Abstract
Chlorine is a disinfectant commonly used to treat water. The United States Environmental Protection Agency (USEPA) has set a standard limit of up to 4 mg·L−1 chlorine for drinking water. The objective of this project was to identify chlorine phytotoxicity thresholds on ‘Rex’ lettuce (Lactuca sativa) when the water source contained chlorine levels within the USEPA standard limits. The nutrient solution to grow lettuce was prepared with reverse osmosis–treated water treated with 0, 0.5, 1, 1.5, 2, and 4 mg·L−1 chlorine and then fertilizers were added. Lettuce plants were grown in a deep-water culture hydroponic system. Visual toxicity symptoms on leaves, relative leaf greenness, and fresh and dry biomass were measured. Our results indicate that irrigation water sources with ≥1 mg·L−1 chlorine used to prepare nutrient solutions can cause phytotoxicity in lettuce plants in just 3 days. Compared with the untreated control, lettuce shoot biomass was lower by 30%, 55%, 66%, 83%, and 92% at 0.5, 1, 1.5, 2, and 4 mg·L−1 of chlorine, respectively. Water sources with ≥ 1 mg·L−1 chlorine can cause significant marketable yield reduction in lettuce grown in deep-water culture.
The suitability of irrigation water sources depends on chemical (i.e., concentration of inorganic elements, oxidizers, and agrochemicals), physical (i.e., suspended solids), and biological (e.g., pathogens, biofilm-forming bacteria, and algae) parameters that limit plant growth. Municipal, well, pond, and rainwater are common sources of irrigation for greenhouse growers in the United States (Raudales et al. 2017). According to the USEPA, chlorine is added to water to reduce microbial populations in the form of chlorine gas, liquid sodium hypochlorite, or solid calcium hypochlorite (USEPA 1999). In the United States, drinking water can have up to 4 mg·L−1 chlorine residual (USEPA 2022). Chlorine of ≈2 mg·L−1 has been determined as the optimum dose to control zoospores from waterborne pathogens, such as Pythium and Phytophthora species (Hong and Richardson 2004; Hong et al. 2003). The discrepancy between the effective dose and phytotoxicity threshold suggests that the suitability of drinking water for application in hydroponic systems might be compromised and further treatments and testing might be needed.
The form and concentration of chlorine applied to a solution will change based on the interaction with organic and inorganic compounds. Residual chlorine is the concentration of chlorine after the reaction stops or after a given contact time (White 1992). The difference between applied chlorine and residual chlorine is the chlorine demand of a solution. Factors in the irrigation, such as peat-based substrates or fertilizers, can exert chlorine demand. For example, chlorine levels dropped from 2 to 0 mg·L−1 in 30 min when 0.6 g of peat was added to 1 L of water (Huang et al. 2011). Chlorine is typically applied as free chlorine: hypochlorous acid or hypochlorite (White 1992). Free chlorine can interact with ammonia and form chloramines, a combined form of chlorine. In irrigation, solutions with 75 mg·L−1 nitrogen (N) from ammonium-N resulted in free chlorine depletion after 2-min contact time, while total chlorine (the sum of free and combined chlorine) remained unchanged (Meador and Fisher 2013). Hence, chlorine in the water sources, such as drinking water, could quickly deplete when interacting with organic residues in solution or form into chloramine when interacting with fertilizers. However, it is difficult to predict if either reaction would be fast or strong enough to affect the risk of crop phytotoxicity in hydroponics.
