminet, and pearl-colored shade nets (Green-Tek, Janesville, WI, USA) with 50% shade intensity plus a no-shade control. Seedlings were then transferred to ebb-and-flow tables (1.5 m × 1.8 m), which had a Styrofoam sheet with 5-cm holes spaced at 27 cm between holes. Net pots (Cz Garden Supply, Seattle, WA, USA) with 5-cm-diameter openings were used. Ecoplus fixed flow water pumps (Sunlight Supply, Vancouver, WA, USA) with 1500 L per hour pumping capacity were used to pump the water. Polyvinyl chloride (PVC) pipes of 2.5 cm diameter were used to make frames of 0.762 m in height to hold the colored shade nets along the top and sides. A 20N–8.6P–17.4K general-purpose water-soluble fertilizer (JR Peters Inc., Allentown, PA, USA) was used. The electrical conductivity (EC) (1.5 to 2.0 mS·cm−1) and pH (5.5 to 6.5) were maintained using an EC/pH meter (HI9813-6; Hanna Instruments, Woonsocket, RI, USA). A pH modifier (pH Down; General Hydroponics, Santa Rosa, CA, USA) was used to lower the pH.

Data collection.

All data were collected 4 weeks after transplanting seedlings into tables. Data were collected on shoot and root fresh weight, shoot and root dry weight, plant height, relative chlorophyll content, and photosynthesis rate. Plants were harvested using clippers at the surface level of oasis cubes and fresh shoot and root weight was measured. Plant material was oven-dried for 2 d at 53.9 °C and after that shoot and root dry weight was measured. Chlorophyll measurements were made using a Soil Plant Analysis Development (SPAD) meter (Minolta SPAD 502; Spectrum Technologies, Aurora, IL, USA). Data were collected from every plant using one upper, middle, and base leaf by inserting the middle portion of the leaf in the sensor. Net photosynthesis rate was measured using a portable photosynthesis system (LI-COR 6400XT; LI-COR Biosciences Lincoln, NE, USA) at a light intensity of 1000 μmol·m−2·s−1 PAR using leaf from middle portion of plant with 6400–18 RGB light source. Spectral data for transmittance was measured after 2 weeks of transplanting near solar noon using a spectrometer (HL-2000 FHSA; Ocean Optics, Orlando, FL, USA), with having sensor under the colored shade nets.

Nutrient analysis.

At 4 weeks of transplanting, leaves of three plants per treatment were oven-dried for 2 d at 53.9 °C and submitted to the Soil, Water, and Forage Analytical Laboratory (SWAFL), at Oklahoma State University (Stillwater, OK, USA) for nutrient analysis. Inductively coupled plasma mass spectrometry (Thermo Fisher Scientific Waltham, MA, USA) was used to analyze samples for all nutrients except nitrogen (N). An elemental analyzer (836 series; LECO Europe, Geleen, Netherlands) was used to analyze N.

Carbohydrate analysis.

Six leaves of two basil plants per treatment for each cultivar from top, middle, and bottom part of plant or one central leaf of two lettuce plants per treatment for each cultivar were collected as sub-samples and dried as previously described. The dried sub-samples were ground into fine powder using a grinder (Mini-Bead Beater 96; Biospec Products, Bartlesville, OK, USA). Subsequently, 25 mg of the powdered sample was analyzed for carbohydrate concentration using the anthrone reagent method in which samples were dehydrated and depolymerized by concentrated sulfuric acid (H2SO4) to form furfural or hydroxymethyl furfural. The active form of the reagent is anthronol, the enol tautomer of anthrone, which reacts with the carbohydrate furfural derivative to give a color that is either green in diluted solutions or blue in concentrated solutions. This color may be seen by measuring the absorbance at 620 nm. After being incubated in 1 mL of ultra-pure (UP) water at 70 °C for 15 min, fine powder samples (25 to 27 mg) were centrifuged for 10 min at 15,000 gn. Anthrone was used as a reagent to measure the amount of soluble sugars in the supernatant after dilution with UP water (1:20 v/v). The remaining pellet was cleaned with water and 95% ethanol (v/v) before being heated to 100 °C for 10 min to allow the starch to gelatinize. After that, the pellet was digested for 4 h at 37 °C using amylo-glucosidase (700 units/mL), alpha-amylase (70 units/mL), and sodium acetate (0.2 M, pH 5.5) in a Roto-ThermTM Plus Incubated Rot (H2024; Benchmark Scientific, Sayreville, NJ, USA). Following incubation, samples were centrifuged at 15,000 gn for 5 min. The supernatant was used for measurement after being diluted with UP water (1:4 v/v) (Kaur 2021). A microplate reader (Epoch, Biotek Instruments Inc. Winooski, VT, USA) was used to read sample plates at 620 nm wavelength, which gives sugar and starch content in leaves. °Brix was also measured using a handheld refractometer (Fjdynamics, Chinatown, Singapore) in which a single leaf was taken from the middle portion of a plant and ground to a liquid.

