Abstract
Algae is not desirable in hydroponics and creates problems such as reduced yield and decreased dissolved oxygen, and affects the physiology of plants and, thus, needs to be controlled. An experiment was conducted in Ebb and Flow hydroponic systems to investigate the application timing and rates of two hydrogen peroxide products (Zerotol and PERpose Plus). Treatments included 35 mL weekly, 35 mL biweekly, 70 mL weekly, 70 mL biweekly, and a control with no application of hydrogen peroxide using a 40-gallon reservoir of water. Pepper ‘Early Jalapeno’ and ‘Lunchbox Red’ and tomato ‘Geronimo’ and ‘Little Sicily’ were used. The study was conducted in a split-plot design with two replications over time. Plant growth parameters, including plant height, flower number, net CO2 assimilation, fresh weight, and dry weight were recorded. Algae data, including dry weight, algae cell counts, and chl a were also measured. Results indicated that with increasing rate and timing of either product decreased algae counts, dry weight, and chl a values. However, weekly and biweekly application of 70 mL of both products were not different for algae quantification. In pepper, plant height, shoot fresh and dry weight, and root fresh and dry weight were found to be significantly greater with Zerotol 35 mL biweekly, Zerotol 70 mL weekly, PERpose Plus 35 mL biweekly, and PERpose Plus 70 mL weekly compared with the control. ‘Lunchbox Red’ was significantly greater than ‘Early Jalapeno’ in all growth parameters, except soil plant analysis development (SPAD). ‘Lunchbox Red’ had the greatest flower number, with weekly application of 70 mL PERpose Plus. In tomato, greatest flower number and SPAD were observed in ‘Geronimo’ with a weekly application of 70 mL PERpose Plus and 70 mL Zerotol, respectively. Greater shoot and root fresh and dry weight for both tomato cultivars were recorded with 35 mL biweekly or 70 mL weekly application with either product. The results from both plants as well as algae analysis suggest that weekly application of 70 mL of either Zerotol or PERpose Plus produced the best results in terms of controlling algae and improving the growth of pepper and tomato plants.
Hydroponics is the method of growing plants under soilless conditions with nutrients, water, and an inert medium (Savvas, 2003). Systems can be circulating, in which the nutrient solution is recirculated and nutrient levels are manipulated, or noncirculating, in which the nutrient solution is not recirculated and flows through the system only once (Singh, 2017). Although there are different types of hydroponic systems, every system must deliver water, nutrients, and oxygen to achieve success for plant production (Aires, 2018). Algae needs water, nutrients, and light to grow and because a hydroponic system provides it all, algae growth is often seen in hydroponics (Morgan, 2017), especially those with recirculating nutrient solution (Schwarz and Gross, 2004).
Algae enters the system through irrigation water and is commonly present in the physical structures that make up the hydroponic system. Under uncontrolled conditions, algae can cause organic loading and clogging of pipes (Supraja et al., 2020). Algae poses several threats to hydroponic crops by competing for the nutrients and releasing harmful toxins that might inhibit or even stop crop growth (Borowitzka, 1995; Schwarz and Gross, 2004). It is generally accepted that algae growth can affect the water quality parameters, such as pH, dissolved oxygen, and nutrients in the water, and may compete with the target vegetables (Abdel-Raouf et al., 2012). Thus, it is critical that algal levels are kept to a minimum in hydroponic systems (Tesoriero et al., 2010).
