Recycled Nutrient Solution Research Articles

Decision-tree-based ion-specific dosing algorithm for enhancing closed hydroponic efficiency and reducing carbon emissions.

The maintenance of ion balance in closed hydroponic solutions is essential to improve the crop quality and recycling efficiency of nutrient solutions. However, the absence of robust ion sensors for key ions such as P and Mg and the coupling of ions in fertilizer salts render it difficult to effectively manage ion-specific nutrient solutions. Although ion-specific dosing algorithms have been established, their effectiveness has been inadequately explored. In this study, a decision-tree-based dosing algorithm was developed to calculate the optimal volumes of individual nutrient stock solutions to be supplied for five major nutrient ions, i.e., NO3, K, Ca, P, and Mg, based on the concentrations of NO3, K, and Ca and remaining volume of the recycled nutrient solution. In the performance assessment based on five nutrient solution samples encompassing the typical concentration ranges for leafy vegetable cultivation, the ion-selective electrode array demonstrated feasible accuracies, with root mean square errors of 29.5, 10.1, and 6.1 mg·L-1 for NO3, K, and Ca, respectively. In a five-step replenishment test involving varying target concentrations and nutrient solution volumes, the system formulated nutrient solutions according to the specified targets, exhibiting average relative errors of 10.6 ± 8.0%, 7.9 ± 2.1%, 8.0 ± 11.0%, and 4.2 ± 3.7% for the Ca, K, and NO3 concentrations and volume of the nutrient solution, respectively. Furthermore, the decision tree method helped reduce the total fertilizer injections and carbon emissions by 12.8% and 20.6% in the stepwise test, respectively. The findings demonstrate that the decision-tree-based dosing algorithm not only enables more efficient reuse of nutrient solution compared to the existing simplex method but also confirms the potential for reducing carbon emissions, indicating the possibility of sustainable agricultural development.

Impact of Sodium Hypochlorite Applied as Nutrient Solution Disinfectant on Growth, Nutritional Status, Yield, and Consumer Safety of Tomato (Solanum lycopersicum L.) Fruit Produced in a Soilless Cultivation

Soilless crop production is spread worldwide. It is a cultivating technique that enhances yield quality and quantity, thus contributing to both food safety and food security. However, in closed-loop soilless crops, the risk of spreading soil-borne pathogens through the recycled nutrient solution makes the establishment of a disinfection strategy necessary. In the current study, sodium hypochlorite was applied to the recycled nutrient solution as a chemical disinfectant to assess its impact on plant growth, leaf gas exchange, fruit yield, tissue mineral composition, and possible accumulation of chlorate and perchlorate residues in tomato fruits. The application of 2.5, 5, and 7.5 mg L−1 of chlorine three times at fortnightly intervals during the cropping period had no impact on plant growth or gas exchange parameters. Furthermore, the application of 2.5 mg L−1 of chlorine led to a significant increase in the total production of marketable fruits (total fruit weight per plant). No consistent differences in nutrient concentrations were recorded between the treatments. Moreover, neither chlorate nor perchlorate residues were detected in tomato fruits, even though chlorate residues were present in the nutrient solution. Therefore, the obtained tomatoes were safe for consumption. Further research is needed to test the application of chlorine in combination with crop inoculation with pathogens to test the efficiency of chlorine as a disinfectant in soilless nutrient solutions.

