Effects of Elevated Water Temperature on Growth of Basil Using Nutrient Film Technique

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Teal HendricksonDepartment of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078

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Bruce L. DunnDepartment of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078

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Carla GoadDepartment of Statistics, Oklahoma State University, 301F MSCS Building, Stillwater, OK 74078

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Bizhen HuDepartment of Horticulture and Landscape Architecture, Oklahoma State University, 358 Ag Hall, Stillwater, OK 74078

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Hardeep SinghWest Florida Research and Education Center, Department of Agronomy, University of Florida, 4253 Experiment Dr., Hwy. 182, Jay, FL 32565

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Hydroponic systems have become increasingly popular for growers in recent years for year-round local production. Whereas optimal air temperature for plant growth has been considered, optimal root zone temperatures have not been examined as thoroughly. The objective of this research was to determine the optimal water temperature for growing different types of basil hydroponically. Research was conducted at the greenhouses in Stillwater, OK. Seventeen cultivars were selected from six main types of basil and transplanted into Nutrient Film Technique hydroponic systems, and three water temperature treatments were applied: 23, 27.5, and 31 °C. Height, width, average leaf area, leaf number, chlorophyll concentration (chlorophyll readings obtained with the Minolta-502 SPAD meter), shoot fresh weight, shoot dry weight, and root dry weight were evaluated. In general, the 27.5 and 31 °C treatments were not greater than each other in terms of leaf number and root dry weight but were greater than the 23 °C treatment. The 31 °C treatment had the greatest height, whereas width, average leaf area, shoot fresh weight, and shoot dry weight were not different from the 27.5 °C treatment. The 23 °C treatment had the greatest chlorophyll concentration (SPAD) value. Cultivar differences were significant in average leaf area and SPAD, with ‘Spicy Bush’ having the smallest leaf area and purple basil having the greatest SPAD value. For all cultivars except purple basil and ‘Large Leaf Italian’, a 27.5 °C water temperature would be recommended for greater plant growth.

Abstract

Hydroponic systems have become increasingly popular for growers in recent years for year-round local production. Whereas optimal air temperature for plant growth has been considered, optimal root zone temperatures have not been examined as thoroughly. The objective of this research was to determine the optimal water temperature for growing different types of basil hydroponically. Research was conducted at the greenhouses in Stillwater, OK. Seventeen cultivars were selected from six main types of basil and transplanted into Nutrient Film Technique hydroponic systems, and three water temperature treatments were applied: 23, 27.5, and 31 °C. Height, width, average leaf area, leaf number, chlorophyll concentration (chlorophyll readings obtained with the Minolta-502 SPAD meter), shoot fresh weight, shoot dry weight, and root dry weight were evaluated. In general, the 27.5 and 31 °C treatments were not greater than each other in terms of leaf number and root dry weight but were greater than the 23 °C treatment. The 31 °C treatment had the greatest height, whereas width, average leaf area, shoot fresh weight, and shoot dry weight were not different from the 27.5 °C treatment. The 23 °C treatment had the greatest chlorophyll concentration (SPAD) value. Cultivar differences were significant in average leaf area and SPAD, with ‘Spicy Bush’ having the smallest leaf area and purple basil having the greatest SPAD value. For all cultivars except purple basil and ‘Large Leaf Italian’, a 27.5 °C water temperature would be recommended for greater plant growth.

Hydroponic systems have become increasingly popular in recent years due to the changing climate and decreased abundance of agricultural water (Sambo et al., 2019). Hydroponic systems provide greater control of experimental variables, such as water temperature, than traditional soil-based research (Shrestha and Dunn, 2016). The Nutrient Film Technique (NFT) system, one type of hydroponic system, is ideal for herbs and other short crops because water is pumped in a thin layer, or film, of constantly recirculating nutrient solution (Dholwani et al., 2018; Mohammed and Sookoo, 2016; Resh, 1978). Because hydroponics systems are generally used in greenhouses, more absolute control over environmental variables, such as temperature, is an essential component of production (Nxawe et al., 2009).

Air temperature has long been considered as an experimental variable while growing crops because extreme cold and heat can cause a multitude of issues. Temperature influences most plant processes, such as photosynthesis, transpiration, respiration, germination, and flowering (Van Der Zanden, 2008). Generally, increasing temperature increases the rate of photosynthesis, transpiration, and respiration to a point (Van Der Zanden, 2008). Rice (Oryza sativa L.) has been shown to increase photosynthesis and yield in high temperatures, but, conversely, cotton (Gossypium hirsutum L.) decreased in photosynthesis and therefore boll filling (Mondal et al., 2016). For many leafy and vegetable crops, lower temperatures are desired to reduce energy use and increase sugar storage, creating a sweeter fruit or leafy green, whereas adverse temperatures can create bitterness in greens (Van Der Zanden, 2008). However, air temperature is only one factor in examining how temperature affects crops overall throughout their growth cycle, and, as the cost of fossil fuels and winter heating climbs, heating the root zone or hydroponic water may be an adequate solution (Kawasaki et al., 2014).