In soil or substrate-based production systems, when chlorine is injected into water sources and irrigation is applied directly to the root zone, chlorine reacts with the organic matter of the substrate and significantly reduces residual chlorine levels. Organic load in hydroponic solutions is low, and while chlorine may react with plant roots or ammonium from the fertilizers, chlorine might remain as combined or free chlorine. Lettuce (Lactuca sativa) exposed to chloramine, prepared with 0.3 to 0.5 mg of sodium hypochlorite and 0.67 mm ammonium, with 1-h contact time resulted in growth inhibition and wilting (Date et al. 2005). Ninebark (Physocarpus sp.), willow (Salix integra), and hydrangea (Hydrangea sp.) exhibited growth reduction when exposed to 2.4 mg·L−1 of residual chlorine in the irrigation water (Cayanan et al. 2009). Lettuce grown in sandy soils and irrigated with 10 mg·L−1 residual chlorine resulted in chlorosis, leaf necrosis, and reduced biomass compared with the control (Lonigro et al. 2017).
In this project, we aim to understand if drinking water, which may contain up to 4 mg·L−1 chlorine, used as a water source to prepare nutrient solutions could affect hydroponically grown lettuce. The objective of this project was to identify chlorine thresholds in water sources used to prepare nutrient solutions that result in lettuce phytotoxicity in hydroponic systems. We hypothesized that phytotoxicity would develop in lettuce when water that contains chlorine concentrations accepted for drinking water is used to prepare nutrient solutions for production in hydroponic systems.
Materials and methods
Greenhouse parameters and measurements
‘Rex’ lettuce (Johnny’s Selected Seeds, Winslow, ME, USA) was sown in 1-inch-square phenolic foam cubes (OASIS Grower Solutions, Kent, OH, USA) with one seed per cube. Foam cubes with germinated lettuce seedlings were maintained in black propagation trays (11 × 21 inches) for 14 d under fluorescent and incandescent (14.6 mol·m−2·d−1) lamps in a growth chamber. The growth chamber was set at 24/18 °C (day/night) during propagation. After 14 d, the lettuce seedlings were transplanted into deep-water culture (DWC) systems and moved to a greenhouse. Each system had a 4 × 2 × 2-inch air stone (Vivosun, Ontario, CA, USA) connected to an aerator (General Hydroponics, Santa Rosa, CA, USA) with an output for each container. Four lettuce plants in an independent 21.5 × 17.8 × 7-inch DWC reservoir (Rubbermaid, Atlanta, GA, USA) constituted an experimental unit. Each DWC reservoir had 24 L of nutrient solution with reverse osmosis water containing 5N–5.2P–21.6K at 31.2 mg·L−1 N and 15.5N–0P–0K at 88.8 mg·L−1 N (JR Peters Inc., Allentown, PA, USA). The plants were grown in the hydroponic system for 4 weeks.
The experiment took place in a polycarbonate greenhouse in Storrs, CT, USA with a heating set point of 18.3 °C and a ventilation set point of 26.7 °C under natural photoperiod from Aug to Sep 2021 for experimental run one and from Sep to Oct 2021 for experimental run two. DWC reservoirs were covered with extruded polystyrene foam (Styrofoam™; DuPont de Nemours, Inc., Wilmington, DE, USA) and placed on benches. The polystyrene foam was cut into 15 × 19-inch dimensions with four 2-inch holes 4.5 inches apart per container with a net-basket each containing a seedling.
Chlorine treatment
The water used for this experiment was treated with an ultrapure reverse osmosis water system (Hydro Service and Supplies, Durham, NC, USA) with a 0.22-µm high-capacity hydrophobic polyvinylidene fluoride membrane. The water pH was adjusted to 5.8 before adding the chlorine. Chlorine solutions prepared using 5% sodium hypochlorite (Thermo Fisher Scientific Inc., Waltham, MA, USA) were applied directly to water to reach target concentrations (0, 0.5, 1, 1.5, 2, and 4 mg L−1 of chlorine). Subsequently, the chlorine concentration was verified (at this point, free and total chlorine were virtually identical), the fertilizers were added, and the pH of the solution was adjusted back to 5.8 with citric acid (Thermo Fisher Scientific Inc.). Chlorine was applied a second time to a fresh batch of water that was used to refill the reservoirs to maintain a 24-L volume and each reservoir required a different volume between 1000 and 1500 mL. Electrical conductivity (EC) was maintained at ∼1.5 mS·cm−1 for each experimental run. From now on, the chlorine levels applied to the water, before adding fertilizers, will be referred as “chlorine dose” and the chlorine measured in nutrient solutions with plants will be referred as “residual chlorine.”
