Sustainable Strategies Based on Reused Leachates and Hydrogen Peroxide Supply to Fertigate Cordyline fruticosa var. ‘Red Edge’ Plants

: Leachate reuse is a helpful tool that contributes to the sustainability of agricultural systems, but it requires previous disinfection. Hydrogen peroxide can be found among the disinfect-ants frequently applied in ecological production systems. Moreover, it can improve the oxygenation of the root system. The objective of this work was to study its effect on C. fruticosa plants fertigated with leachates. A split-plot design with six treatments, three without an H 2 O 2 supply (S 0 ) and three with an H 2 O 2 supply dosage at 2% (S H2O2 ), was arranged: raw leachate from C. lanatus (L 100 ), raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m −1 (L WD ), and raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m −1 (L NSD ). The results produced data about the evolution of the nutrient and leachate solutions throughout the cultivation period. Morphological (height, leaf number, leaf area, total fresh weight, relative water status, and dry weight) and physiological (chlorophyll a, chlorophyll b, carotenoids, chlorophyll a+b, and proline) parameters were studied to reveal the plant response. The efficiency of nutrient utilization was higher with the L WD treatment, and water and nitrogen utilization efficiency decreased under the H 2 O 2 supply. In conclusion, the reuse of diluted leachate is advised for nutritionally undemanding crops, such as C. fruticosa ; moreover, the H 2 O 2 supply improved tolerance to salinity and enhanced root growth and Red-Green-Blue (RGB) values.


Introduction
Currently, three objectives are related to sustainability in intensive production: (1) to cultivate without deteriorating the agricultural environment, (2) to get rid of the food insecurity generated by the contamination of products, and (3) to obtain a production that allows owners of small and medium-size farms to have a decent income in disadvantaged areas [1].
In the province of Almería (located in the southeast of Spain), there are 30,600 ha of protected crops [2], of which 2000 ha are occupied by biological and ecological crops. The progressive increase of organic farming during the last 10 years has allowed producers to obtain a higher yield free of pollutants and food of high nutritional and organoleptic value, thanks to an environmentally friendly production system based on biological interactions and beneficial natural processes [3].
In this area, the reuse of leachates is an advantageous technique for saving water, allowing a more rational utilization of resources and contributing to the sustainability of the intensive production system in the Mediterranean area [4][5][6]. Nevertheless, more research on the disinfection of leachates and the effects of disinfectants (which prevent the development of pathogens) on crops is needed [4,7].
The disinfection of effluents can be achieved by a variety of methods, mostly using chlorine, hypochlorite, chlorine dioxide, ozone, and UV-light; other means employed to a lesser extent are treatments by chloramines, bromine, bromine chloride, iodine, hydrogen peroxide, ozone, metal ions, heat, ultrasound, electrostatic processes, gamma-and X-rays, etc. All these methods were developed primarily for their application with potable water [5,8].
Nevertheless, the need of achieving greater efficiency in production systems compels us to focus this research on aspects that may limit production, such as those generated by deficient aeration in the rhizosphere, which causes serious problems in plants due to radical hypoxia and can be avoided by utilizing oxygen releasers [6,9]. In the presence of hydrogen peroxide, a greater absorption of oxygen takes place, as has been demonstrated with horseradish [10,11].
Peroxides, which have been widely used for more than 50 years in the pharmaceutical and food industry, hospitals, agriculture, wastewater treatment, water purification, etc., act as oxidants. Peroxides have a proven disinfectant activity against all types of microorganisms (bacteria, viruses, protozoa, coccidia, algae, fungi, and spores), causing bacterial proteins to lose their functionality and attacking the cell membrane, DNA, and other components, causing cell death [12][13][14]. The decomposition products of peroxides are water and oxygen, so they are highly biodegradable products that leave no residue of any kind [15,16]. Hydrogen peroxide has the additional advantage of releasing oxygen in the disinfection process, and its use is allowed in biological agriculture in Europe [1].
In terms of water and nutrient requirements, leachates from horticultural crops can be reused for the fertigation of other tolerant crops, thereby achieving a more sustainable production system for growers by reducing the inputs of water and fertilizers [9,10]. The aim of this work was thus to study the effect of oxifertigation (H2O2 supply) on Cordyline fruticosa var. 'Red Edge' plants fertigated with pure or diluted leachates, as well as with a standard nutrient solution, in a soilless culture of rock wool of Citrullus lanatus in southeastern Spain. To this point, there is a dearth of research on the addition of H2O2 in fertigation in terms of plant growth, a lack that this study aims at remedying.