Root zone temperature (RTZ) is often a concept less considered than air temperature in relation to plant growth; however, RZTs have been shown to correlate to an increase in rooting percentages and shoot growth in both rootstocks and crops such as tomato (Solanum lycopersicum L.), cucumber (Cucumis sativus L.), and pepper (Capsicum annuum L.) (Nxawe et al., 2009; Rosik-Dulewska and Grabda, 2002). A greater RZT allows more and quicker nutritional uptake, whereas a lower RZT can slow nutritional uptake (Moorby and Graves, 1980; Yan et al., 2012). Optimized root conditions can also substitute for poor air temperature, as seen in lettuce with a RZT of 24 °C grown in 31 °C air temperature, which still produced a marketable product, in contrast to a higher RZT of 31 °C, which caused wilting (Thompson et al., 1998). In soil environments, time of year and air temperature can have a significant impact on RZT. Tomatillos (Physalis ixocarpa Brot. ex Hornem) grown in the spring had an increase in dry shoot weight as the RZT increased; however, in the summer, dry shoot weight decreased with increased RZT during the establishment phase of growth (Diaz-Perez et al., 2005). The roots themselves are also highly sensitive to temperature change, especially root depth and root width, as roots adapt to their environment (Luo et al., 2020). High temperature may significantly accelerate the root meristem cell division, thus the development of lateral root primordium, creating more branches but possibly thinner roots depending on species (Luo et al., 2020). This concept is applicable not only in soil media but also in hydroponic nutrient solutions.

In previous research, using chilled water temperatures to mimic cold air temperatures to convince cool season crops to grow has been the done (Thompson et al., 1998). However, other research has looked at heating water to encourage greater nutrient uptake, increased transpiration, nutritional absorption, root length, and leaf area (Moorby and Graves, 1980). Plant such as tomatoes and spinach (Spinacia oleracea L.) often thrive with warmer root temperatures as a result of increased metabolism that allows for more nutrient uptake and shoot growth (Wang et al., 2022). In a study using condensation irrigation in pots with soil, pots treated with the elevated temperatures secondary to condensation irrigation stimulated root growth in green basils, though specific cultivars were not reported (Arabnejad et al., 2021).

Unlike some hydroponic crops (e.g., lettuce, Lactuca sativa L.) that prefer cooler temperatures, basil (Ocimum basilicum L.) is commonly found in tropical and subtropical regions, preferring high light and an optimal air temperature ≈26 °C (Currey, 2020; Raimondi et al., 2006; Thakulla et al., 2021). Basil is a highly popular crop due to its usefulness in a variety of fields, such as a culinary herb, a medicinal plant, and an agent to control pests (Raimondi et al., 2006). Nevertheless, controlling the phenolic content and phytochemical concentration is difficult, and basil has been cultivated more frequently in controlled environments in recent years (Dou et al., 2018). Unfortunately, many herbs may exhibit low yield in hydroponic systems for unknown reasons (Bulgari et al., 2016). Thus, the objective of this research was to ascertain whether various cultivars of basil would produce better growth under different RZTs that more accurately match their preferred tropical and subtropical climate.

Materials and Methods

Plant materials and growth conditions.

The experiment was conducted at the Oklahoma State University Department of Horticulture and Landscape Architecture research greenhouses in Stillwater, OK. Average air temperatures for each run were 23.27, 27.97, and 30.11 °C. Average humidity for each run was 45.86%, 70.44%, and 66.28%. Daily light integral (DLI) averaged 26.6, 19.2, and 29.9 mol·m−2·d−1 for each run. Seeds of 17 cultivars of basil—Sweet Thai, Cardinal, Red Rubin, Prospera, Cinnamon, Nufar, Italian Large Leaf, Karpoor Tulsi, Lemon, Amethyst, Dark Opal, Spicy Bush, Lime, Rutgers Devotion, Genovese, and Elide—were obtained from Johnny’s Selected Seeds (Winslow, MN) and planted in Horticubes Grow Cubes (Smithers Oasis, Kent, OH) on 1 Apr. 2021, and repeated on 7 May 2021 and 1 July 2021, and placed under misters for 4 weeks. Ten seedlings in cubes per cultivar per table were transplanted to an NFT hydroponic table (Botanicare, Vancouver, WA) on 22 Apr. 2021, and was repeated on 4 June 2021 and 29 July 2021. The 40-gal tanks were filled with tap water and 147.41 g Jack’s 5N–5.2P–21.6K (J.R. Peters, Allentown, PA) along with 97.52 g of calcium nitrate (American Plant Products, Oklahoma City, OK), which were added initially according to the recommended rates. The pH and electrical conductivity (EC) of the solution were checked every other day, and full-strength fertilizer mix (as described earlier) was added as needed to maintain the pH between 5.5 and 6.5 and the EC at 1.5 to 2.5 mS·cm−1, as recommended by Singh and Dunn (2016).

Treatments and data collection.

The NFT tanks were heated or cooled to three temperatures: 23, 27.5, and 31 °C. Two heaters, Orlushy Submersible Aquarium Heater at 300 W (NOVA Pet Appliance CO., Guangdong, China) and the Aqua Heat Titanium Reservoir and Aquarium Heater at 200 W (Sunlight Supply Inc., Vancouver, WA), were used in tandem to maintain temperatures within a single tank due to heat retainment issues. For the 23 °C table, an Active Aqua Water Chiller (Hydrofarm, Petaluma, CA) was used to maintain a cooler temperature. Temperature readings were taken every 2 h using HOBO thermocouple data loggers (Onset, Cape Cod, MA). Dissolved oxygen (DO) was measured daily using a DO meter (Milwaukee Instruments, Rocky Mount, NC).