Data analysis.

This experiment was arranged in a randomized complete block design with two replications at the same time. There were five treatments, and the experiment was repeated over late spring and fall. The experimental unit was nine plants per cultivar of each crop. Data analysis was done by using SAS 9.4 (SAS Institute, Cary, NC, USA). Tests of significance were reported at 0.05, 0.01, and 0.001 levels. The data were analyzed using generalized linear mixed model methods. Tukey’s highly significant difference multiple comparison methods were used to separate the means.

Results

Environmental conditions.

There were significant differences among shade nets for DLI, air temperature, and relative humidity during the late spring and fall seasons (Table 1). During late spring and fall, no-shade exhibited 20.6 mol·m−2·d−1 DLI, which was almost double as compared with black (9.6 mol·m−2·d−1). The temperature during late spring was the greatest under red (30.6 °C). In the fall, the temperature was highest in no-shade (26.8 °C), which was not different from pearl and red treatments having 25.3 °C and 25.2 °C, respectively. During late spring, relative humidity was greatest under aluminet, which was not different from black. In fall, relative humidity was greatest under aluminet. Aluminet and pearl created a neutral shade with uniform reduction in light within the visible spectrum (Fig. 1). Light spectra varied with shade net resulting in 90%, 65%, 50%, 30%, and 70% of incident light occurring between 400 nm and 700 nm for nonshaded, pearl, aluminet, black, and red shade nets, respectively.

Table 1.

Greenhouse conditions for daily light integral, temperature, and relative humidity under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment for 2021 late spring and fall season, Stillwater, OK, USA.

Table 1.
Fig. 1.
Fig. 1.

Reflectance percentage in different wavelengths of light under no-shade (A) and 50% shading intensity of aluminet (B), black (C), pearl (D), and red colored shade nets during 2021 late spring and fall seasons with basil and lettuce in ebb-and-flow hydroponic systems under greenhouse conditions in Stillwater, OK, USA.

Citation: HortScience 58, 11; 10.21273/HORTSCI17252-23

Plant growth.

There was a season × cultivar × treatment interaction in basil and lettuce for plant growth. In basil, plant height showed a significant three-way interaction between season × cultivar × treatment (Table 2). In late spring, the greatest plant height was exhibited by no-shade (31.7 cm), which was 70% and 50% greater than black and aluminet for ‘Aroma-2’, respectively (Table 3). For ‘Genovese’, plant height was greatest for pearl (34.1 cm), but there was no significant difference between any colored shade nets. During fall in ‘Aroma-2’, red exhibited the greatest plant height, which was 25% greater than black. For ‘Genovese’ in fall, red showed the greatest plant height (45.9 cm), which was 30%, 28%, and 22% greater than aluminet, black, and pearl, respectively.

Table 2.

Summary analysis of variance table for different growth and quality parameters under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment in leaves of basil and lettuce cultivars grown in ebb-and-flow tables under greenhouse conditions during 2021 late spring and fall season, Stillwater, OK, USA.

Table 2.
Table 3.

Interaction between season, cultivar, and shade net for plant height and relative chlorophyll concentration in basil and lettuce cultivars grown under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment in ebb-and-flow hydroponic systems under greenhouse conditions during 2021 late spring and fall season, Stillwater, OK, USA.