Nutrient solution and plant measurements
Nutrient solution measurements were made every other day for pH, EC, dissolved oxygen (DO), and temperature (°C) and weekly for free and total residual chlorine and oxidation-reduction potential (ORP)—an indicator of oxidizing strength of a solution. Free and total residual chlorine were measured using environmental test kits with rapid dissolving reagents (OrionTM AQUAfast IV Powder Chemistries, Thermo Fisher Scientific Inc.) in a colorimeter (OrionTM AQ4000, Thermo Fisher Scientific Inc.). The other parameters, pH, EC, DO, temperature, and ORP, were measured with a portable meter (Orion Star A329, Thermo Fisher Scientific Inc.). The values obtained in solutions with 0 mg·L−1 were used as a background absorbance and these values were subtracted from the measurements obtained in solutions dosed with > 0.5 mg·L−1 (Table 1). Residual chlorine and ORP measurements in week 3 reflect the values of the solution 1 d after the water was added to the reservoirs. Plant measurements were recorded daily for visual phytotoxicity symptoms and weekly for relative greenness of the leaves. Relative greenness was measured using a chlorophyll meter (SPAD 502 Plus; Spectrum Technologies, Inc., Aurora, IL, USA), while the visual symptoms including necrosis, marginal necrosis, chlorosis, and distortion/cupping were evaluated by counting the number of leaves with symptoms and the total number of leaves per plant. Then, the value was reported as the percentage of leaves with symptoms in relation to the whole plant. Shoots were cut at the substrate line and shoots and roots were weighed fresh and dry (dried at 70 °C for 2 weeks).
Weekly measurements of residual free and total chlorine and oxidation-reduction potential (ORP) in nutrient solutions for hydroponic culture of lettuce. Fresh solutions were added on weeks 1 and 3.
Statistical analysis
The experiment was a complete randomized design. The experimental unit consisted of four lettuce seedlings in a DWC reservoir with one experimental unit per replicate, and four replicates per treatment (n = 4), and the experiment was run twice (n = 8). DWC reservoirs were randomly distributed in the greenhouse on metal benches. Data were analyzed by analysis of variance with statistical software (SAS version 9.4; SAS Institute Inc., Cary, NC, USA) to establish significance of the effects of all factors (α = 0.05). Means were separated by Tukey’s Studentized range honestly significant difference test (α = 0.05) at the 95% confidence interval using PROC MIXED. Homogeneity of variance and normality were checked for all measured variables using Kolmogorov-Smirnov test, Cramer-von Mises test, and Kuiper test. Correlations were analyzed using the CORR procedure to compare variables set at α = 0.05.
Results
Nutrient solution and plant measurements
Data obtained from both experimental runs were analyzed together because there was homogeneity between runs (P > 0.05) for all measurements, except for pH, DO, and temperature. Interactions between chlorine dose and week were significant for each dependent variable measured weekly—relative greenness, free and total residual chlorine, ORP, pH, and phytotoxic symptoms (P < 0.001). DO was between 7.6 and 8.3 mg·L−1 and pH was adjusted to 5.8 every other day. For experimental run one, pH was statistically the same across all chlorine doses for weeks 1 and 2. Statistically significant differences were observed in pH, but they were within a range of 5.6 to 6.0 for run one and 5.8 to 5.9 for run two. Temperature of the nutrient solution for experimental run one was 25 to 28 °C, and temperature of the nutrient solution for experimental run two was 21 to 25 °C. Residual total chlorine with ≥1.5 mg·L−1 chlorine dose was higher than 0.5 mg·L−1 in the nutrient solution during weeks 1 and 3 measured 1 d after chlorine was added to the water (Table 1). ORP was greatest in the nutrient solution with chlorine doses ≥1 mg·L−1 during weeks 1 and 3 measured 1 d after chlorine was added to the water (Table 1).