Each plant on the table was scanned using a chlorophyll meter (SPAD-502, Konica Minolta, Japan) 30 d after transplanting. SPAD readings were taken from each plant from the middle of the top and bottom leaves and were averaged to determine the chlorophyll concentration. Plants were harvested 30 d after transplanting. Fresh shoot weight and leaf area were assessed on the day of harvest. Leaf area was measured using a LI-COR L1-3100C Area Meter (LI-COR Biosciences, Lincoln, NE), selecting the third leaf from the top of each plant. Each leaf was measured twice and averaged. Shoots and roots were dried at 59 °C for 2 d. Three samples from each treatment per replication of five cultivars, Genovese, Dark Opal, Kapoor Tulsi, Lemon, and Prospera, were chosen to be analyzed for total nitrogen, phosphorus, magnesium, sulfur, calcium, potassium, boron, zinc, copper, and manganese and were then sent to the Soil, Water and Forage Analytical Laboratory at Oklahoma State University for analysis of leaf mineral element concentrations using a nutrient analyzer (TruSpec Carbon and Nitrogen Analyzer; LECO Corp., St. Joseph, MI).

Experimental design and statistical analysis.

The study was conducted using a split-plot in a randomized complete block design with three replications. The whole main plot was water temperature (three levels), and cultivar of basil (17 levels) was the subplot factor. Statistical analysis was performed using SAS/STAT software (Version 9.4; SAS Institute, Cary, NC). Tests of significance were reported at the 0.05, 0.01, and 0.001 levels. The data were analyzed using generalized linear mixed models methods. Tukey multiple comparison methods were used to separate the means.

Results

Temperature and cultivar effect and their interaction on plant growth parameters.

There was a significant cultivar × temperature interaction for number of leaves and root dry weight (Table 1). The cultivar Cinnamon in the 31 °C heated treatment had the greatest number of leaves but was not different from the 27.5 °C treatment (Table 2). For ‘Lime’, ‘Sweet Thai’, ‘Kapoor Tulsi’, ‘Nufar’, and ‘Prospera’ either 27.5 °C or 31 °C treatments also resulted in greater leaf numbers. For ‘Red Rubin’, ‘Mrs. Burns Lemon’, and ‘Large Leaf Italian’, greater number of leaves were recorded with the 31 °C treatment, whereas ‘Cardinal’ had greater numbers with the 27.5 °C treatment. Within the purple type, ‘Amethyst’ has the least number of leaves compared with ‘Red Rubin’ and ‘Dark Opal’. ‘Mrs. Burns Lemon’ had a significantly greater number of leaves in the 23 °C treatment than ‘Lime’, although the 27.5 °C and 31 °C treatments were not significantly different. ‘Sweet Thai’ and ‘Cinnamon’ had the greatest number of leaves in the Asian basils but were only significantly greater than ‘Cardinal’. In the large leaf basils, ‘Nufar’ had significantly greater number of leaves overall than ‘Italian Large Leaf’. The number of leaves for classic basils was not significantly different from one another but was significantly lower than citrus basils and some Asian basils, such as ‘Sweet Thai’ and ‘Cinnamon’.

Table 1.

Tests of effects for water temperature (23, 27.5, and 31 °C) and 17 basil cultivars grown in Nutrient Film Technique hydroponics systems at Oklahoma State University research greenhouses in Stillwater, OK.

Table 1.
Table 2.

Least square means interaction between basil cultivar and water temperature (23, 27.5, and 31 °C) for number of leaves and root dry weight for 17 basil cultivars in Nutrient Film Technique hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.

Table 2.

‘Nufar’ in the 31 °C heated treatment had the greatest root dry weight and was not different from the 27.5 °C, but both were greater than the 23 °C treatment (Table 2). ‘Dark Opal’ and ‘Mrs. Burns Lemon’ were greater at 31 °C than 23 °C but were not different from 27.5 °C. For ‘Amethyst’, root dry weight under 27.5 °C was greater than 23 °C but was not different from the 31 °C treatment. Purple, citrus, and fine leaf basils had the lowest root dry weight compared with the other types. Large leaf basils had greater root dry weights than other types. Asian and classic basils tended to have greater root dry weights at warmer temperatures but were not different from other types at the lowest temperature.

Plant height, average leaf area, shoot fresh weight, shoot dry weight, and SPAD showed significant main effects for cultivar (Table 1). ‘Kapoor Tulsi’ was the tallest cultivar but was not significantly different from any other cultivar except ‘Spicy Bush’ (Table 3). Average leaf area was greatest for ‘Nufar’ but not significantly different from any of the other cultivars except ‘Spicy Bush’. Regarding shoot fresh weight and dry weight, ‘Cinnamon’ had the greatest weight, but was only significantly different from ‘Amethyst’. Greatest value for SPAD was observed in ‘Amethyst’, which was not significantly different from ‘Red Rubin’, ‘Dark Opal’, ‘Cinnamon’, ‘Sweet Thai’, ‘Kapoor Tulsi’, and ‘Rutgers Devotion’. In general, purple and Asian varieties had greater values, whereas fine leaf and large leaf varieties were lower.