It is a common practice in hydroponics to clean the system manually by basic cleaning or using black plastic sheets to reduce algal growth (Vänninen and Koskula, 1998), but time for application and high use of labor adds to costs (Caixeta et al., 2018). Alternatively, chemical algaecides are also used occasionally (Nonomura et al., 2001). In recent years, interest in environmental-friendly chemicals has been on the rise. One such chemical is hydrogen peroxide (H2O2), which decomposes rapidly into harmless products water and oxygen (Randhawa et al., 2012). Hydrogen peroxide is generated via superoxide, presumably in a noncontrolled manner, during electron transport processes such as photosynthesis and mitochondrial respiration, and increases in response to environmental stresses such as excess excitation (light) energy, drought, and cold (Dat et al., 2000). Many studies have reported application of H2O2 resulted in effective and selective inhibition of algal growth with low or no effects on other phytoplankton groups in natural waters (Barrington and Ghadouani, 2008). In plants, H2O2 plays an important physiological role as a reactive oxygen species (ROS), which is produced under both biotic and abiotic conditions and plays a key role against O2-derived cell toxicity (Almeida et al., 2005). Exogenous application of H2O2 was found to increase plant growth, physiological activities, and biochemical properties of wax apple (Syzygium samarangense L. var. Jambu Madu) fruits (Khandaker et al., 2012). Similarly, Orabi et al. (2015) reported that a low level of H2O2 can have a significant positive effect on plant growth, endogenous growth regulators, antioxidant enzyme activity, fruit yield, and quality of tomato (Solanum lycopersicum L.). Many studies have demonstrated that seed priming by H2O2 can enhance abiotic stress tolerance by modulating ROS detoxification and by regulating multiple stress-responsive pathways and gene expression (Mohammad et al., 2015). Alternatively, high concentration of H2O2 can promote stomatal closure in plants, reducing net photosynthesis and growth (Xia et al., 2014), and produce carcinogenic effects and induce cell death (Nurnaeimah et al., 2020). At higher levels, H2O2 has also been recognized as a toxic molecule that can cause damage at different levels of cell organization and thus losses in cell viability (Gechev et al., 2005). Among ROS, H2O2 is recognized as comparatively long-lived ROS that can diffuse easily through membranes and reach targets far from production sites; however, H2O2 has strong oxidizing capacities that render it capable of interacting with most biomolecules resulting in oxidative stress (Wojtyla et al., 2016).
Although H2O2-based products have been used widely as aquatic herbicide and algaecide in wastewater treatment, its application in hydroponic crop production is still limited (Caixeta et al., 2018). The signaling role of H2O2 in plants is well established, particularly with reference to plant processes like stress acclimation, cell wall cross-linking, stomatal behavior, phytoalexin production, regulation of the cell cycle, and photosynthesis. However, if not applied within the optimum range, H2O2 can result in decreased net photosynthesis and plant dry mass (Khan et al., 2016). So, the toxicity or danger associated with H2O2 on one hand and signaling cascades on other makes it a versatile chemical whose rate and concentration need to be tightly controlled in plants during the application (Petrov and Breusegem, 2012). The objective of this study was to find the optimum amount and timing of application of two broad-spectrum algaecide-labeled H2O2 products to reduce algae while limiting effects on growth, including plant height, flower number, net CO2 assimilation, SPAD, and fresh and dry weight of pepper (Capsicum annuum L.) and tomato plants using an Ebb and Flow hydroponic system.
Materials and Methods
Plant materials and growth conditions.
The experiment was conducted at the Department of Horticulture and Landscape Architecture Research Greenhouses in Stillwater, OK. Seeds of tomato ‘Geronimo’ and ‘Little Sicily’ and pepper ‘Early Jalapeno’ and ‘Lunchbox Red’ were obtained from Johnny’s Selected Seeds (Winslow, MN). Pepper seeds were sown in oasis Horticubes (Harris Seeds, Rochester, NY) of size 1.5 cm3 on 22 Apr. 2020 and kept on the mist bench. Tomatoes were planted 3 weeks later on 13 May 2020 in oasis Horticubes of the same size. Nutrients were applied once when the seedlings were 3 weeks old. Nutrient solution was made by mixing 4.9 g 5N–5.2P–21.6K (Jack’s; J.R. Peters, Allentown, PA) and 3.2 g of calcium nitrate (American Plant Products, Oklahoma City, OK) diluted in 10 L of water and watered over the seedlings. Both peppers and tomatoes were transplanted to an Ebb and Flow table [Gro Master, Maple Park (Virgil), IL] on 17 June 2020. A Styrofoam sheet containing 5-cm-diameter slots spaced 28 cm apart was placed on each table to support the plants. A 5-cm net pot was placed on each slot of the Styrofoam and one plant was placed into each net pot. Each Ebb and Flow bench was supplied with a 40-gallon-capacity tank. Tanks were filled with tap water and 147.41 g of 5N–5.2P–21.6K listed previously along with 97.52 g of calcium nitrate listed previously was added initially as recommended by Singh et al. (2019). The pH and electrical conductivity (EC) of the solution were checked every day to maintain the pH between 5.5 and 6.5 and the EC at 1.5 to 2.5 mS·cm−1.