Table 3.

In lettuce, plant height and relative chlorophyll content exhibit a significant three-way interaction between season × cultivar × treatment (Table 2). Plant height was not different during late spring for both cultivars of lettuce. During fall, plant height was similar for ‘Lollo Antonet’, whereas ‘Green Forest’ showed greatest plant height under red (20.8 cm), which was almost double no-shade and pearl shade treatment (Table 3). For both late spring and fall, relative chlorophyll content was not different for any cultivar or treatment, but there were differences among seasons and cultivars. Overall, the fall season performed best as compared with late spring season under different shade nets in both cultivars of lettuce for relative chlorophyll content.

In basil, there was a significant cultivar × treatment interaction for shoot dry weight, root fresh weight, and net photosynthesis rate (Table 2). No-shade showed the highest shoot dry weight and for ‘Aroma-2’ and ‘Genovese’ at 6.2 g and 8.3 g, respectively. Root fresh weight was also greatest under no-shade having and for ‘Aroma-2’ and ‘Genovese’ respectively (Table 4). Aluminet showed the greatest net photosynthesis rate for ‘Aroma-2’, which was 25% greater than no-shade and black and for ‘Genovese’ aluminet also showed the greatest net photosynthesis rate (20.1 g), which was 65% greater than red shade nets.

Table 4.

Interaction between cultivar and shade net on shoot fresh weight, shoot dry weight, root fresh weight, and photosynthesis rate of two cultivars of basil and lettuce grown under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK, USA.

Table 4.

In lettuce, shoot fresh and dry weight, root fresh weight, and net photosynthesis rate showed significant cultivar × treatment interaction (Table 2). ‘Lollo Antonet’ showed greatest shoot fresh weight (66.8 g) under no-shade, which was 60% and 65% greater than black and pearl shade nets, respectively (Table 4). In ‘Green Forest’ shoot fresh weight was greatest under aluminet (95.4 g), which was 30% greater than no-shade and red, and 45% and 30% greater than black and pearl, respectively. Shoot dry weight was greatest under aluminet (8.1 g), which was 40% and 25% greater than black and pearl, respectively, in ‘Lollo Antonet’, whereas in ‘Green Forest’ shoot dry weight was greatest in no-shade (10.8 g), which was 35%, 55%, 20%, and 30% greater than aluminet, black, pearl, and red shade nets, respectively. The no-shade showed the greatest root fresh weight (17.3 g) for ‘Lollo Antonet’, which was almost double from aluminet, black, and red. In ‘Green Forest’ also, root fresh weight was greatest in no-shade (33.2 g), which was double from red shade net and 20% greater than pearl. For ‘Lollo Antonet’ net photosynthesis rate was greatest in aluminet (10.5 µmol·m−2·s−1), which was 35% and 50% greater than no-shade and red. In ‘Green Forest’, black (18.2 µmol·m−2·s−1) showed the greatest net photosynthesis rate, which was 20% greater than red.

Basil showed a significant interaction between season × treatment for shoot fresh and dry weight, net photosynthesis rate, and relative chlorophyll content (Table 2). Shoot fresh weight and shoot dry weight were greatest for no-shade during both seasons (Table 5). Net photosynthesis rate was greatest in aluminet (20.2 µmol·m−2·s−1), which was 25% and 45% greater than no-shade and red, respectively, in late spring. During fall, net photosynthesis rate was greatest in aluminet (23.8 µmol·m−2·s−1), which was 15%, 25%, and 40% greater than black, pearl, and red, respectively. In late spring, relative chlorophyll content did not show any significant differences among treatments, but during fall, relative chlorophyll content was greatest with no-shade (42.6), which was 10% and 18% greater than aluminet and red, respectively.

Table 5.

Interaction between season and shade net on shoot fresh weight, shoot dry weight, photosynthesis rate, and relative chlorophyll concentration of basil and lettuce grown under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK, USA.

Table 5.