Table 3.

Least square means of six types and 17 cultivars on growth and quality of basil grown in Nutrient Film Technique hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.

Table 3.

Root zone temperature main effects were observed for parameters including plant height, width, average leaf area, shoot fresh weight, shoot dry weight, and SPAD (Table 1). The 31 °C treatment produced the tallest plants, whereas there was no difference between the other two temperature treatments (Table 4). The basil plant widths under 31 and the 27.5 °C treatments were not different from each other but were significantly greater than that under 23 °C treatments. The 31 °C had the greatest average leaf area but was not different from 27.5 °C. The 27.5 and 31 °C shoot fresh and dry weights were not significantly different from each other but were greater than the 23 °C treatment. The greatest SPAD value was found in 23 °C treatment but was not significantly different from the 27.5 °C treatment.

Table 4.

Least square means of three temperatures on growth and quality of basil grown in Nutrient Film Technique hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.

Table 4.

For nutrient content, cultivar effect was significant for magnesium, sulfur, and iron content, whereas temperature treatment effect was significant for total nitrogen, copper, and manganese (Table 5). Total nitrogen and copper content were the greatest in the 27.5 °C treatment, and this treatment was significantly greater than the 23 and 31 °C treatments. Similarly, manganese content was found to be the greatest in the 27.5 °C treatment, which was significantly greater than the other two treatments (Table 6). ‘Dark Opal’ had the greatest iron content but was only significantly different from ‘Mrs. Burns Lemon’ (Table 7). ‘Mrs. Burns Lemon’ had reduced amounts of sulfur compared with all other cultivars. ‘Kapoor Tulsi’ had the greatest magnesium content, which was significantly greater than all other cultivars.

Table 5.

Analysis of variance for nutrient content in five basil cultivars grown in Nutrient Film Technique hydroponics systems at Oklahoma State University research greenhouses in Stillwater, OK.

Table 5.
Table 6.

Least square means of water temperature on nutrient levels in five basil cultivars (Kapoor Tulsi, Dark Opal, Mrs. Burns Lemon, Genovese, and Prospera) grown in Nutrient Film Technique hydroponics systems at Oklahoma State University research greenhouses in Stillwater, OK.

Table 6.
Table 7.

Least square means cultivar effects of nutrient content of select basil cultivars grown in a Nutrient Film Technique hydroponic system at the Oklahoma State University research greenhouses in 2021.

Table 7.

Water temperature and dissolved oxygen.

Daily water temperature in the 23 °C chilled control treatment ranged from 19.0 to 23.0 and 22.8 to 25.4 °C during the first and second replications, respectively (Fig. 1). Average temperature for the 23 °C chilled control treatment was 21.6 and 23.8 °C during the first and second replications, respectively. Data for the third repetition of 23 °C was lost due to a data logger malfunction. Daily water temperature in the 27.5 °C heated treatment ranged from 22.0 to 30.3, 24.9 to 31, and 23.2 to 30.4 °C during the first, second, and third replications, respectively. Average temperature for the 27.5 °C heated treatment was 26.4, 28.5, and 27.9 °C during the first, second, and third replications, respectively. Daily water temperature in the 31 °C heated treatment ranged from 21.7 to 33.1, 24.9 to 33.8, and 29.8 to 34.1 °C during the first, second, and third replications, respectively. Average temperature for the 31 °C heated treatment was 29.1, 31.2, and 31.9 °C during the first, second, and third replications, respectively. In terms of daily dissolved oxygen within each NFT tank, the 23 °C treatment was significantly greater than the 31 °C treatment at a mean of 6.47 mg·L−1 throughout the growing season (Fig. 2).

Fig. 1.
Fig. 1.

Average daily temperature of nutrient solution for three treatments (23, 27.5, and 31 °C) using a Nutrient Film Technique hydroponic systems over three replications (Spring, Summer, and Fall) in Stillwater, OK, in 2021.

Citation: HortScience 57, 8; 10.21273/HORTSCI16690-22

Fig. 2.
Fig. 2.

Effects of temperature difference (23, 27.5, and 31 °C) on average dissolved oxygen levels of nutrient solution in Nutrient Film Technique hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK, in 2021.

Citation: HortScience 57, 8; 10.21273/HORTSCI16690-22

Discussion

Effects of cultivar and water temperature on plant growth.

Generally, 27.5 °C is considered the ideal air temperature for growing basil (Currey, 2020). Air temperatures in this research averaged 23.27, 27.97, and 30.11 °C in each repetition, respectively. Maintaining constant higher greenhouse temperatures is usually not practical due to equipment and input costs. Root growth can show an increase under optimal air conditions; however, root zone heating allows for the same root growth under suboptimal air temperatures (Kawasaki et al., 2014).