Integrated management of hydroponic wastewater for complete water recycle and cyanobacteria cultivation using an electric conductivity-based tool

This work presents an integrated management process for hydroponic wastewater (HWW) reuse, by developing an electric conductivity-based tool that calculates recycled nutrient solutions' enrichment requirements and indicates their proper treatment. Treatment includes advanced methods and Synechococcus elongatus PCC 7942 (S7942) cyanobacteria cultivation for the removal/recovery and utilization of nutrients contained in HWW. The results showed that HWW can be utilized as substrate for S7942 cultivation. Increasing initial substrate nitrate‑nitrogen concentration from 389 ± 3 mg L−1 to 2132 ± 17 mg L−1 increases nitrogen assimilation from 0.131 mgNO3-N mgVSS−1 to 0.163 mgNO3-N mgVSS−1. Similarly, increasing initial substrate phosphate-phosphorous concentration from 8.4 ± 0.2 mg L−1 to 138.2 ± 5.0 mg L−1 increases phosphorous assimilation from 0.022 mgPO4 mgVSS−1 to 0.133 mgPO4 mgVSS−1. The advanced ion exchange treatment had efficiency >98 %. Adsorption with activated carbon and nanomaterials showed selective removal of nitrates, phosphates, calcium and magnesium. Sonication further increased nanomaterials' selectivity.

Responses of sweet pepper (Capsicum annum L.) cultivated in a closed hydroponic system to variable calcium concentrations in the nutrient solution.

The use of water containing calcium bicarbonate (Ca(HCO3 )2 ) at excessively high concentrations in closed hydroponic crops can cause calcium ion (Ca2+ ) accumulation in the recycled nutrient solution (NS) and concomitantly negatively affect yield and product quality. The aim of the study was to determine maximum Ca2+ concentrations that do not harm the crop and to simulate the pattern of Ca2+ accumulation when the Ca2+ concentration in the irrigation water, and concomitantly in the replenishment nutrient solution (RNS), is excessive. In the current study, irrigation water containing 1.5, 3.0, 4.5 and 6.0 mmol L-1 Ca2+ was used to prepare the RNS supplied to pepper cultivated in a closed hydroponic system. At 1.5 mmol L-1 Ca2+ , no Ca2+ accumulation was observed in the recirculating NS. However, at 3.0, 4.5 and 6.0 mmol L-1 in the irrigation water, the Ca2+ concentration in the recirculating NS, increased by the latter cropping stages to 17, 28 and 37 mmol L-1 , corresponding to 6.4, 9.0 and 10.8 dS m-1 . The accumulation of Ca2+ in the recirculating NS affected both tissue nutrient concentrations and uptake concentrations of Ca2+ , sulphate ion (SO4 2- ) and magnesium ion (Mg2+ ), but this was not the case for nitrogen (N) or potassium ion (K+ ). Growth, yield and plant water uptake were restricted at moderate (3.0 and 4.5 mmol L-1 ) and high (6.0 mmol L-1 ) external Ca2+ levels. In soilless pepper crops with zero discharge of fertigation effluents, the Ca2+ concentration in the irrigation water and the RNS should be lower than 3.0 mmol L-1 to avoid yield restrictions due to salinity. © 2021 Society of Chemical Industry.

Growth and Bioactive Compound Content of Glehnia littoralis Fr. Schmidt ex Miquel Grown under Different CO2 Concentrations and Light Intensities.

This study aims to determine the effect of different CO2 concentrations and light intensities on the growth, photosynthetic rate, and bioactive compound content of Glehnia littoralis Fr. Schmidt ex Miquel in a closed-type plant production system (CPPS). The plants were transplanted into a deep floating technique system with recycling nutrient solution (EC 1.0 dS·m-1 and pH 6.5) and cultured for 96 days under a temperature of 20 ± 1 °C, a photoperiod of 12/12 h (light/dark), and RGB LEDs (red:green:blue = 7:1:2) in a CPPS. The experimental treatments were set to 500 or 1500 µmol∙mol−1 CO2 concentrations in combination with one of the three light intensities: 100, 200, or 300 µmol∙m−2∙s−1 photosynthetic photon flux density (PPFD). The petiole length of G. littoralis was the longest in the 500 µmol∙mol−1 CO2 concentration with the 100 µmol∙m−2∙s−1 PPFD. The fresh weight (FW) and dry weight (DW) of shoots and roots were the heaviest in the 300 µmol∙m−2∙s−1 PPFD regardless of the CO2 concentration. Higher CO2 concentrations and light intensities produced the greatest photosynthetic rates. However, the SPAD value was not significantly different between the treatments. Higher light intensities produced greater content per biomass of chlorogenic acid and total saponin, although the concentration per DW or FW was not significantly different between treatments. The first and second harvest yields were the greatest in the 300 µmol∙m−2∙s−1 PPFD, regardless of the CO2 concentration. These results show that the 300 µmol∙m−2∙s−1 PPFD enhanced the growth, photosynthetic rate, and bioactive compound accumulation of G. littoralis, regardless of the CO2 concentration in a CPPS.