Phytotoxic symptoms
Chlorine dose and day had a significant effect on percentage of leaves with ph-ytotoxic symptoms on lettuce plants (P < 0.001). All plants treated with ≥0.5 mg·L−1 of chlorine dose exhibited different degrees of phytotoxic symptoms. The onset of symptoms was observed 3 d after the plants were transplanted into the hydroponic system, and by day 10 all the leaves presented symptoms (Fig. 1). The symptoms started as a translucent interveinal, marginal chlorosis or purple specks on the leaves, which eventually turned necrotic (Fig. 2). No phytotoxic symptoms were observed on plants treated with 0 mg·L−1 of chlorine (Fig. 1). Three days after transplanting, plants treated with ≥0.5 mg·L−1 of chlorine dose began exhibiting marginal necrosis, and 100% of leaves treated with 4 mg·L−1 of chlorine dose were exhibiting phytotoxic symptoms (Fig. 1). Discoloration, necrosis, and reduced growth of leaves continued to increase in lettuce plants treated with ≥1 mg·L−1 of chlorine dose. After 7 d, lettuce treated with ≥ 1 mg·L−1 of chlorine dose had significantly higher phytotoxic symptoms than all other treatments. On days 8 and 9, the percentage of leaves with phytotoxic symptoms was between 98.8% and 100% for all treatments except for plants treated with 0 and 0.5 mg·L−1 of chlorine dose. Lettuce plants treated with 0.5 mg·L−1 of chlorine dose had a lower percentage of leaves with phytotoxic symptoms by day 4 because newly developed leaves did not present symptoms (Fig. 1). Lettuce plants treated with 0 and 0.5 mg·L−1 of chlorine dose had lower phytotoxic symptoms than lettuce treated with ≥1 mg·L−1 of chlorine dose during the 28-d growing period. Strong positive correlations were observed between percentage of leaves with phytotoxic symptoms and free residual chlorine (R2 = 0.832, P < 0.001), and foliar phytotoxicity and total chlorine residual (R2 = 0.827, P < 0.001).
Phytotoxic symptoms on ‘Rex’ lettuce represented by the percentage of leaves per plant with foliar injury for both experimental runs in response to chlorine dose concentrations in a deep-water culture floating raft. Phytotoxic symptoms included necrotic margins, necrotic or bleached spots, cupping, distortion, chlorosis, and wilt. Treatments consisting of chlorine dose concentrations ranging from 0 to 4 mg·L−1 are expressed on the x-axis. Photos near the x-axis represent lettuce plants 8 d post-treatment with chlorine dose concentrations. Means with the same letters by day (vertical row) are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8); 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Phytotoxic symptoms on ‘Rex’ lettuce represented by the percentage of leaves per plant with foliar injury for both experimental runs in response to chlorine dose concentrations in a deep-water culture floating raft. Phytotoxic symptoms included necrotic margins, necrotic or bleached spots, cupping, distortion, chlorosis, and wilt. Treatments consisting of chlorine dose concentrations ranging from 0 to 4 mg·L−1 are expressed on the x-axis. Photos near the x-axis represent lettuce plants 8 d post-treatment with chlorine dose concentrations. Means with the same letters by day (vertical row) are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8); 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Phytotoxic symptoms on ‘Rex’ lettuce represented by the percentage of leaves per plant with foliar injury for both experimental runs in response to chlorine dose concentrations in a deep-water culture floating