In this study, root dry weights increased as the temperature increased for most basil cultivars, with the exception of purple basils and some large leaf basils. These results are similar to what Kawasaki et al. (2014) saw in heating the root zone of tomatoes. On the 7th and the 14th days of heating the root zone, root growth and dry weight were greater than the ambient control under suboptimal air conditions. Unlike in this study, when exposed to low or high RZTs, Korean mint (Agastache rugosa L.) exhibited significantly lower fresh and dry root weight due to temperature stress (Lam et al., 2020). In a study with cucumber cultivar Super N3, Haghighi et al. (2015) found that 25 °C was ideal for increasing dry root weight because lower and higher temperatures caused stress on the plants. Al-Rawahy et al. (2019) found that cooled RZTs of 22 and 25 °C produced significantly more leaves than the 28 °C treatment or the 33 °C control in cucumber ‘Reema F1’, once again stressing the importance of optimal temperature to produce better yield is dependent on crop and cultivar. Sakamoto and Suzuki (2015b) found that root zone heating to 33 °C significantly reduced leaf number in carrots (Daucus carota L. cv Tokinashigosun), whereas in lettuce, cooling root temperatures did not significantly change leaf number (Sun et al., 2016). However, in soybeans (Glycine max L.), increasing RZT increased leaf number (Dashti et al., 2016). Root weight and architecture are largely dependent on species because different species have different optimal temperatures (Luo et al., 2020).

Cultivar itself can also play a significant role in root biomass, magnifying the effect of elevated water temperature (Lazarević et al., 2020). Cultivar differences were seen in height, average leaf area, shoot fresh weight, shoot dry weight, and SPAD in this research. Walters and Currey (2015) found that citrus basils had significantly greater height when compared with sweet, purple, Asian, large leaf, and fine leaf basil, which was contrary to what was found in this study. Like this research, Spicy Bush has been found to be one of the shortest cultivars, often being ≈11 to 15 cm tall (Svecov and Neugebauerova, 2010; Walters and Currey, 2015). Asian basils, such as ‘Kapoor Tulsi’, are generally among the taller cultivars, although usually not significantly different from other cultivars, unlike in the present study (Simon et al., 1999; Walters and Currey, 2015). Upadhyay (2017) reported that ‘Kapoor Tulsi’ is usually 30 to 60 cm tall, which matches the findings of our study, leading to the conclusion that ‘Kapoor Tulsi’ is a taller cultivar than other cultivars of basil. In the present study, ‘Spicy Bush’, a fine leaf basil, had the smallest leaf area, and ‘Nufar’, a large leaf basil, had the largest average leaf area. Marotti et al. (1996) and Walters and Currey (2015) found similarly that average leaf area was found to be greatest in large leaf varieties, whereas fine leaf, bush, and compact varieties had the smallest leaf size. This was due to genetic differences between the plants.

Basil shoot fresh weight and dry weight are often gauged by basil color. In this study, ‘Amethyst’ had the lowest shoot fresh and shoot dry weight, with ‘Cinnamon’, a green Asian basil, having the greatest shoot weight. In general, purple basils have been found to have less fresh and dry shoot weight compared with green basils (Abbas, 2014; Marotti et al., 1996; Walters and Currey, 2015; Yaldiz et al., 2015). However, Svecov and Neugebauerova (2010) found that ‘Lime’ had the lowest fresh weight, and ‘Spicy Bush’ had the lowest dry weight yield. This was contrary to what we found but is likely due to the differences between hydroponic and field studies regarding basil.

Among basil cultivars, there are significant differences in chlorophyll content due to genetic variability causing differences in pigmentation (Lazarević et al., 2020). SPAD readings for the green varieties (citrus, Asian, fine leaf, large leaf, and classic) from this study correspond to similar studies, the average reading being between 23 and 31 (Lazarević et al., 2020; Singh et al., 2019; Teliban et al., 2020; Yang and Kim, 2020). The purple basil had greater SPAD values compared with most of the other green varieties; however, these numbers match the values found in previous studies of red basil, which are generally between 32 and 40 (Saha et al., 2016; Teliban et al., 2020; Yang and Kim, 2020). SPAD readings and total chlorophyll content of leaves have been found to be significantly correlated (Jiang et al., 2017).

Water temperature was found to have significant effects on plant growth and development. At ideal temperatures (27.5 °C), water temperature improved growth. White jute (Corchorus capsularis L.), an annual herb native to tropical Asia, when grown at high temperatures, showed an increase in stem length but reduced leaf area, because high temperature can increase photosynthetic rate which make leaves less important (Luo et al., 2020). If the temperature strays too high or too low, damage can occur to the plant and its metabolism (He et al., 2002). Low temperature can affect the imbalance between growth inhibitors and promoters in plants such as abscisic acid, cytokinin, and gibberellins, which are mainly synthesized in the root apical meristems (Lam et al., 2020). Korean mint was thought to suffer water stress at the 10 °C treatment due to the possibility that water uptake was inhibited from root to shoot during the treatment’s duration (Lam et al., 2020). Spinach (Spinacia olearacea L.) was found to have increased leaf number and length, as well as shoot fresh weight, at higher temperatures of 24, 26, and 28 °C compared with the 10 °C control (Nxawe et al., 2009). At 25 °C, Pak choy (Brassica rapa L.) produced the greatest growth and yield at 38 °C compared with the control of 25 °C (Maludin et al., 2020). Low temperatures can also precede to low rates of cell expansion, leading to many small cells per given area, making the leaves denser and smaller, reducing single leaf area (Carotti et al., 2021). Sakamoto and Suzuki (2015a) found in a study with red leaf lettuce ‘Red Wave’ that the exposure of lettuce roots to a low temperature of 10 °C significantly reduced leaf area, stem diameter, and fresh weight of tops and roots compared with 20 °C.