In lettuce, shoot fresh weight, shoot dry weight, and net photosynthesis rate showed significant season × treatment interaction (Table 2). Shoot fresh weight in late spring season was greatest in no-shade (117.1 g), which was 50%, 52%, and 20% greater than black, pearl, and red, respectively (Table 5). During fall, shoot fresh weight under aluminet (49.1 g) was 85%, 130%, and 45% greater than black, pearl, and red nets, respectively. No-shade showed greatest shoot dry weight (9.7 g) in late spring, which was 33%, 45%, and 25% greater than aluminet, black, and red, respectively. During fall, shoot dry weight was greatest in no-shade (8.9 g), which was double that of black and 17% greater than pearl. In aluminet, net photosynthesis rate was greatest in late spring (13.8 µmol·m−2·s−1) and fall (13.9 µmol·m−2·s−1), which was not different from any other treatment except red.

There was a significant season × cultivar interaction for shoot fresh weight and relative chlorophyll content in basil (Table 2). During late spring, ‘Genovese’ showed greatest shoot fresh weight (29.9 g), which was 70% greater than ‘Aroma-2’, whereas relative chlorophyll content did not show any differences among cultivars (Table 6). In fall, there were no significant differences between cultivars for shoot fresh weight, whereas relative chlorophyll content was greatest for ‘Aroma-2’ (40.5) compared with ‘Genovese’ (38.8). In lettuce, significant interaction between season × cultivar was seen for shoot fresh weight and shoot dry weight (Table 2). Shoot fresh and dry weight did not show any significant difference in both cultivars of lettuce during late spring season (Table 6), but in the fall, ‘Lollo Antonet’ showed greater shoot fresh weight (43.9 g) and shoot dry weight (8.0 g) than ‘Green Forest’. Main effects of treatment and cultivar were significant for root dry weight in basil (Table 2). No-shade had the greatest root dry weight (4.2 g) as compared with all other treatments in basil (data not shown). Root dry weight was significantly greater in ‘Genovese’ (3.0 g) than ‘Aroma-2’ (1.9 g) (data not shown).

Table 6.

Interaction between season and cultivar on shoot fresh weight, shoot dry weight, and relative chlorophyll concentration of two cultivars of basil and lettuce grown under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK, USA.

Table 6.

Nutrients.

Basil plants showed cultivar × treatment interaction for potassium under different colored shade nets for both cultivars (Table 7). ‘Aroma-2’ showed the greatest potassium concentration (6.9%) under black treatment, which was 40% greater than aluminet treatment (Table 8). ‘Genovese’ had the greatest potassium concentration under red (6.7%), but was not significantly different from any other treatment.

Table 7.

Summary analysis of variance table for leaf foliar nutrients under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in basil and lettuce nets in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK.

Table 7.
Table 8.

Cultivar and treatment interaction for leaf foliar analysis of potassium, magnesium, iron, copper, and manganese under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in basil and lettuce nets in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK, USA.

Table 8.

Magnesium (Mg), iron (Fe), copper (Cu), and manganese (Mn) showed a significant interaction between cultivar × treatment for lettuce cultivars under different colored net treatments (Table 7). ‘Green Forest’ showed greatest concentration of Mg (0.66%) and Fe (248.5 mg·L−1) under the no-shade treatment, which was not different from pearl (Table 8). In ‘Lollo Antonet’ Mg concentration was greatest under aluminet treatment (0.72%), which was 12% greater than black treatment and iron concentration did not show any significant differences among different treatments. The no-shade treatment showed the greatest Cu (10.0 mg·L−1) concentrations in ‘Green Forest’, which were double than black and red treatments, whereas ‘Lollo Antonet’ showed the greatest Cu concentration under pearl treatment (7.6 mg·L−1) but not different from all other treatments. In ‘Lollo Antonet’, Mn concentration was greatest with no-shade. ‘Green Forest’ under no-shade showed the greatest concentration of Mn (283.7 mg·L−1), which was 80%, 75%, 50%, and 70% greater than aluminet, black, pearl, and red treatments, respectively.