Treatments and data collection.
Both Zerotol (Biosafe Systems, East Hartford, CT) and PERpose Plus (Bioworks, Victor, NY) had four different combinations of rate and timing, 35 mL weekly and twice weekly (biweekly), 70 mL weekly and biweekly, and a control without any H2O2 application. Zerotol active ingredient includes hydrogen peroxide and peroxyacetic acid, whereas PERpose Plus uses hydrogen peroxide and hydrogen dioxide. Each plant on the table was scanned using a chlorophyll meter (SPAD-502; Konica Minolta, Tokyo, Japan) 63 d after transplanting. SPAD readings were taken from each plant from top, middle, and bottom leaves and then averaged to determine the chlorophyll concentration. Data on photosynthesis rate was taken using a LI-COR 6400 (LI-COR, Lincoln, NE). The LI-COR with 6400-02B LED light source chamber was used by keeping the reference CO2 at 400 ppm. The block temperature was set at 28 °C and the light level was set at 1000 μmol·m−2·s−1. The third leaf from the top was selected per plant and was used as a nondestructive sample for the LI-COR measurement. Plants were harvested 70 d after transplanting and data were collected on number of flowers (fully opened), plant height, and root and shoot fresh weight and dry weight (oven dried for 2 d at 53.9 °C). Dissolved oxygen (D.O.) level of the solution was measured every day with a HI9146 D.O. meter (Hanna Instruments, Woonsocket, RI).
Quantification of algae.
A hemocytometer (Hausser Scientific, Horsham, PA) was used to count the number of algae cells. A 100-µL water sample was collected from each table and 100 µL of trypan blue dye was added to make the solution for the slide before 1 µL homogeneous solution was added to the hemocytometer slide. The slide was examined under a compound microscope (Olympus, Waltham, MA) at ×40 and the average number of viable algae cells were counted. The average cell count was multiplied by 10,000X the dilution factor (2) to calculate the algae concentration (viable cells/mL) according to LeGresley and McDermott (2010). A water sample from each treatment was collected to measure the chlorophyll (chl)-a of algae with a spectrophotometer (GENESYS 30; The Laboratory Depot, Dawsonville, GA) to measure the absorbance of the samples at various wavelengths (750 nm, 665 nm, 647 nm, and 630 nm) as reported by Kumar and Saramma (2013).
Experimental design and statistics.
Eight plants per cultivar per species were randomly planted in tables and treatments were arranged in a split-plot design with two replications over time. The experiment was replicated by planting another set of pepper seeds on 28 Aug. 2020 and tomatoes on 18 Sept. 2020 adopting the similar methods listed previously. Statistical analysis was performed using SAS/STAT software (version 9.4; SAS Institute, Cary, NC). Tests of significance were reported at the 0.05, 0.001, and 0.0001 level. The data were analyzed using generalized linear mixed models methods. Tukey multiple comparison methods were used to separate the means.
Results
Quantification of algae.
Dry weight of algae, chl a, and algal cell counts were significantly affected by the rate and application timing of H2O2 products (Table 1). As H2O2 rate increased, dry weight, algal cells, and chl a were found to decrease. Lowest algae dry weight was recorded with 70 mL biweekly application of either product; however, this was not different from 70 mL weekly treatment, and 35 mL biweekly treatment. A similar trend was observed in algal cell counts as well, with the lowest cell counts in 70 mL biweekly treatment of either product. Lowest chl a was recorded in samples collected from 70 mL biweekly treatment of either product, which was similar to all other treatments except control and 35 mL weekly treatment for either product. Algae species were still found in treatments that were sent for analysis with Chlamydomonas spp., Gleocystis vesiculosa, and Scenedesmus acutus being recorded in the greatest numbers collectively from all treatments (Table 2). Chlamydomonas spp. was found in all treatments, Gloeocystis vesiculosa was found in all treatments except Zerotol 70 mL biweekly treatment, and Scenedesmus spp. was found in all treatments except PERpose Plus 70 mL biweekly treatment. Average cells/mL of water for Scenedesmus acutus decreased 99.9%, 99.8%, and 94.8% in Zerotol 35 mL weekly, PERpose Plus 35 mL weekly, and Zerotol 70 mL biweekly treatment, respectively, compared with the control. Average cells/mL of pennate diatom species decreased 99.8%, 98%, 66.3%, and 52.1% in Zerotol 70 mL biweekly, Zerotol 35 mL weekly, PERpose Plus 70 mL biweekly, and PERpose Plus 35 mL weekly treatment, respectively compared with the control. Average cells/mL of centric diatom species was reduced 17.5%, 62.1%, and 99.2% compared with the control in Zerotol 35 mL weekly, PERpose Plus 35 mL weekly, and Zerotol 70 mL biweekly treatment, respectively. Average cells/mL of Leptolyngbya spp. was recorded to be 2016 in the control treatment but was not found in any other treatments.