Greenhouse Experiment
The trial was performed in two greenhouses over a 2-month period from April to June. The first site was a commercial plastic greenhouse located on a farm called "Coto Espinosa" in La Cañada de San Urbano, Almería, Spain (36°89′ N; 2°36′ W), with C. lanatus Thunb. (watermelon) plants cultivated in a rockwool soilless system, where leachates were collected from 4 collection trays (each one with 2 slabs). The trays were placed randomly in the greenhouse, and the leachates were mixed and analyzed weekly during the cultivation cycle. The second site was a tunnel greenhouse located at the University of Almería, Spain, (36°83′ N; 2°41′ W), where C. fruticosa L. var. 'Red Edge' plants were cultivated with peat moss and perlite (3:1 v/v) in 10 cm pots. The average daily temperature was 22.15 °C, with a relative humidity (RH) 70.43% and photosynthetically active radiation (PAR) 2.74 mol m −2 d −1 .

Treatments and Experimental Design
The experimental design was a split plot (3 × 2 factorial). The different factors assessed were as follows: the type of fertigation treatment (L100: raw leachate from C. lanatus, LWD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m −1 ; LNSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m −1 ) and levels of supplies of H2O2 as OX-VIRIN in the leachate (without H2O2 supplied to the leachate and with H2O2 supplied to the leachate at 2% (v/v)) resulting in a total of 6 treatments ( Figure 1) with 4 randomized complete blocks, and 3 plants per block, giving, plus border plants, a total of 94 plants. The threshold of EC of 2.5 dS m −1 during the cultivation cycle was the most suitable for the growth of this species [17]. Irrigation frequency depended on crop water requirements, with an average daily dose of 20 mL per plant. Table  1 shows the chemical composition of the tap water, the ranges of nutrient solution supplied to watermelon, and the nutrient solution supplied to C. fruticosa.

Nutrient Solutions Analysis
The parameters determined in the nutrient solutions and leachates tested were pH, EC, NO3 − , SO4 2− , H2PO4 − , Cl − , Ca 2+, Mg 2+ , K + and Na + . Samples were collected weekly. The values of pH and EC were measured with a Crison MicropH 2001 pH-meter and with a Crison Micro CM 2200 conductivity meter (Crison, Barcelona, Spain). Nitrate was analyzed according to the colorimetric determination by nitration of salicylic acid [18]. Sulphate was determined following the method proposed by Novozamsky and Eck [19]. Phosphate was determined by the vanadomolybdophosphoric colorimetric method [20], using a Helios Gamma spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). Chloride was analyzed following the method proposed by Kolthoff and Kuroda [21]. Total Ca 2+ and Mg 2+ were determined by atomic absorption spectrophotometry [22] utilizing a 3300 atomic absorption spectrophotometer (PerkinElmer Waltham, MA, USA). The total K + and Na + were directly measured by flame spectrophotometry [23] utilizing a flame photometer (Evans Electro Selenium LTB, Halstead, Essex, UK).

Plant Morphological Parameters
Four random plants per treatment were sampled at 60 days after the beginning of the trial. Plant height (H) was measured with a ruler from the base of the plant to the tip. Leaf area (LA) was measured at the end of the experiment, using an area integrator ΔT Area Meter MK2 (Delta-T Devices, Burwell, UK). The number of leaves (LN) was obtained through manual counting.
Afterwards, the plant material was washed [24] and separated into absorption organs (roots), conductive organs (stems and petioles) and photosynthetic organs (leaves). The different fresh vegetal fractions were weighted (FW) and dried in a forced-air oven at 70 °C for 48 h and weighed on a Mettler Toledo PB-303-S scale (Columbus, OH, USA) to obtain the respective dry weight (DW). The relative water status (RWS) (g of water/g of dry matter) was calculated on an FW basis [25], following the formula where TFW is the total plant fresh weight and TDW is the total plant dry weight.
The color index in the leaves for red (R), green (G), and blue (B) values was assessed using an optical scanner and the program Adobe Photoshop.

Plant Physiological Parameters
At the end of the experimental period, four plants were randomly selected per treatment for the determination of the concentration in the leaves of proline (Pro) and the pigments chlorophyll a (Chla), chlorophyll b (Chlb), chlorophyll a + b (Chla + b), and carotenoids (Ct).
The fresh material of the leaves (0.5 g) was crushed with ethanol (5 mL at 96%) and then washed with ethanol (5 mL at 70%). The alcoholic extract was centrifuged for 10 min (3500× g), and then the supernatant was stored at 4 °C for further analysis. The ninhydrin reagent method was used to determine the free proline concentration (expressed in µg g −1 FW) in the alcoholic extract supernatant, following the recommendations given by Irigoyen et al. [26].
Fresh samples of the leaves (0.2 g) were submerged in methanol for 24 h under dark conditions at room temperature. The extraction procedure does not include any crushing/homogenization steps, and the supernatant was separated by filtration when the leaf samples were completely colorless. The concentration of pigments in the supernatant was recorded spectrophotometrically utilizing the protocol reported by Wellburn [27].