raft. Phytotoxic symptoms included necrotic margins, necrotic or bleached spots, cupping, distortion, chlorosis, and wilt. Treatments consisting of chlorine dose concentrations ranging from 0 to 4 mg·L−1 are expressed on the x-axis. Photos near the x-axis represent lettuce plants 8 d post-treatment with chlorine dose concentrations. Means with the same letters by day (vertical row) are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8); 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Phytotoxic symptoms on ‘Rex’ lettuce leaves exposed to chlorine doses in a deep-water culture hydroponic system: (A) lettuce with silver-translucent, necrotic spots 4 d post-treatment with 4 mg·L−1 chlorine dose; (B) purple specks on the leaf surface 8 d post-treatment with 2 mg·L−1; (C) marginal necrosis and cupping on lettuce 4 d post-treatment with 4 mg·L−1 chlorine dose; (D) chlorotic discoloration on lettuce 8 d post-treatment with 2 mg·L−1 chlorine dose; (E) chlorotic spotting on lettuce 13 d post-treatment with 1.5 mg·L−1 chlorine dose; and (F) distortion, necrotic cupping, and chlorosis on lettuce 28 d post-treatment with 2 mg·L−1 chlorine dose; 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Phytotoxic symptoms on ‘Rex’ lettuce leaves exposed to chlorine doses in a deep-water culture hydroponic system: (A) lettuce with silver-translucent, necrotic spots 4 d post-treatment with 4 mg·L−1 chlorine dose; (B) purple specks on the leaf surface 8 d post-treatment with 2 mg·L−1; (C) marginal necrosis and cupping on lettuce 4 d post-treatment with 4 mg·L−1 chlorine dose; (D) chlorotic discoloration on lettuce 8 d post-treatment with 2 mg·L−1 chlorine dose; (E) chlorotic spotting on lettuce 13 d post-treatment with 1.5 mg·L−1 chlorine dose; and (F) distortion, necrotic cupping, and chlorosis on lettuce 28 d post-treatment with 2 mg·L−1 chlorine dose; 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Phytotoxic symptoms on ‘Rex’ lettuce leaves exposed to chlorine doses in a deep-water culture hydroponic system: (A) lettuce with silver-translucent, necrotic spots 4 d post-treatment with 4 mg·L−1 chlorine dose; (B) purple specks on the leaf surface 8 d post-treatment with 2 mg·L−1; (C) marginal necrosis and cupping on lettuce 4 d post-treatment with 4 mg·L−1 chlorine dose; (D) chlorotic discoloration on lettuce 8 d post-treatment with 2 mg·L−1 chlorine dose; (E) chlorotic spotting on lettuce 13 d post-treatment with 1.5 mg·L−1 chlorine dose; and (F) distortion, necrotic cupping, and chlorosis on lettuce 28 d post-treatment with 2 mg·L−1 chlorine dose; 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Relative greenness (SPAD) indicating foliar chlorosis or greenness was significantly affected by chlorine doses (P < 0.001). There were no differences in relative greenness on lettuce plants during week 1 after treatment (Fig. 3). During week 2, lettuce plants treated with 0 and 0.5 mg·L−1 chlorine dose were greener compared with plants treated with ≥1.5 mg·L−1 chlorine dose. In weeks 3 and 4, lettuce plants treated with 4 mg·L−1 of chlorine dose had the lowest relative greenness. Plants treated with 1 mg·L−1 of chlorine dose had higher relative greenness than plants treated with 2 and 4 mg·L−1 of chlorine dose. During week 4, lettuce plants treated with 0 and 0.5 mg·L−1 of chlorine dose had the highest relative greenness values. Relative greenness (SPAD) was negatively correlated with residual free (R2 = −0.827, P < 0.001) and total (R2 = −0.833, P < 0.001) chlorine residual.