Least square means for rate and application timing of two hydrogen peroxide products on algae samples from Ebb and Flow hydroponic systems in Stillwater, OK.
Different rates and application timing of two hydrogen peroxide products on taxonomic counts of algae present in Ebb and Flow hydroponic systems in Oklahoma State University research greenhouses, Stillwater, OK.
Plant growth of pepper.
There was a significant cultivar × H2O2 interaction for flower number and CO2 assimilation (Table 3). Greatest CO2 assimilation was observed in ‘Early Jalapeno’ with Zerotol 35 mL biweekly treatment, which was not different from Zerotol 35 mL weekly and Zerotol 70 mL weekly treatments. Within cultivars, application of Zerotol 35 mL biweekly and 70 mL weekly resulted in greatest value for CO2 assimilation in ‘Lunchbox Red’. Between cultivars for CO2 assimilation, ‘Early Jalapeno’ was found to have greater CO2 assimilation in comparison with ‘Lunchbox Red’. Greatest flower number was recorded in ‘Lunchbox Red’ with PERpose Plus 70 mL weekly treatment (Table 4). SPAD, plant height, and fresh and dry root and shoot weight showed significant main effects for both cultivar and H2O2 (Table 3). Within H2O2 treatments, Zerotol 35 mL weekly treatment had the greatest SPAD reading and was found to be similar to all other treatments except the control and Zerotol 70 mL biweekly treatments (Table 5). For plant height, greatest value was recorded with Zerotol 70 mL weekly treatment but was not different from all other treatments except control, Zerotol 35 mL weekly, and Zerotol 70 mL biweekly treatments (Table 5). Greatest root fresh weight was recorded with 70 mL weekly treatment of either products and Zerotol 35 mL biweekly treatment and were different from the control treatment only. Whereas, biweekly application of 35 mL Zerotol produced plants with the greatest root dry weights but was similar to all other treatments except the control and 70 mL biweekly treatments of either product. Weekly application of 70 mL PERpose Plus resulted in the greatest shoot fresh weight. However, 70 mL weekly application of Zerotol and 35 mL biweekly application of either product also resulted in plants with greater shoot fresh weights. Whereas, greatest shoot dry weight was recorded with Zerotol 35 mL biweekly and PERpose Plus 70 mL weekly treatments but were similar to all other treatments except control and 70 mL biweekly treatments of either product (Table 5). ‘Lunchbox Red’ performed better in terms of all the growth parameters except SPAD, which was greater in ‘Early Jalapeno’ (Table 6).
Tests of effects for pepper ‘Early Jalapeno’ and ‘Lunchbox Red’ and rate and application timing of Zerotol and PERpose Plus, both hydrogen peroxide (H2O2) products, in Ebb and Flow hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.
Least squares means for ‘Early Jalapeno’ and ‘Lunchbox Red’ and different rate and application timing of two hydrogen peroxide products for flower number and net CO2 assimilation of peppers grown in Ebb and Flow hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.
Least square means for rate and application timing of two hydrogen peroxide products on growth of pepper ‘Early Jalapeno’ and ‘Lunchbox Red’ grown in Ebb and Flow hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.
Least square means for ‘Early Jalapeno’ compared with ‘Lunchbox Red’ for growth of peppers grown in Ebb and Flow hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.
Plant growth of tomato.