In basil, the main effects of cultivar were significant for phosphorous (P), calcium (Ca), boron (B), zinc (Zn), and manganese (Mn) in both basil cultivars (Table 7). ‘Genovese’ had the greatest concentration of P, Ca, B, Z, and Mn as compared with ‘Aroma-2’ (Table 8). In lettuce, the main effects of cultivar were significant for Ca, potassium (K), sulfur (S), and zinc (Zn) (Table 7). ‘Lollo Antonet’ showed greatest concentrations of Ca, K, S, and Zn, which was 15%, 25%, 33%, and 15% greater than ‘Green Forest’, respectively (Table 9).

Table 9.

Main effects of cultivar for leaf nutrient analysis on phosphorous, calcium, potassium, sulfur, boron, zinc, and manganese under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in basil and lettuce nets in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK, USA.

Table 9.

Main effects of treatment in basil were significant for N, P, Ca, B, Zn, Cu, and Mn (Table 7). Black shade net showed greatest N concentration, and aluminet showed greatest Cu concentration (Table 10). Calcium showed the greatest concentration with no-shade (1.38%), which was 35% greater than pearl and black. Phosphorous (1.23%) and Mn (121.4 mg·L−1) concentration was greatest under black, which was not different from aluminet and no-shade treatment. Aluminet showed the greatest concentration of B (26.0 mg·L−1), which was 30%, 32%, and 38% greater than no-shade, pearl, and red, respectively. Zinc concentration was greatest under aluminet (67.2 mg·L−1), which was 30% greater than pearl.

Table 10.

Main effects of treatment for leaf foliar analysis on nitrogen, phosphorous, calcium, potassium, sulfur, boron, zinc, copper, and manganese under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in basil and lettuce nets in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK, USA.

Table 10.

In lettuce, the main treatment effects were significant for N, P, Ca, K, S, B, and Zn under different treatments (Table 7). N (4.81%), S (0.36 mg·L−1), and B (28.0 mg·L−1) concentrations were greatest under black treatment, which differed from red (Table 10). Pearl showed the greatest phosphorous concentration (0.83%), which was 20% greater than aluminet and black, respectively. Calcium concentration was greatest under no-shade (0.95%), which was 35%, 34%, 32%, and 23% greater than aluminet, black, pearl, and red. Potassium concentration under pearl shade net (8.5%) was 30%, 47%, 31%, and 20% greater than no-shade, aluminet, black, and red, respectively. No-shade treatment showed the greatest Zn concentration (82.6 mg·L−1), which was 40%, 42%, 25%, and 17% greater than aluminet, black, pearl, and red, respectively.

Sugars and starch.

Both basil and lettuce showed a significant three-way interaction between season × cultivar × treatment for sugars and starch (Table 11). For basil during late spring, ‘Aroma-2’ showed greatest sugar concentration under aluminet (54.2 mg/g dry weight), which was 30%, 270%, 60%, and 40% greater than no-shade, black, pearl, and red, respectively. ‘Genovese’ did not show any significant differences in sugars between different treatments (Table 12). In fall, sugar concentration in ‘Aroma-2’ was greatest under pearl, which was not different from the no-shade, and in ‘Genovese’ sugar concentration was greatest under pearl treatment. Starch during late spring, was greatest with the no-shade in ‘Aroma-2’ (30.6 mg/g dry weight) and in ‘Genovese’ (12.9 mg/g dry weight) starch was greatest under pearl, which was different from aluminet and red treatments. In fall, starch in ‘Aroma-2’ did not show any significant differences among treatments, but in ‘Genovese’ starch was greatest with the no-shade (14.2 mg/g dry weight) which was 70%, 10%, 80%, and 40% greater than aluminet, black, pearl, and red, respectively.

Table 11.

Summary analysis of variance table for sugars and starch under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in basil and lettuce nets in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK, USA.

Table 11.
Table 12.

Interaction between season, cultivar, and treatment for sugar and starch under 50% aluminet, black, pearl, and red colored shade nets and a no-shade treatment during 2021 late spring and fall seasons in basil and lettuce nets in ebb-and-flow hydroponic systems under greenhouse conditions, Stillwater, OK, USA.

Table 12.