Growth and phytochemicals of ice plant (Mesembryanthemum crystallinum L.) as affected by various combined ratios of red and blue LEDs in a closed-type plant production system

As a high value added crop, the ice plant (Mesembryanthemum crystallinum L.) as known to be effective in diabetes and is becoming widely used by consumers. This study aimed to improve the growth and phytochemical contents of the ice plant using combinations of red and blue light-emitting diodes (LEDs) in a closed-type plant production system. Seedlings of the ice plant were transplanted into a deep floating technique system with a recycling nutrient solution (EC 4.0 dS m-1, pH 6.5). The plants were cultured under a temperature 25 ± 1 ℃ and a relative humidity 60 ± 5%. The combinations of red and blue LEDs (R9B1, red:blue = 9:1; R7B3, red:blue = 7:3; R5B5, red:blue = 5:5; R3R7, red:blue = 3:7; and R1B9, red:blue = 1:9) were used at 150 ± 5 μmol·m-2·s-1 PPFD with a photoperiod of 14 h/10 h (light/dark) for 28 days. Growth of the ice plant presented significantly higher values under the R9B1 and R7B3 for high ratio of red LED treatments. The fresh and dry weights per plant of shoot were the lowest in the R1B9 at 4.49 g and 0.14 g, respectively. Total phenolic concentration, total flavonoid concentration, and antioxidant activity of the ice plant increased proportionally to the treated blue LED ratio. In addition, the concentration per dry weight of myo-inositol was the highest in the R1B9 at 310.83 mg g-1. While pinitol concentration of the ice plant showed no increasing tendency with blue LEDs, significantly higher values were obtained under the high blue ratios, such as R3B7 and R1B9 treatments. In conclusions, when considering the phytochemical contents per biomass of the ice plant consumed by actual consumers, it was the most effective applying red and blue LEDs in a closed-type plant production system at the 70-90% or 10-30%, respectively.

Simultaneous biodegradation of methane and styrene in biofilters packed with inorganic supports: Experimental and macrokinetic study

Four upflow 0.018 m3 biofilters (3 beds), B-ME, B-200, B-500 and B-700, all packed with inorganic materials, were operated at a constant air flow rate of 0.18 m3 h−1 to eliminate methane (CH4), a harmful greenhouse gas (GHG), and styrene (C8H8), a carcinogenic volatile organic compound (VOC). The biofilters were irrigated with 0.001 m3 of recycled nutrient solution (NS) every day (flow rate of 60 × 10−3 m3 h−1). Styrene inlet load (IL) was kept constant in each biofilter. Different CH4-ILs varying in the range of 7–60 gCH4 m−3 h−1 were examined in B-ME (IL of 0 gC8H8 m−3 h−1), B-200 (IL of 9 gC8H8 m−3 h−1), B-500 (IL of 22 gC8H8 m−3 h−1) and B-700 (IL of 32 gC8H8 m−3 h−1). Finally, the effect of C8H8 on the macrokinetic parameters of CH4 biofiltration was studied based on the Michaelis-Menten model. Average C8H8 removal efficiencies (RE) varying between 64 and 100% were obtained at CH4-ILs increasing from 7 to 60 gCH4 m−3 h−1 and for C8H8-ILs range of 0–32 gC8H8 m−3 h−1. More than 90% of C8H8 was removed in the bottom and middle beds of the biofilters. By increasing C8H8-IL from 0 to 32 gC8H8 m−3 h−1, maximal EC in Michaelis-Menten model and macrokinetic saturation constant declined from 311 to 39 g m−3 h−1 and from 19 to 2.3 g m−3, respectively, which confirmed that an uncompetitive inhibition occurred during CH4 biofiltration in the presence of C8H8.