There was a significant cultivar × H2O2 interaction for all growth parameters except for net CO2 assimilation, whereas CO2 assimilation was found to be affected by H2O2 product only (Table 7). Zerotol application at the rate of 35 mL biweekly and 70 mL weekly resulted in the greatest CO2 assimilation compared with any other treatments (Table 8). Greatest flower number was observed in ‘Geronimo’ with a weekly application of 70 mL PERpose Plus, which was not different from Zerotol 35 mL biweekly and PERpose Plus 35 mL weekly treatments as well as Zerotol 35 mL weekly and 35 mL biweekly treatment of either product in ‘Little Sicily’. Greatest SPAD value was recorded in ‘Geronimo’ within Zerotol 70 mL weekly treatment, which was also similar to PERpose Plus 70 mL weekly treatment and 35 mL biweekly treatment of either product. In ‘Lunchbox Red’, both weekly and biweekly application of 35 mL Zerotol resulted in the greatest SPAD and were similar to all other treatments except the control and 70 mL biweekly treatments of either product (Table 9). In ‘Little Sicily’, Zerotol 35 mL biweekly application resulted in the greatest plant height and was similar to all the treatments except Zerotol 70 mL biweekly treatment, whereas ‘Geronimo’ had the greatest plant height with PERpose Plus 70 mL biweekly treatment and was similar to all the treatments except Zerotol 35 mL biweekly and PERpose Plus 70 mL biweekly treatments. When compared across the cultivars, ‘Little Sicily’ had greater plant height than ‘Geronimo’ (Table 9). Greater fresh and dry root weight were observed in ‘Little Sicily’ in comparison with ‘Geronimo’. In ‘Little Sicily’, greatest root fresh weight was observed with biweekly application of 35 mL Zerotol and was similar to all other treatments except control and 70 mL biweekly treatments of either product. The greatest root dry weight was observed in PERpose Plus 35 mL biweekly treatment, which was similar to 35 mL weekly and biweekly treatments of Zerotol, as well as 35 mL and 70 mL weekly treatments of PERpose Plus. In ‘Geronimo’, 70 mL weekly application of either product resulted in the greatest fresh weight of roots and was similar to all the treatments except the control, Zerotol 35 mL weekly, and PERpose Plus 70 mL biweekly treatments. However, greater root dry weight was recorded in PERpose Plus 70 mL weekly treatment compared with any other treatments (Table 9). ‘Little Sicily’ had the greatest shoot fresh weight with PERpose Plus 70 mL weekly treatment, which was similar to all other treatments except the control and Zerotol 70 mL biweekly treatments. The shoot dry weight was found to be greatest with PERpose Plus 70 mL biweekly treatment but was only different from the control treatment. In ‘Geronimo’, PERpose Plus 35 mL biweekly and 70 mL weekly treatments resulted in greatest shoot fresh weight but was not different from PERpose Plus 35 mL weekly, Zerotol 35 mL biweekly, and Zerotol 70 mL weekly treatments. For shoot dry weight, PERpose Plus 70 mL weekly had the greatest value but was similar to PERpose Plus 35 mL biweekly treatment (Table 9).
Tests of effects for tomato ‘Little Sicily’ and ‘Geronimo’ and rate and application timing of Zerotol and PERpose Plus, both hydrogen peroxide (H2O2) products, in Ebb and Flow hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.
Least square means for rate and application timing of two hydrogen peroxide products on net CO2 assimilation of tomato ‘Little Sicily’ and ‘Geronimo’ grown in Ebb and Flow hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.
Least square means for tomato ‘Little Sicily’ and ‘Geronimo’ and different rate and application timing of two hydrogen peroxide products for growth of tomatoes grown in Ebb and Flow hydroponic system at Oklahoma State University research greenhouses in Stillwater, OK.
Dissolved oxygen.