For lettuce during late spring, ‘Lollo Antonet’ showed the greatest sugar concentration (129.9 mg/g dry weight), which was 55%, 37%, 120%, and 79% greater than no-shade, black, pearl, and red, respectively (Table 12). ‘Green Forest’ showed greatest concentration of sugars under aluminet. In fall, there were not any significant differences in sugars among treatments in ‘Lollo Antonet’, whereas in ‘Green Forest’ sugar concentration was greatest with no-shade, which was not different from black. Starch during late spring did not show any significant difference among treatments for both cultivars. In fall, ‘Lollo Antonet’ showed greatest concentration of starch under pearl (19.4 mg/g dry weight), whereas for ‘Green Forest’ (7.9 mg/g dry weight) starch concentration showed no significant differences among treatments.

In basil, only the main effects of treatment and cultivar were significant (Table 11). ‘Aroma-2’ showed greater °Brix values (4.3) as compared with ‘Genovese’ (4.0) (data not shown). Among colored treatments, pearl (4.4), aluminet (4.4), and no-shade (4.3) were greater than red (4.1) and black (3.7). In lettuce, cultivar × treatment interaction and season main effects were significant for °Brix (Table 11). Aluminet showed the greatest °Brix values for ‘Lollo Antonet’ (2.8) and ‘Green Forest’ (3.5) cultivars of lettuce as compared with other treatments (data not shown). °Brix values were greater during late spring season (3.0) as compared with fall season (2.7).

Discussion

Environmental conditions.

Shade nets reduced direct solar radiation reaching plants and maintained lower air temperatures for both late spring and fall seasons. Among colored shade nets, pearl had the greatest DLI for both seasons. The greater DLI for pearl shade nets is consistent with the manufacturer descriptions, which indicate a design to minimize the absorption of visible light and instead transmit direct radiation into diffuse or scattered light (Ilic et al. 2019; Shahak et al. 2008). Gaurav (2014) also found that pearl shade net had the greatest light intensity compared with red and black shade nets. Similar to our studies, the reduction in transmitted solar radiation also helps to reduce canopy and air temperature under these nets (Ilic et al. 2017). Ilic et al. (2019) also found that shade nets help to reduce solar radiation from 40% to 60% depending on the time of the day compared with open field conditions. Counce (2021) found that different shade nets helped to reduce solar radiation by up to 30% to 45% compared with no-shade conditions. In the present study, aluminet shade net had the lowest temperature during late spring, and black had the lowest temperature during fall. Black shade nets were effective at cooling at greater temperatures. Ahmed et al. (2016) found air temperature reduction of 3 to 4 °C under a black shade net compared with greenhouse air temperature. Black shade nets can absorb all wavelengths of light and reduce its intensity, thus helps to reduce temperature under them. Ilic et al. (2017) also found that pearl shade nets can reduce air temperature by 1 °C, and black shade nets helped to reduce up to 3 °C. In the present study, there was an increase in relative humidity under shade nets, probably because shade nets trap the water that is transpired from the plant. Similar to the present study, Ahmed et al. (2016) reported relative humidity was nearly double under shade nets than in ambient greenhouse conditions.

Plant growth and quality.

Plants can sense variation in the light spectrum, including reductions in blue or red light (Franklin 2008). An increase in red light and loss of blue light is commonly associated with rapid vertical growth, termed shade avoidance syndrome. Ovadia et al. (2015) reported shading with red shade nets increased branches and internode length in cut flowers. Oren-Shamir and Gussakovsky (2001) reported that red shade nets increased the branching and height of plants in Japanese pittosporum (Pittosporum tobira T.). Light is known to influence specific biological pathways, which further can affect the balance between shoot and branch growth. Red light can activate phytochrome pigment which converts indole acetic acid (IAA) hormone into its inactive forms. When plants absorb red light, they start to produce kaempferol derivatives, which further bind to cofactors of IAA oxidase and keep them inactive (Mumford et al. 1961). Decrease in concentration of IAA results in the suppression of lateral branches and increase in apical dominance and plants grow vertically compared with sides.