‏Methane biofiltration under different strategies of nutrient solution addition

Two biofilters B1 and B2 packed with non-porous inorganic materials were operated for 283 days to eliminate methane (CH4), an important greenhouse gas. The biofilters ran under a constant empty bed residence time of 6 min at CH4 inlet loads of 7–63 g m−3.h−1. B1 was fed with a fresh nutrient solution every day (2 L d−1, flow rate of 1 L min−1) whereas B2 was supplied by a recycled nutrient solution from a 10 L tank. The flexibility of both biofilters were evaluated and compared in absence of nutrient solution for 35 days. Also, the performance of both biofilters under a different nutrient solution addition frequency (0.5 L every 6 h, flow rate of 1 L min−1) was investigated. B2 performed appropriately with an average removal efficiency of 62 ± 3% for an inlet load of 13 g m−3.h−1. B1 showed an average removal efficiency of 66 ± 4% for the same inlet load as B2. Analysis of nutrient solution in the recycle tank showed that the highest nutrient consumption over the time was at an inlet load of 63 g m−3.h−1. B1 and B2 were tolerant to nutrient solution deprivation for 25 and 21 days, respectively. Frequent addition of nutrient solution (0.5 L every 6 h, flow rate of 1 L min−1) improved the performance with corresponding average removal efficiency of 64 ± 4% for B1, compared to 57 ± 4% with nutrient solution addition of 2 L every 24 h with a flow rate of 1 L min−1, while no significant change was observed in B2.

Water management in a controlled ecological life support system during a 4-person-180-day integrated experiment: Configuration and performance

Water management subsystem (WMS) is a major component of the controlled ecological life support system (CELSS). For guaranteeing the water requirement of crop growth and crewmember's daily life, a WMS was established in a 4 person 180-day integrated experiment (carried out in Shenzhen, China, 2016) to maintain a closed cycle with a total water amount of ~23 m3. The design and operation of the WMS was summarized as follows: (1) Collection and allocation of condensate water. About 917 L/d condensate water (>98% was from plants' evapotranspiration) was collected, and ~866 L/d of which was reused as plant nutrient solution after ultraviolet (UV) disinfection, and 50.6 L/d was used as the raw water for the domestic water supply module (DWS). (2) Domestic water supply. The condensate water from the plant cabin was purified through the DWS, a modified membrane bioreactor (MBR) system, and then provided hygiene and potable water to 4 crewmembers with different water quality standards. (3) Wastewater recovery. 51.4 L/d wastewater from urination and personal hygiene were treated together via a biological wastewater treatment process to complete the conversion of nitrogen and organic matters, and then recycled to plant nutrient solution. (4) Nutrient solution recycling. In the overall water cycle process, the plant nutrient solution was continuously self-circulated and the water quality of which was maintained at a relatively stable level with total organic carbon of 20–30 mg/L and NH4+-N < 1.0 mg/L. The 180-day continuous operation demonstrated that a 100% water closure was achieved. Based on the results of this study, an upgraded water cycle system for larger-scale and longer-term CELSS has been proposed.