D.O. levels varied from 5.2 to 6.6, 6.2 to 7.5, 6.2 to 7.9, and 6.8 to 8.2 mg·L−1 for PERpose Plus treatments of 35 mL weekly, 35 mL biweekly, 70 mL weekly, and 70 mL biweekly, respectively (Fig. 1). Average D.O. levels were 6.0, 6.6, 7.0, and 7.7 mg·L−1 for PERpose Plus treatments of 35 mL weekly, 35 mL biweekly, 70 mL weekly, and 70 mL biweekly, respectively. D.O. levels varied from 5.4 to 6.4, 6.0 to 7.6, 6.1 to 7.8, and 6.1 to 8.1 mg·L−1 for Zerotol treatments of 35 mL weekly, 35 mL biweekly, 70 mL weekly, and 70 mL biweekly, respectively (Fig. 1). Average D.O. levels were 5.9, 6.6, 6.9, and 7.6 mg·L−1 for Zerotol treatments of 35 mL weekly, 35 mL biweekly, 70 mL weekly, and 70 mL biweekly, respectively, whereas the control levels varied from 5.1 to 5.7 mg·L−1 and averaged 5.5 mg·L−1. Spikes in D.O. levels were observed on days in which H2O2 was added to the tanks, then levels decreased within a couple of days until the next application. The trend showed that with increasing dose and application of H2O2, there was an increase in D.O. level.
Effects of rate and application timing of two H2O2 products Zerotol (Zer) and PERpose Plus (PP) (35 mL weekly and biweekly, 70 mL weekly and biweekly, and control) on dissolved oxygen (D.O.) levels of nutrient solution in Ebb and Flow hydroponic systems at Oklahoma State University research greenhouses in Stillwater, OK.
Citation: HortScience 57, 1; 10.21273/HORTSCI16193-21
Discussion
In the present study, dry weight of algae, chl a, and algal cell counts were found to decrease with increasing rate and application timing of H2O2 products. Hydrogen peroxide is a strong oxidant that has been used in industrial application and water treatment processes (Ismail et al., 2015). When catalyzed in water, H2O2 can generate a wide variety of free radicals and other reactive species that are capable of transforming or decomposing organic chemicals (Petri et al., 2003). Hydrogen peroxide–based products have been used as aquatic herbicides and microbiocide to manage algae, cyanobacteria, fungi, and microorganisms in water (Randhawa et al., 2012). However, the efficiency of H2O2 can be influenced by abiotic factors such as light intensity and nutrient availability, as well as biotic factors like phytoplankton and bacterioplankton composition (Santos et al., 2021). According to a study conducted by Vänninen and Koskula (1998), a single application of 1 dL of 125 ppm H2O2 or daily applications over 3 weeks of 1 dL of 100 ppm peroxide reduced algal growth by 40% to 60%, which was similar to our findings. However, our results showed that both Zerotol and PERpose Plus reduced algae by as much as 95% even at lower rates. Exposure of Planktothrix agardhii to H2O2 resulted in the degeneration of cells and filaments. Swollen cell walls and some degree of cytoplasmatic alteration were observed after 24 h of treatment with 0.83 mg·L−1 H2O2, whereas, after 48 h under the highest concentration (3.33 mg·L−1) resulted in cell wall and plasmatic membrane disruption, disorganized and degraded thylakoids, and changes in the cytoplasmatic inclusions (Bauzá et al., 2014). The same study also reported that the chl a decreased 40%, 86%, 86%, 78%, and 79%, respectively, with application of 0.17, 0.33, 0.83, 1.67, and 3.33 mg·L−1 H2O2, which corresponds to our finding that chl a decreases with increasing rate of H2O2. Santos et al. (2021) evaluated the effects of a single application of H2O2 (10 mg·L−1) over 120 h in mesocosms introduced in a reservoir and reported that H2O2 efficiently decreased the biomass of cyanobacteria, green algae, and diatoms over 72 h along with the chlorophyll concentration, leading to an increase in transparency. However, after 120 h, subsequent dominance of green algae was observed. In some cases, suppression of one algae species might benefit the growth of another species, which could be the reason why some of the treatments in our experiment had new species that were not recorded in the control treatment. Sinha et al. (2018) observed that a reduction in cyanobacteria biomass was followed by an increase in the abundance of eukaryotic diatom Synedra sp. and green algae Cladophora sp., suggesting that these species benefited from the collapse of cyanobacteria and used the available nutrients. In another study conducted by Wang et al. (2019), in which a Microcystis aeruginosa bloom was suppressed with the application of H2O2, growth of Chlamydomonas spp. was found to be promoted.