However, if the water temperature is greater than the optimal level, roots may be damaged, affecting stem length and diameter (Lam et al., 2020). High temperatures, such as 32 °C, have been known to cause tip-burn and other physiological issues in lettuce (Carotti et al., 2021). High root-zone temperatures, such as 27.5 °C, resulted in water and nutrient loss, which ultimately led to reduction of plant growth in tomatoes (Díaz-Pérez et al., 2007). In cucumbers, a high temperature of 35 °C was found to lower fresh and dry shoot weight, as well as decreasing chlorophyll and antioxidant content (Haghighi et al., 2015). Higher temperatures can also be greatly beneficial to plants. Elad et al. (2017) found that heating sweet basil allowed for the repression of disease, specifically white and gray mold. Wang et al. (2016) found that in newly unfurled cucumber leaves, warming of the root zone caused an increase in leaf area expansion and reduced the stomatal limitation of photosynthesis, while improving water supply from the roots. The present study found that warmer water temperatures led to a greater leaf count and average leaf area, which are often the most important factors in basil quality.

Water temperature can also cause variability in chlorophyll content. Higher RZTs can affect the physiological processes of the plant, such as chlorophyll content, and, consequently, plant metabolism, while any resultant heat stress can trigger significant changes in plant physiological processes, such as water uptake and leaf photosynthesis, as well as decreased SPAD values due to a decrease in chlorophyll content (Al-Rawahy et al., 2019). SPAD readings are a quick, nondestructive gauge of relative chlorophyll concentration, and Ruiz-Espinoza et al. (2008) found that SPAD readings were strongly correlated with chlorophyll content in basil. Lower SPAD values at high temperatures may be due to cessation of chlorophyll production or chlorophyll degradation under higher temperatures (Haghighi et al., 2015). However, Kalisz et al. (2016) found that lower temperatures increased the chlorophyll-a to chlorophyll-b ratio, increasing SPAD values.

Nutrient content is often an important indicator of plant nutrient uptake and quality. Differences in nutrient content among cultivar are common in basil, often expressing in magnesium, iron, calcium, and zinc, aligning similarly with the differences found in this study (Lazarević et al., 2020). Values for iron were similar to previous studies that averaged between 80 to 100 ppm, whereas this study averaged 135 ppm due to high outliers such as ‘Dark Opal’ and ‘Prospera’; however, ‘Kapoor Tulsi’ and ‘Genovese’ were within the normal average range (Lazarević et al., 2020; Saha et al., 2016). In the case of sulfur, known values and values from the current study both stayed at ≈0.30 ppm (Saha et al., 2016). Magnesium levels in the current study were somewhat greater than found in some reports, but this is thought to be due to differences in cultivars (Saha et al., 2016). Root zone heating has been shown to increase the uptake of total nutrients and promote their transport to the shoot of the plant (Kawasaki et al., 2014; Lam et al., 2020). In a study on RZTs and their influence on tomatoes in winter, increased temperatures allowed for greater total nutrient uptake and lowered uptake in cooler temperatures; however, at high air temperatures, root cooling allowed for better total nutrient uptake (Kawasaki et al., 2014). Lam et al. (2020) reported that in cucumbers, a higher RZT allowed for an increase in nutrient uptake. As each species has its own optimal temperature, a study on melons (Cucumis melo L. cv. Arava), cucumbers, and rape (Brassica napus cv. Emerald) found that each species had a different optimal RZT for optimal nutrient uptake (Yan et al., 2012). In a study comparing hydroponic and aquaponic basil growth with the green basil cultivar Aroma II, Saha et al. (2016) found that magnesium content was similar to the 23 °C treatment, due to their average water temperature of 23.4 °C. The warmer temperatures investigated in this study are suspected to be the cause of the increase compared with the Saha et al. (2016) study.

Dissolved oxygen and water temperature.

In the present study, dissolved oxygen was significantly greater in the coolest temperature treatment, but increased dissolved oxygen did not increase plant growth. Dissolved oxygen levels are inversely related to water temperature, with lower temperatures holding more oxygen and higher temperatures holding less oxygen (Falah et al., 2010; U.S. Environmental Protection Agency, 2012). As reported by Al-Rawahy et al. (2019), the temperature of a nutrient solution has a direct relationship to the amount of oxygen consumed by plants and an inverse relation to the oxygen dissolved. Nutrient solution cooling to 22 and 25 °C provided positive effects on the availability of dissolved oxygen levels in the nutrient solutions as well as on all growth parameters (plant height, leaf number, chlorophyll content, leaf area) and production parameters (number of fruits and yield) in cucumber. Low levels of oxygen in the water can reduce plant growth and development, as well as increase the likelihood of pathogens in hydroponic systems (Yan et al., 2012). However, according to Ruso et al. (2021), basil can persist with dissolved oxygen levels as low as 4 mg·L−1, with optimal levels at 6.5 mg·L−1. In the present study, dissolved oxygen rates were well above the minimum requirements for basil, thus elevated water temperature provided more benefits than could be accounted for by increased oxygen.