Ice plant growth and phytochemical concentrations are affected by light quality and intensity of monochromatic light-emitting diodes

The ice plant (Mesembryanthemum crystallinum L.), widely known to be an effective cure for diabetes mellitus, is also a functional crop. This study was conducted to examine the effects of light quality and intensity of monochromatic light-emitting diodes (LEDs) on ice plant growth and phytochemical concentrations in a closed-type plant production system. Ice plant seedlings were transplanted into a deep floating technique system with a recycling nutrient solution (EC 4.0 dS m−1, pH 6.5). Fluorescent lamps, as well as monochromatic red (660 nm) and blue (450 nm) LEDs, were used at 120 ± 5 or 150 ± 5 µmol m−2 s−1 PPFD with a photoperiod of 14 h/10 h (light/dark) for 4 weeks. Ice plants showed higher growth under the high light intensity treatment, especially under the red LEDs. Furthermore, the SPAD value and photosynthetic rate were higher under the red LEDs with 150 µmol m−2 s−1 PPFD. The ice plant phytochemical composition, such as antioxidant activity and myo-inositol and pinitol concentrations, were highest under the blue LEDs with 150 µmol m−2 s−1 PPFD. Total phenolic concentration was highest under the blue LEDs with 120 µmol m−2 s−1 PPFD. Despite a slightly different dependence on light intensity, phytochemical concentrations responded positively to the blue LED treatments, as compared to other treatments. In conclusion, this study suggests that red LEDs enhance ice plant biomass, while blue LEDs induce phytochemical concentrations.

Light-emitting diodes and exogenous amino acids application improve growth and yield of strawberry plants cultivated in recycled hydroponics

Abstract Strawberry plants in recycled hydroponics exhibit growth and yield reduction due to autotoxicity. Strawberry plants were grown under light-emitting diodes (LED) and sprayed with amino acids to investigate their influence on the growth and yield under autotoxicity. In the first study, plants were grown under three LED light conditions [Red : Blue (R : B) = 8:2, 5:5, and 2:8 adjusted to similar light intensity of 106–117, 107–125, and 105–121 μmol m−2 s−1, respectively] and under white light provided by fluorescent lamps [104–129 μmol m−2 s−1] and also treated with two amino acids [hydroxyproline (Hyp) and glutamic acid (Glu)] and water (control). This study was conducted under relatively high temperature (30/25 °C; day/night) in order to enhance the occurrence of autotoxicity. Further, the nutrient solution was recycled for the duration of the crop cycle to allow the accumulation of autotoxic compounds. Greater growth and fruit yield, higher ascorbic acid content in fruits and also higher calcium and iron content in leaves, crowns and roots of strawberry plants were observed due to R : B = 8:2 LED lighting and Glu spray. In the second study, the selected LED (R : B = 8:2) from the first study was used with three different intensities (i.e., 149, 269, and 567 μmol m−2 s−1) and either with or without Glu spray under controlled environment condition (25/20 °C; day/night). Results showed that plants exposed to 567 μmol m−2 s−1 of R : B = 8:2 LED showed greater performances on growth and minerals content in leaves, crown and roots of strawberry plant supplied either with or without Glu whereas higher number of fruits per plant and fruit yield were observed with Glu spray. Therefore, we propose that combining Glu spray with exposure to R : B = 8:2 LED light of 567 μmol m−2 s−1 may improve the growth, yield and quality of strawberry cultivated in a hydroponic system with a recycled nutrient solution.

A Study on Inactivation of Pathogenic Bacteria for Nutrient Solution Recycling Using Advanced Oxidation Processes

This study was conducted to test the applicability of hydroxy radical reactor system, which applied advanced oxidation processes, to sterilize pathogenic bacteria for nutrient solution recycling in closed hydroponics. Removal efficiency was tested on 25 L of nutrient solution maxed with 10 mL culture solution of bacteria, E. coli, and R. solanacearum in a pilot tank. The testing conditions included various levels of hydroxy radicals resulting from air flow rates of 40, 80, and 120 L min-1, and 12 hours processing time. The removal of bacteria, E. coli, and R. solanacearum by hydroxy radical in nutrient solution was significantly increased with an increase in the flow rate of the air from 40 L min-1 to 120 L min-1. The optimum removal efficiency was achieved at an air flow rate of 120 L min-1 for 2 hours treatment. There were no significant differences in removal efficiency among bacteria, E. coli, and R. solanacearum for tested level and time of hydroxy radical. These results verified the efficiency of hydroxy radical in removing the pathogenic bacteria and the applicability of hydroxy radical reactor system in the field. Removal efficiency of pathogenic bacteria by hydroxy radical resulting from air flow level of 40, 80, and 120 L min-1. Removal efficiency of bacteria, E. coli, and R. solanacearum by hydroxy radical from air flow level of 120 L min-1.