Relative greenness (SPAD) of ‘Rex’ lettuce plants over 4 weeks treated with various chlorine dose concentrations (0, 0.5, 1, 1.5, 2, and 4 mg·L−1) in a deep-water culture system. Means with the same letters across each week are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8). Error bars represent SE; 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Relative greenness (SPAD) of ‘Rex’ lettuce plants over 4 weeks treated with various chlorine dose concentrations (0, 0.5, 1, 1.5, 2, and 4 mg·L−1) in a deep-water culture system. Means with the same letters across each week are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8). Error bars represent SE; 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Relative greenness (SPAD) of ‘Rex’ lettuce plants over 4 weeks treated with various chlorine dose concentrations (0, 0.5, 1, 1.5, 2, and 4 mg·L−1) in a deep-water culture system. Means with the same letters across each week are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8). Error bars represent SE; 1 mg·L−1 = 1 ppm.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Yield
Chlorine dose at all treatment levels had a significant effect (as compared with the control at 0 mg·L−1) on shoot and root dry weight of lettuce plants [P < 0.001 (Fig. 4)]. Compared with the untreated control, lettuce shoot biomass was lower by 30%, 55%, 66%, 83%, and 92% at 0.5, 1, 1.5, 2, and 4 mg·L−1 of chlorine, respectively (Fig. 4A).
Dry weight in grams of averaged four ‘Rex’ lettuce shoots and roots in response to chlorine dose concentrations in deep-water culture floating raft. (A) Dry shoot and root weight of lettuce 42 d after sowing in combined experimental runs. Means with the same letters are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8). Bars on the graph represent SE. Dry root weight values and SE were multiplied by three for visual representation. (B) Lettuce plants grown in a deep-water culture system 42 d post exposure to 0, 0.5, 1, 1.5, 2, and 4 mg·L−1 concentrations of chlorine dose (from left to right); 1 mg·L−1 = 1 ppm, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Dry weight in grams of averaged four ‘Rex’ lettuce shoots and roots in response to chlorine dose concentrations in deep-water culture floating raft. (A) Dry shoot and root weight of lettuce 42 d after sowing in combined experimental runs. Means with the same letters are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8). Bars on the graph represent SE. Dry root weight values and SE were multiplied by three for visual representation. (B) Lettuce plants grown in a deep-water culture system 42 d post exposure to 0, 0.5, 1, 1.5, 2, and 4 mg·L−1 concentrations of chlorine dose (from left to right); 1 mg·L−1 = 1 ppm, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Dry weight in grams of averaged four ‘Rex’ lettuce shoots and roots in response to chlorine dose concentrations in deep-water culture floating raft. (A) Dry shoot and root weight of lettuce 42 d after sowing in combined experimental runs. Means with the same letters are not significantly different according to Tukey’s Studentized range honestly significant difference test at P ≤ 0.05 (n = 8). Bars on the graph represent SE. Dry root weight values and SE were multiplied by three for visual representation. (B) Lettuce plants grown in a deep-water culture system 42 d post exposure to 0, 0.5, 1, 1.5, 2, and 4 mg·L−1 concentrations of chlorine dose (from left to right); 1 mg·L−1 = 1 ppm, 1 g = 0.0353 oz.
Citation: HortTechnology 33, 1; 10.21273/HORTTECH05091-22
Discussion
Our study demonstrates that lettuce yields and marketable quality are significantly reduced when nutrient solutions are prepared with water that contains ≥0.5 mg·L−1 chlorine and the symptoms are visible after 3 d. The chlorine levels tested in this project are levels within the USEPA standards for drinking water. We observed that measuring chlorine after mixing with the fertilizers might provide an inaccurate depiction of the risk of phytotoxicity, because although the measured levels might be low, phytotoxicity symptoms still develop (Table 1, Fig. 1). This is particularly important when trying to diagnose the cause of the problem. Operations that use drinking water from public water facilities should have a dechlorination system in place and monitor chlorine in the incoming water. Our results also suggest that chlorine may not be a compatible technology to use in hydroponics because the phytotoxicity threshold is lower than the plant pathogen efficacy levels (Raudales et al. 2014).