Better performance of basil under no-shade in late spring suggests that shade nets are less beneficial to warm-season crops, as they are more likely not to reach minimum light requirements (20 to 25 mol·m−2·d−1) for optimum growth (Currey et al. 2020). Shaded plants use more resources to increase the size of their organs to get more sunlight and under full light conditions, plants produce a greater number of branches and leaves, which increases biomass production (Pierson et al. 1990). Basil is a warm-season crop, thus requiring more light and reduced light intensity under shade nets might be the cause that basil plants showed greater biomass production under no-shade net treatments. Brown et al. (1995) also concluded that plant biomass will decrease, and height will increase under increased red light and the absence of blue light. Pierson et al. (1990) found similar results, which showed increased biomass in cheat grass (Bromus tectorum L.) grown under no-shade as compared with shade nets. Tafoya et al. (2018) in cucumber (Cucumis sativus L.) also found increased biomass production under aluminet colored shade nets. Contradictory to our findings, Yavari et al. (2021) found that red shade helps to increase while aluminet shade decreases plant biomass production in arabidopsis (Arabidopsis thaliana L.) accession, and they hypothesized plants were from different geographical accessions for that study and may have had different light spectrum needs.

Chlorophyll and photosynthesis.

When phytochromes and cryptochromes receive light, these photoreceptors trigger a series of biochemical reactions that further affect phosphorylation and protonation of different proteins (Belkov et al. 2019). This phosphorylation and protonation further affect genes involved in chlorophyll synthesis and ultimately affect photosynthesis. Light under aluminet and pearl shade contains high levels of red, blue, and green wavelengths compared with black, which are required by plants and increase the efficiency of light due to diffusion. Increased photosynthesis under aluminet colored shade net may be due to the presence of red and blue light (Kong et al. 2012), along with reduced temperatures, which reduced heat stress. Similar to our finding, Tafoya et al. (2018) found that photosynthesis and stomatal conductance increased under aluminet and pearl-colored nets in cucumber.

Magnesium concentration in plant leaves is a critical factor that affects relative chlorophyll content and photosynthesis in plants. The central part of plant chlorophyll contains Mg atoms, which play an important role in increasing chlorophyll synthesis (Bohn et al. 2004). Mg is a central atom in chlorophyll ring structure and Mg is responsible for absorbing light energy and converting it into chemical energy. Dorenstouter et al. (2008) found that magnesium helps in the activation of ribulose 1,5-biphosphate carboxylase enzyme, which is the main enzyme in photosynthesis. This process helps in the fixation of CO2, which was facilitated by the activation of phosphoenolpyruvate with the help of Mg ions (Cakmak and Kirkby 2008). The greater leaf Mg concentration under aluminet shade net in lettuce suggests these plants likely have greater capacity for photosynthesis. Cakmak and Yazici (2010) found that high light intensities and high temperature can adversely affect and reduce Mg concentration in leaves, which ultimately affects relative chlorophyll content in plant leaves, which might explain why it was longer in other colored shade nets.

Nutrients.

Mou (2009) found that nutritional quality of lettuce leaves is affected by light, temperature, and growing conditions. Iron, Zn, Ca, and K are major nutrients that are required by humans in their daily diet (Eaton et al. 1996). The present study demonstrated increased concentrations for several nutrients under no-shade and pearl, which also allowed the greatest amount of light under them. Light is the main factor that controls the opening and closing of stomata, which further affects transpiration rate (Aikman and Houter 1990). Vapor pressure deficit and CO2 concentration in atmosphere and inside the greenhouse is also a significant contributor of the opening and closing of stomata (Ochsner 2022). High vapor pressure deficit increases transpiration rate and if relative humidity goes down inside the greenhouse, vapor pressure deficit goes up and vice versa. High transpiration rate in turn affects uptake, translocation, and distribution of nutrients in plant roots and leaves because roots are in direct contact with nutrients in hydroponic systems (Savvas and Passam 2002). Xu et al. (2021) found that changes in the light spectrum and increased light intensity increase the nutrient uptake and crop productivity in Arabidopsis. Counce (2021) found that nutrient concentration is dependent on season and shade net treatments in romaine lettuce grown in ebb-and-flow hydroponics tables. Zhou et al. (2019) also found that increased light intensity and temperature affect N, P, and K uptake in lettuce.