Our results indicated that either a 70-mL weekly or 35-mL biweekly application of either H2O2 product resulted in improved growth performance of both pepper and tomato plants. This may be due to the positive role of H2O2 in plant growth and development. H2O2 is said to be the most stable ROS, and therefore plays a crucial role as a signaling molecule in various physiological processes, including photosynthesis, respiration, translocation, and transpiration (Slesak et al., 2007). Growth (root and shoot length; fresh and dry weight) and photosynthetic performance of cowpea (Vigna unguiculata L.) was found to increase by foliar application of H2O2 at 0.5 to 1.0 mm solution (Hasan et al., 2016). Similarly, root dipping treatment of H2O2 to tomato plants under copper stress and stress-free conditions improved growth, including shoot and root length, shoot and root fresh weight, shoot and root dry weight, and leaf area (Nazir et al., 2019). Foliar application of diluted H2O2 (at 20 mm) has been shown to increase the dry weight of leaves, and increase fruit set, biomass, and quality of fruit, as well as decreasing bud drop in wax apple (‘Jambu Madu’) crops (Khandaker et al., 2012). Uchida et al. (2002) reported that H2O2 application induces the activity of sucrose phosphate synthase, which is an enzyme important in the formation of sucrose from triose phosphates during and after photosynthesis and positively regulates the metabolic and antioxidant enzyme activities in favor of plant growth and development. Furthermore, application of H2O2 has been found to increase the shoot and root length in two wheat (Triticum aestivum L.) cultivars by increasing the activity of starch hydrolyzing enzymes, which resulted in better root carbohydrate. As the root length increases, absorption of water and nutrients usually increases, leading to increased growth parameters and prevention against the harmful effects of stress (Latef et al., 2019). On the other hand, H2O2 functions by decomposing into an unstable free radical oxygen molecule that can destroy biotic cell tissue and has the potential to indiscriminately damage healthy living root tissue, consequently reducing the fresh weight as well (Lau and Mattson, 2021). Lower root and shoot weight at 70 mL biweekly treatments in our study may be the result of this phytotoxicity.
Khan et al. (2018) studied the effects on roots of two varieties (S-22 and PKM-1) of tomato using three different concentrations of H2O2 (0.01, 0.1, and 0.5 mm). The 0.1 mm application of H2O2 for 4 h proved best and resulted in improved growth and photosynthetic attributes, enhanced activity of various antioxidant enzymes, and greater accumulation of proline. However, the effects were more prominent in ‘S-22’ compared with ‘PKM-1’. The induced activity of antioxidant enzymes and proline content by H2O2 was recorded to be more in ‘S-22’ than ‘PKM-1’, which helped in protecting photosynthesis and maintaining high plant fresh and dry mass than in PKM-1. They concluded that both the concentration of H2O2 as well as cultivars influenced the effects of H2O2 on growth of tomato, which supports our findings.
Physiological effects of exogenous application of H2O2 and calcium (Ca) were studied on sweet and hot pepper under heat stress and a significant change in biochemical, physiological, and fatty acid contents in both cultivars was reported; however, sweet peppers absorbed greater amounts of Ca in comparison with the hot peppers (Motamedi et al., 2019). Ca is an essential macronutrient in plants and provides structural stability to cell walls and membranes and acts as a secondary messenger in cellular signaling (White and Broadley, 2003). This could be a reason for greater height and shoot and root weights in ‘Lunchbox Red’ compared with ‘Early Jalapeno’.