Before this project, the chlorine phytotoxicity thresholds for hydroponic greenhouse production systems were unclear. Premuzic et al. (2007) reported that 0.55 and 5.5 mg·L−1 chlorine dose had no effect on lettuce biomass and some toxicity between 0.5 and 11 mg·L−1. However, the net effect of chlorine damage in that study was unclear because the control also presented leaf damage in slightly under 20% of the leaves, whereas the other treatments (0.55 and 5.5 mg·L−1) were slightly above 20%. In our experiment, we observed a significant reduction in biomass with chlorine dose concentrations ≥0.5 mg·L−1, and 100% foliar visual injury after 10 d with ≥1 mg·L−1 chlorine, and no injury on the controls. Others have observed phytotoxicity on container-grown ornamentals with levels above 2.5 mg·L−1 (Cayanan et al. 2008, 2009; Donovan et al. 2015). Premuzic et al. (2007) also reported that 11 mg·L−1 of sodium hypochlorite did not affect the biomass, whereas in our study, lettuce plants exposed to 4 mg·L−1 of chlorine dose had 92% and 78% lower shoot and root dry weight compared with the control. Differences in application method, type of soilless system, and lettuce cultivar might explain the discrepancy. Premuzic et al. (2007) applied the chlorine via drip irrigation into buckets with perlite and recirculated the solution. They did not specify if they tracked the a.i. over time; it is possible that aeration—a known mechanism to remove chlorine (White 1992)—during recirculation resulted in dechlorination. Thus, the levels in our study represent the thresholds at which phytotoxicity is likely to be observed in lettuce grown hydroponically in DWC when the solution is prepared with water containing up to 4 mg·L−1.
Residual chlorine in irrigation sources can be removed by either using activated carbon filters or deactivating chlorine. Carbon filtration, such as granular activated carbon (GAC) filtration, can assist in filtering out chlorine from municipal sources. Grant et al. (2019) reported GAC filters successfully removed sodium hypochlorite at 2 mg·L−1 of free and total residual chlorine from greenhouse irrigation water. Chemical dichlorination can be achieved through the use of sulfur dioxide, sodium bisulfate, and sodium metabisulfate (USEPA 1999). Including a dechlorination method in the irrigation system is essential to reduce the risk of phytotoxicity in operations producing hydroponic lettuce with drinking water from water treatment facilities.
We observed oxidation and reduced root development on lettuce in a commercial greenhouse, which led us to investigate this issue. In 2018, we tested city water at a hydroponic operation in Connecticut that had free residual chlorine ranging from 1.8 to 2 mg·L−1, and free residual chlorine was still detected over 0.5 mg·L−1 after passing through a GAC filter. Another facility in Ohio experienced damage in their hydroponic lettuce that had 0.85 mg·L−1 of free residual chlorine detected from the municipal water and 0.45 mg·L−1 of free residual chlorine detected from the solution in their DWC reservoir. Each grower questioned a Pythium outbreak when they saw reduced growth of the lettuce; however, Pythium was not detected. Pythium root rot (Pythium sp.) in lettuce, similar to chlorine toxicity, causes reduced plant growth, foliar chlorosis and necrosis, and root necrosis (McGehee et al. 2018). In our experiments, we observed significant reduction in root development (Fig. 4). Growers who confuse the symptoms caused by oxidation of chlorine with those of Pythium root rot could potentially use more oxidizers in the solution and augment the problem. Therefore, proper diagnosis of the causal agent of the symptoms is essential. Growers using drinking water from water treatment facilities should have in place a system to monitor chlorine in the water and have a dechlorination system to treat all incoming water to reduce the risk of phytotoxicity.
Water sources with chlorine levels that are equal to or greater than 0.5 mg·L−1 used to prepare nutrient solutions for hydroponic systems may cause phytotoxic symptoms on lettuce after 3 d and reduce crop yields by 30% or more. Growers using drinking water from water treatment facilities should track chlorine to prevent injury. Operations that use municipal water should have a system to monitor chlorine levels in the water and proactively treat water with a GAC or ozone system to prevent chemical injury. There is a discrepancy between chlorine levels required to control Pythium species and phytotoxicity thresholds in hydroponic culture. Therefore, chlorine should not be recommended as a water treatment option in hydroponic lettuce.
Units
References cited
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