Water reuse in closed hydroponic systems: Comparison of GAC adsorption, ion exchange and ozonation processes to treat recycled nutrient solution

In this study, the removal of root exudates from recycled nutrient solution (RNS) in hydroponic systems was investigated by using alternative advanced treatment processes. In particular, the performance of adsorption on granular activated carbon (GAC adsorption), ion exchange (using Amberlite IRA96) and ozonation was evaluated. The characterization of a RNS from a lettuce hydroponic company showed the occurrence of several organic acids including benzoic, maleic, palmitic, stearic, phthalic, succinic, acrylic, adipic, myristic, mannonic, oleic, arachidic and acetic acid. Furthermore, benzoic, acrylic, myristic and palmitic acid were detected even after the RNS treatment by GAC adsorption, ion exchange or ozonation. Further experiments were performed to evaluate the removal efficiency of root exudates from RNS by the selected treatment processes. For this purpose benzoic acid (BA), which is a potent growth inhibitor, was selected as target compound. Benzoic acid occurring at 23μg/L in the RNS was completely removed by GAC adsorption, ion exchange or ozonation process. In addition, GAC adsorption was the most efficient technique for organic matter removal. Indeed, the COD removal efficiency was 74%, 67% and 22% for GAC adsorption, ozonation and ion exchange treatments, respectively. The results from UV–vis absorbance measurements corroborated the best performance of activated carbon in organic compounds removal from RNS. For instance, the absorbance removal at 280nm reached 82%, 51% and 27% after GAC adsorption, ozonation and ion exchange treatments, respectively.

Double use of water treatment in soilless growing systems: disinfection of recirculating solution and removal of plant protection products from discharge water

Worldwide increase of water shortage and competition for fresh water is raising the awareness that an increase in water use efficiency in horticulture is a necessity. At the same time, environmental awareness and legislation (e.g. the EU Water Framework Directive 2000/60/EC) restricts the emissions of nutrients and plant protection products (PPPs). In the Netherlands in soilless growing systems much effort is put into measures to enable maximal recycling of nutrient solutions, thereby decreasing the required amount of fresh water needed and the emission of nutrients and PPPs. Recycling of water fluxes also implicates that the drain water has to be disinfected to prevent rapid dispersal of pathogens. Heat treatment, filtration, Ozone and UV treatment are commonly used disinfection technologies. Some of these technologies (Ozone and UV) are known for their ability to break down PPPs and therefore their joint application (disinfection and end-of-pipe purification of discharged drain water) have been studied. In this paper we present the results of the integration and optimisation of ozone technology in a cucumber and a sweet pepper soilless cultivation. An Agrozone ozone unit that alternately treated the recycled nutrient solution for the elimination of pathogens and removed PPPs from discharge water was investigated. The ozone unit eliminates pathogens and PPPs by direct oxidation of organic material batchwise. It was shown that both disinfection of the recirculating nutrient solution and removal of PPPs from discharge water are possible with the same ozone equipment, if installed correctly. Preliminary tests with Tomato Mosaic Virus have shown that this technology can completely disinfect (from 402 to 0 lesions per plant), and remove up to 98% of PPPs from standardised greenhouse discharge water. For disinfection purposes shorter treatment times and lower ozone doses can be applied.

A Targeted Management of the Nutrient Solution in a Soilless Tomato Crop According to Plant Needs.