Carbohydrates.

Aluminet, pearl, and no-shade were all found to increase sugars and °Brix concentrations. Carbohydrates are made through the process of photosynthesis using light energy (Ma et al. 2016). Huber (1981) also mentioned that plants use photosynthesis to convert carbon into sugars and starches. Plants use the Calvin cycle in the process of photosynthesis, which provides energy for plants and also generates triosephosphate, which initiates carbohydrate formation (Taiz and Zeiger 1991). Triosephosphate and dihydroxyacetone phosphate translocated from chloroplasts combine with aldol to produce fructose, which turns into glucose (Halford et al. 2010).

In our study, the photosynthesis rate was greatest under aluminet and pearl nets and photosynthesis is known to correlate to sugar concentration. Unterschuetz (2022) noted that increased bitterness in lettuce makes it less favored by consumers and thus leads to reduced consumption. He further mentioned this greater sugar concentration in lettuce leaves help to increase the sweetness of lettuce leaves and bitterness in lettuce starts when bolting occurs, and sugars are transported toward reproductive growth. Li et al. (2013) found that increased sugar content during vegetative growth in lettuce leaves increases sweetness, which favors consumer preference. Unterschuetz (2022) and McLemore (2022) also found that temperature plays a significant role in the accumulation of sugar concentration in lettuce leaves. Zhian et al. (1994) found increased sugar concentration under greater light intensity in ginseng (Panax quinquefolius L.). The inverse relationship between sugars and starches in the present study is illustrative of carbohydrate allocation patterns in most plants. Namely, starches are stored as energy and can be converted to sugars (maltose), and these sugars provide energy for plant growth and development (Halford et al. 2010). Halford et al. (2010) also found that sugar and starch content is highly dependent on genetic constituent of different cultivars. In our study °Brix and carbohydrates were not correlated (data not shown). Plants contain different pools of soluble sugars (glucose, fructose, sucrose, galactose, and maltose) and polysaccharides like starch (Chow and Landhausser 2004). The anthrone regent method analyzed all soluble sugars and starches, whereas °Brix only measures sucrose values in plant leaves. °Brix measures the percent weight of total soluble sugars present in a sucrose solution (Dongare et al. 2015; Thakulla et al. 2021), and is commonly used to measure total soluble solids in different fruits and vegetables.

Conclusion

In conclusion, this study is consistent with other findings that colored shade nets increase plant height, photosynthesis, and relative chlorophyll content in lettuce and basil. In contrast, biomass, yield, and leaf nutrient concentration of lettuce and basil were greatest under no-shade. Colored shade nets (aluminet, pearl, and red), having more transmittance of red light, are best at increasing photosynthesis and sugar concentration, while no-shade was best to increase biomass and nutrient concentration in lettuce and basil leaves. Basil ‘Genovese’ and lettuce ‘Green Forest’ cultivars had the greatest amount of nutrients. Both basil cultivars had greater shoot fresh and dry weight in fall, whereas both lettuce cultivars grew better in the late spring. Future studies should evaluate different locations in terms of light levels and temperature effects using these shade nets or different shade intensities on other species and cultivars, as those factors are known to influence plant growth and quality.

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  • Fig. 1.

    Reflectance percentage in different wavelengths of light under no-shade (A) and 50% shading intensity of aluminet (B), black (C), pearl (D), and red colored shade nets during 2021 late spring and fall seasons with basil and lettuce in ebb-and-flow hydroponic systems under greenhouse conditions in Stillwater, OK, USA.

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    • Export Citation
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    • Export Citation
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    • Search Google Scholar
    • Export Citation
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    • Search Google Scholar
    • Export Citation
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Harpreet Singh Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Bruce L. Dunn Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Charles Fontanier Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Hardeep Singh Department of Agronomy, University of Florida, 4253 Experiment Drive, Highway 182, Jay, FL 32565, USA

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Amandeep Kaur Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Lu Zhang Department of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078-6027, USA

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Contributor Notes

B.L.D. is the corresponding author. E-mail: bruce.dunn@okstate.edu.

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