Hydrogen peroxide treatment also improved photosynthesis as a result of improved gas exchange parameters, chlorophyll content, and net photosynthesis along with improved scavenging of ROS and recovery of photosynthetic efficiency in tomato (Nazir et al., 2019). In addition, H2O2 treatment also improved chlorophyll content in mung bean (Vigna radiata L.) and tomato by modulating endogenous plant hormones (Fariduddin et al., 2014; Orabi et al., 2015). Root dipping treatment with H2O2 significantly increased photosynthesis of tomato plants because of increase in the activity of Rubisco and PS II resulting in increased stomatal conductance (gS) and intercellular CO2 concentration (Khan et al., 2018). In a similar study conducted by Nurnaeimah et al. (2020), plant height, leaf area, chlorophyll content, net photosynthetic rate, gS, and quantum yield of mistletoe fig (Ficus deltoidea L.) increased after treatment with 16 and 30 mm H2O2. They concluded that the application of H2O2 increased stomatal opening, CO2 concentration, and accumulated photosynthetic pigments, which ultimately resulted in the increased photosynthetic rate. However, increasing the concentration of H2O2 to 60 mm led to a decrease in growth parameters and increased the accumulation of arsenic, iron, and sodium content in the leaves of mistletoe fig. These results can be supported by the fact that H2O2 at low concentration acts as a regulator of some major processes, such as assimilation, photosynthesis, respiration, gS, cell cycle, growth and development, and plant response to biotic and abiotic pressure; however, it can increase the oxidative damage, and ultimately cause cell death above a certain threshold (Latef et al., 2019). The study of Xia et al. (2014) also reported that a low concentration of H2O2 promotes stomatal opening, whereas a high concentration of H2O2 promotes stomatal closure in tomatoes. This might be a reason why a decrease in growth parameters of pepper and tomato was observed in our study with 70 mL biweekly application of either H2O2 products. In addition, H2O2 has also been reported to promote reproductive growth in fruits by inhibiting the growth of rudimentary leaves as well as by promoting the expression of the flower-related gene, LcLFY and reducing bud drop (Ismail et al., 2015).
An increase in D.O. levels with increased rate and application timing of H2O2 was observed. In hydroponic nutrient solutions, decomposition of H2O2 gives rise to byproducts H2O and O2, and this released O2 can increase the D.O. concentration in the root zone (Lau and Mattson, 2021). Hydrogen peroxide, when added into nutrient solution, breaks down, readily releasing a molecule of water and a reactive O-molecule, which can bind with another O-molecule. This O-molecule can also react with organic compounds, which often results in degradation of the compound (Fredrickson, 2014). In addition, reduced amounts of algae in water can result in greater D.O. level as the result of increased permeability of light in water (Lauguico et al., 2020). Plant growth of hydroponic crops has been shown to increase when irrigated with additional oxygen in the solution (Soffer et al., 1990). Butcher (2016) reported that the treatment with high-frequency application of H2O2 alone produced the greatest level of D.O., whereas combination of high-frequency application (every 3 d) of H2O2, vortex oxygenation, and air-pump injection treatment yielded the greatest production of chlorophyll in peppermint geranium (Pelargonium tomentosum L.). The increased production of chlorophyll was likely due to healthy root growth and respiration within a nutrient solution where high levels of D.O. and turbulence were facilitated. According to Chérif et al. (1997), tomato plant roots were susceptible to Pythium infection when the oxygen in the root zone dropped below 2.8 mg·L−1. Poor root and plant performance as well as an increase in the incidence of disease were reported when the root zone was oxygen deficient (Chérif et al., 1997). Marfà et al. (2005) reported greater yield of pepper plants grown in perlite with nutrient solution enriched with oxygen (16 mg·L−1) compared with unenriched solution (6 mg·L−1). In contrast to our result, Ouyang et al. (2021) reported a significant increase in growth, photosynthesis, yield, and quality of tomato when the D.O. of irrigation water was increased (9 mg·L−1) compared with the control (4 mg·L−1). This could be because they used aeration as a method of increasing D.O., whereas the high level of H2O2 in our study might have caused phytotoxicity and resulted in poor plant growth.
Conclusion
With increasing rate and timing of either H2O2 product, algae counts, dry weight, and chl a values decreased. However, weekly and biweekly application of 70 mL of the products were not significantly different from each other for controlling algae growth, so weekly would be recommended. Surprisingly, Zerotol 35 mL biweekly, Zerotol 70 weekly, PERpose Plus 35 mL biweekly, and PERpose Plus 70 mL weekly resulted in better growth in both tomato and pepper. Our results also indicated that 70 mL biweekly application of either product caused phytotoxicity and resulted in decreased growth. Results from both plants as well as algae analysis suggested that weekly application of 70 mL of either Zerotol or PERpose Plus produced the best results in terms of controlling algae and improving the growth of pepper and tomato plants. Future research should investigate other plants using these two products and rates, effects on fruiting, and overall nutrition of fruits.
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