The adoption of closed soilless systems is useful in minimizing the environmental impact of the greenhouse crops. Instead, a significant problem in closed soilless systems is represented by the accumulation of ions in the recycled nutrient solution (NS), in particular the unabsorbed or poorly absorbed ones. To overcome such problem, we: (1) studied the effect of several values of the electrical conductivity (EC) of NS in a NFT (Nutrient Film Technique) system on a cherry type tomato crop, and (2) define a NS (called recovery solution), based on the concept of “uptake concentration” and transpiration–biomass ratio, that fits the real needs of the plant with respect to water and nutrients. Three levels of EC set point (SP), above which the NS was completely replaced (SP5, SP7.5, and SP10 for the EC limit of 5, 7.5, and 10 dS m-1, respectively), were established. The SP10 treatment yield was not different from other treatments, and it allowed a better quality of the berries (for dry matter and total soluble solids) and higher environmental sustainability due to a lower discharge of total nutrients into the environment (37 and 59% with respect to SP7.5 and SP5, respectively). The recovery solution used in the second trial allowed a more punctual NS management, by adapting to the real needs of the crop. Moreover, it allowed a lesser amount of water and nutrients to be discharged into the environment and a better use of brackish water, due to a more accurate management of the EC of the NS. The targeted management, based on transpiration–biomass ratio, indicates that, in some stages of the plant cycle, the NS used can be diluted, in order to save water and nutrients. With such management a closed cycle can be realized without affecting the yield, but improving the quality of the tomato berries.

Producción de lechuga en invernadero con y sin recirculación de la solución nutritiva

Hydroponic systems with nutrient solution recycling provide more efficient water and fertilizer use, but over time it becomes difficult to sustain the nutritional balance and control diseases that attack plant roots, which eventually results in lower yield compared to systems where nutrient solution is not recycled. The aim of this study was to evaluate the yield and earliness of butter lettuce (Cortesana M1 variety) grown with different hydroponic systems and their efficiency in terms of water and nutrient use. A randomized block experimental design with four replicates and three treatments (T) was used: T1: tezontle substrate beds without recycling the drained nutrient solution; T2: tezontle substrate beds with recycling the drained nutrient solution; and T3: floating root system. Yield and earliness variables were measured and also the savings and efficiency in water and fertilizer use were estimated. Data were subjected to analysis of variance and Tukey’s multiple comparisons of means test (P = 0.05). The best yield, earliness and efficiency in water and nutrient use occurred with the floating root system. There were no differences in yield or earliness between the substrate systems with or without nutrient solution recycling.

Effects of Renewal Pattern of Recycled Nutrient Solution on the Ion Balance in Nutrient Solutions and Root Media and the Growth and Ion Uptake of Paprika (Capsicum annuum L.) in Closed Soilless Cultures

Ion imbalance in recycled nutrient solutions is caused by selective ion uptake of plants, which occurs at different rates in different growth stages. The objectives of this study were to investigate the ion balances in both recycled nutrient solutions and rockwool media using different renewal patterns for the nutrient solutions, and to analyze the subsequent effects on uptake of water and nutrients. Over 12 weeks of paprika cultivation, two different renewal patterns (week units) of 6-4-2 and 8-2-2 weeks were compared with a constant renewal pattern of 4-4-4 weeks (control). The nutrient solution in the reservoir tank was constantly maintained at EC <TEX>$2.5dS{\cdot}m^{-1}$</TEX> and pH 5.5-6.5. The changes in the ion balance with the 4-4-4 week pattern were smaller than those with the other treatments. In the early growth stage, however, the ion balances similarly changed among all treatments. Greater changes were subsequently observed for the 6-4-2 week pattern. Although fruit yield and shoot fresh weight of paprika were the lowest with 6-4-2 renewal pattern, no significant differences were observed. Our results indicate that renewal intervals can be extended in consideration of growth stage for more efficient and practical operations in closed soilless cultures.