Conclusion

In this study, most types of basil grown in nutrient solution heated to 27.5 and 31 °C had overall better growth than basil grown at a 23 °C nutrient solution. Purple basil and some large leaf basils, such as ‘Italian Large Leaf’ were unaffected. SPAD values were the greatest in the 23 °C treatment, however, but were not significantly different from the 27.5 °C treatment. Total nitrogen content was greatest in the 27.5 °C treatment. These results suggest that heating nutrient solutions to 27.5 °C will promote basil growth and quality in most types of basil grown in NFT hydroponic systems. Application and economics of root zone heating will depend on grower location, time of year, utility costs, scale of production, and ability to control the greenhouse environment. Further research is needed to determine the most economical and efficient way to heat the nutrient solution and what effect warmer RZT has on phenolic content.

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Contributor Notes

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

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    Fig. 1.

    Average daily temperature of nutrient solution for three treatments (23, 27.5, and 31 °C) using a Nutrient Film Technique hydroponic systems over three replications (Spring, Summer, and Fall) in Stillwater, OK, in 2021.

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    Fig. 2.

    Effects of temperature difference (23, 27.5, and 31 °C) on average dissolved oxygen levels of nutrient solution in Nutrient Film Technique hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK, in 2021.

  • Abbas, M.S. 2014 Assessment of density and cultivation type on growth and yield of two cultivars of basil (Ocimum basilicum L.) Intl. J. Agron. Agr. Res. 5 1 74 79

    • Search Google Scholar
    • Export Citation
  • Al-Rawahy, M.S., Al-Rawahy, S.A., Al-Mulla, Y.A. & Nadaf, S.K. 2019 Effect of cooling root-zone temperature on growth, yield and nutrient uptake in cucumber grown in hydroponic system during summer season in cooled greenhouse J. Agr. Sci. 11 1 47 60 https://doi.org/10.5539/jas.v11n1p47

    • Search Google Scholar
    • Export Citation
  • Arabnejad, H., Mirzaei, F. & Noory, H. 2021 Greenhouse cultivation feasibility using condensation irrigation (studied plant: Basil) Agr. Water Manage. 245 106526 https://doi.org/10.1016/j.agwat.2020.106526

    • Search Google Scholar
    • Export Citation
  • Bulgari, R., Baldi, A., Ferrante, A. & Lenzi, A. 2016 Yield and quality of basil, Swiss chard, and rocket microgreens grown in a hydroponic system N. Z. J. Crop Hort. Sci. 45 2 119 129 https://doi.org/10.1080/01140671.2016.1259642

    • Search Google Scholar
    • Export Citation
  • Carotti, L., Graamans, L., Puksic, F., Butturini, M., Meinen, E., Heuvelink, E. & Stanghellini, C. 2021 Plant factories are heating up: Hunting for the best combination of light intensity, air temperature and root-zone temperature in lettuce production Front. Plant Sci. 11 592171 https://doi.org/10.3389/fpls.2020.592171

    • Search Google Scholar
    • Export Citation
  • Currey, C.J. 2020 Managing basil production throughout the year Produce Grower. 4 May 2022. <https://www.producegrower.com/article/hydroponic-production-primer-managing-basil-production-throughout-the-year>

    • Search Google Scholar
    • Export Citation
  • Dashti, N.H., Cherian, V.M. & Smith, D.L. 2016 Soybean production and suboptimal root zone temperatures 217 240 Miransari, M. Abiotic and Biotic Stresses in Soybean Production. Academic Press New York, NY

    • Search Google Scholar
    • Export Citation
  • Dholwani, S., Marwadi, S., Patel, V. & Desai, V.P. 2018 Introduction of hydroponic system and it’s methods Intl. J. Res. Trends Innovation 3 3 69 73

    • Search Google Scholar
    • Export Citation
  • Díaz-Pérez, J., Gitaitis, R. & Mandal, B. 2007 Effects of plastic mulches on root zone temperature and on the manifestation of tomato spotted wilt symptoms and yield of tomato Scientia Hort. 114 90 95 https://doi.org/10.1016/j.scienta.2007.05.013

    • Search Google Scholar
    • Export Citation
  • Diaz-Perez, J.C., Phatak, S.C., Giddings, D., Bertrand, D. & Mills, H. 2005 Root zone temperature, plant growth, and fruit yield of tomatillo as affected by plastic film mulch HortScience 40 5 1312 1319 https://doi.org/10.21273/HORTSCI.40.5.1312

    • Search Google Scholar
    • Export Citation
  • Dou, H., Niu, G., Gu, M. & Masabni, J.G. 2018 Responses of sweet basil to different daily light integrals in photosynthesis, morphology, yield, and nutritional quality Amer. Soc. Hort. Sci. 53 4 496 503 https://doi.org/10.21273/HORTSCI12785-17

    • Search Google Scholar
    • Export Citation
  • Elad, Y., David, D.R., Israeli, L. & Fogel, M. 2017 Passive heat treatment of sweet basil crops suppresses white mould and grey mould Plant Pathol. 66 105 114 https://doi.org/10.1111/ppa.12550

    • Search Google Scholar
    • Export Citation
  • Falah, M.A.F., Wajima, T., Yasutake, D., Sago, Y. & Kitano, M. 2010 Responses of root uptake to high temperature of tomato plants (Lycopersicon esculentum Mill.) in soil-less culture J. Agr. Technol. 6 3 543 558