Response of hydroponically grown head lettuce on residual monomer from polyacrylamide

The aim was to assess acrylamide monomer (AMD) uptake by hydroponically grown lettuce. Lettuce was cultivated by applying plant tissue testing in a recycled system by the use of nutrient solutions prepared with two water-soluble flocculants F3 and F4 containing 176 and 763 mg kg−1 of AMD, respectively. The effects on growth, fresh weight and plant leaf quality were evaluated by comparing these treatments and one control standard nutrient solution typically recommended for lettuce hydroponic cultivation. To assess the nutritional status of lettuce, samples were collected and lyophilised before determination of the selected micro- and macro-element contents. An HPLC with photodiode array detector method was applied to determine AMD in both selected flocculants and dried plant samples. Results show that lettuces cultivated under the conditions described above absorb AMD from nutrient solutions into their leaves. The AMD presence in recycled nutrient solutions has a negative influence on the growth of lettuce, reducing their average fresh weight and average number of leaves. The study confirmed that the problem of AMD mobility and its accumulation risk in plants should to be an important topic with respect to safe polyacrylamide (PAM) handling in the agro food area.

DEVELOPMENT AND EVALUATION OF A DSS FOR DRAINAGE MANAGEMENT IN SEMI-CLOSED HYDROPONIC SYSTEMS

The recycling of greenhouse effluents in soilless cultivation systems is environ-ment-friendly, since the waste of water and the disposal of nutrients to the environ-ment is drastically reduced or even eliminated. However, a long-term recycling of the effluents may result in accumulation of Na+ and Cl- in the recycled nutrient solution. The major aim of the present research was to test the performance of a Na+ mass balance model operating as part of a decision support system in a tomato crop grown in a closed-loop hydroponic system. Three treatments were compared: a completely open, a completely closed and a semi-closed hydroponic system during one experimental period lasting from October 2011 to January 2012. The fresh water used for the preparation of the nutrient solution contained 2 mM NaCl. During the total experimental period, the water uptake by the crop was relatively low, near to 130 L m--2, and thus, the concentration of Na+ in the closed system did not surpass the level of about 20 mM during the total experimental period. Comparison between measured and simulated Na+ values gave fair results. The system was efficient at minimizing the discharge of nutrient solution effluents in semi-closed hydroponic systems, thereby maximizing the water use efficiency.

Uptake of nutrients and water by paprika (Capsicum annuum L.) as affected by renewal period of recycled nutrient solution in closed soilless culture

One of the problems in closed soilless cultures is to find a solution in order to reduce ion imbalance in recycled nutrient solutions when the drained nutrient solution is re-used. To reduce ion imbalance in recycled nutrient solutions, determination of renewal period is required, since uptake of individual ion by plants changes throughout the growing period. The aims of this study are to determine the appropriate renewal period to reduce ion imbalance in the recycled nutrient solution and to investigate the uptake of nutrients and water by the paprika plant and to measure fruit yield and mineral contents in plant as influenced by renewal period of recycled nutrient solution in a closed soilless culture. Paprika plants were grown in rockwool slabs with either 4-, 8-, or 12-week renewal period in the closed as compared to the open hydroponic system (control). The electrical conductivity (EC), pH, and volume of the nutrient solutions were measured every three days. The nutrient solution in the reservoir tank was constantly maintained at EC 2.5 dS·m−1 and pH 5.5–6.5. As results, the uptake of K+ was significantly influenced by the renewal period. Changes of cation and anion ratio in the recycled nutrient solution could be reduced by discharging every 4 weeks. The significantly lowest fruit yield was observed in a closed hydroponic system using a 12-week renewal period. Regarding the ion balance in the recycled nutrient solution, renewing every 4 weeks was found to be the most effective in this study. However, considering the yield along with nutrients and water uptake, a 8-week interval could also be used.