    • Search Google Scholar
    • Export Citation
  • Haghighi, M., Mozafariyan, M. & Abdolahipour, B. 2015 Effect of cucumber mycorrhiza inoculation under low and high root temperature grown on hydroponic conditions J. Crop Sci. Biotechnol. 18 2 89 96 https://doi.org/10.1007/s12892-014-0083-4

    • Search Google Scholar
    • Export Citation
  • He, L., Nada, K., Kasukabe, Y. & Tachibana, S. 2002 Enhanced susceptibility of photosynthesis to low-temperature photo inhibition due to interruption of chill-induced increase of S-adenosylmethionine decarboxyllase activity in leaves of Spinach (Spinacea oleraceae L) Plant Cell Physiol. 43 196 206 https://doi.org/10.1093/pcp/pcf021

    • Search Google Scholar
    • Export Citation
  • Jiang, C., Johkan, M., Hohjo, M., Tsukagoshi, S. & Maruo, T. 2017 A correlation analysis on chlorophyll content and SPAD value in tomato leaves Shoku to Midori No Kagaku 71 37 42 https://doi.org/10.20776/S18808824-71-P37

    • Search Google Scholar
    • Export Citation
  • Kalisz, A., Jezdinský, A., Pokluda, R., Sękara, A., Grabowska, A. & Gil, J. 2016 Impacts of chilling on photosynthesis and chlorophyll pigment content in juvenile basil cultivars Hort. Environ. Biotechnol. 57 330 339 https://doi.org/10.1007/s13580-016-0095-8

    • Search Google Scholar
    • Export Citation
  • Kawasaki, Y., Matsuo, S., Kanayama, Y. & Kanahama, K. 2014 Effect of root-zone heating on root growth and activity, nutrient uptake, and fruit yield of tomato at low air temperatures J. Jpn. Soc. Hortic. Sci. 83 4 295 301 https://doi.org/10.2503/jjshs1.MI-001

    • Search Google Scholar
    • Export Citation
  • Lam, V.P., Kim, S.J., Bok, G.J., Lee, J.W. & Park, J.S. 2020 The effects of root temperature on growth, physiology, and accumulation of bioactive compounds of Agastache rugosa Agriculturae 10 162 https://doi.org/10.3390/agriculture10050162

    • Search Google Scholar
    • Export Citation
  • Lazarević, B., Carović-Stanko, K. & Šatović, Z. 2020 Physiological responses of basil (Ocimum basilicum L.) cultivars to Rhizophagus irregularis inoculation under low phosphorus availability Plants 9 1 14 https://doi.org/10.3390/plants9010014

    • Search Google Scholar
    • Export Citation
  • Luo, H., Xu, H., Chu, C., He, F. & Fang, S. 2020 High temperature can change root system architecture and intensify root interactions of plant seedlings Front. Plant Sci. 11 160 https://doi.org/10.3389/fpls.2020.00160

    • Search Google Scholar
    • Export Citation
  • Maludin, A.J., Lum, M.S., Lassim, M.M. & Gobilik, J. 2020 Optimal plant density, nutrient concentration and rootzone temperature for higher growth and yield of Brassica rapa L. ‘Curly Dwarf Pak Choy’ in raft hydroponic system under tropical climate Transact. Sci. Technol. 7 4 178 188

    • Search Google Scholar
    • Export Citation
  • Marotti, M., Piccaglia, R. & Giovanelli, E. 1996 Differences in essential oil composition of basil (Ocimum basilicum L.) Italian cultivars related to morphological characteristics J. Agr. Food Chem. 44 3926 3929 https://doi.org/10.1021/jf9601067

    • Search Google Scholar
    • Export Citation
  • Mohammed, S. & Sookoo, R. 2016 Nutrient film technique for commercial production Agr. Sci. Res. J. 6 11 269 274

  • Mondal, S., Ghosal, S. & Barua, R. 2016 Impact of elevated soil and air temperature on plants growth, yield and physiological interaction: A critical review Sci. Agr. 14 3 293 305 https://doi.org/10.15192/PSCP.SA.2016.14.3.293305

    • Search Google Scholar
    • Export Citation
  • Moorby, J. & Graves, C.J. 1980 Root and air temperature effects on growth and yield of tomatoes and lettuce Acta Hort. 98 29 44 https://doi.org/10.17660/ActaHortic.1980.98.2

    • Search Google Scholar
    • Export Citation
  • Nxawe, S., Laubscher, C.P. & Ndakidemi, P.A. 2009 Effect of regulated irrigation water temperature on hydroponics production of Spinach (Spinacia oleracea L.) Afr. J. Agr. Res. 4 12 1442 1446

    • Search Google Scholar
    • Export Citation
  • Raimondi, G., Orsini, F., Maggio, A., De Pascale, S. & Barbieri, G. 2006 Yield and quality of hydroponically grown sweet basil cultivars Acta Hort. 723 723 353 359 https://doi.org/10.17660/ActaHortic.2006.723.48

    • Search Google Scholar
    • Export Citation
  • Resh, H.M. 1978 Hydroponic Food Production 5th ed. Woodbridge Press Publishing Company Santa Barbara, CA

  • Rosik-Dulewska, C. & Grabda, M. 2002 Development and yield of vegetables cultivated on substrate heated by geothermal waters part I: Bell pepper, slicing cucumber, tomato J. Veg. Crop Prod. 8 133 144 https://doi.org/10.1300/J068v08n01_14

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