Water and Nutrient Use Efficiency of the System Crop
At the end of the experimental period, we determined the water and nutrient use efficiency of the system crop, as frequently used in agronomic research, following the procedure established by Dobermann [28] defined as the total dry weight (g) per liter or gram of nutrient applied (g).

Statistical Analysis
The significance of the effect of the treatments was assessed by standard analyses of variance (two-way ANOVA) and Least Significant Difference (LSD) test carried out using Statgraphics Plus V.4.0 (Statpoint Technologies, Inc. Warrenton, VA, USA). Differences were considered significant at p < 0.05.

Plant Morphological and Physiological Parameters
During the experimental period, neither mortality of plants nor incidence of pests or diseases for the treatments were assessed. The fertigation with diluted leachates did not have significant effects on H, LN, LA, TFW, RGB values, but the RWS increased significantly under dilution (LWD and LNSD). On the other hand, H2O2 supply significantly decreased LN, LA, and TFW and increased the RGB values (Table 2). Moreover, the application of diluted leachates did not cause noteworthy effects on the TDW and the distribution ratios of biomass, but the H2O2 supply decreased the TFW. Different trends for biomass partitioning were found among the different organs assessed. The RWR increased, and the LWR decreased under H2O2 supply (Table 3).  Leachate dilutions and H2O2 supply did not significantly affect the photosynthetic pigment concentration in C. fruticosa plants (Table 4). Nevertheless, proline concentration presented significant differences for the two studied factors. The highest proline concentration was detected under L100 (71.75 µg g −1 FW), followed by LNSD, and the lowest was found in LWD (62.00 µg g −1 FW). The application of H2O2 also reduced the proline concentration in leaves. Table 4. Effects of fertigation treatments on chlorophyll a (Chla), chlorophyll b (Chlb), chlorophyll a + b (Cha + b) and carotenoids (Ct) (mg g −1 FW) and proline (Pro) (µg g −1 FW) in the leaves of C. fruticosa at the end of the experimental period.  Figure 2 showed the evolution of the nutritional parameters (pH, EC, NO3 − , H2PO4 − , SO4 2− , Cl − , K + , Na + , Ca 2+ , Mg 2+ ) in the fertigation treatments throughout the experimental period. The highest values of the nutritional parameter's concentrations were found under L100, except for H2PO4 − . Moreover, pH, EC, SO4 2− , Cl − , Na + , and Mg 2+ presented similar values in LWD and LNSD. Nitrate, H2PO4 − , K + and Ca 2+ were higher in LNSD. Nevertheless, when nutrient solutions were supplied with H2O2, important changes occurred. The pH, EC, SO4 2− , Cl − , K + , Na + , Ca 2+ , Mg 2+ decreased and NO3 − increased.  The fertigation with leachates diluted with a standard nutrient solution and without H2O2 supply decreased the accumulated volume of leachate ( Table 5). The accumulated volume of the nutrient solution applied throughout the trial was 1.14 mL per plant, so the drainage percentage was 18.00, 17.00, 17.00, 19.00, 21.00 and 17.00% for the fertigation treatments L100, LWD, LNSD, L100 + H2O2, LWD + H2O2 and LNSD + H2O2, respectively. There is an interesting interaction between factors because non-significant differences between oxyfertigation and non-oxyfertigation under LWD treatment were found (Figure 4).

Nutrient Solution and Leachate Characterization
Leachate pH presented a similar behavior to that of nutrient solutions pH ( Figure 2). The oxyfertigated nutrient solutions had more acidic values (4.7-4.9) than leachates (6.26-7.23). There was a decline in leachate pH (Figure 3) throughout the cultivation cycle.
The fertigation treatments LWD, LNSD, LWD + H2O2 and LNSD + H2O2 had a leachate EC trend similar to that of the nutrient solutions EC (Figure 2), while it was higher in L100 and L100 + H2O2, where pure leachates were applied. The application of H2O2 appears to have caused a slight reduction of EC in all the treatments tested from 22 to 43 DAT.
The fertigation treatments L100 and LWD produced leachates with the highest average concentration of NO3 − , while the lowest was found in the fertigation treatment LWD + H2O2. L100 presented the highest quantity of leachate related to the diluted treatments and oxyfertigated versus non-oxyfertigated (Table 5). Furthermore, nitrate concentrations in leachates were greater than the concentrations in the applied nutrient solutions ( Figure 2). During the investigation, the behavior of leachates and nutrient solutions was comparable.
Phosphate concentration in leachates presented a behavior comparable to that found in nutrient solutions (Figure 2), presenting the highest values for the treatments LNSD and LNSD + H2O2. Under these treatments, the leaching of H2PO4 − increasedsubstantially. Oxyfertigated treatments caused a higher quantity of leachate related to non-oxyfertigated treatments (Table 5).
For the evolution of SO4 2− concentration, all the treatments followed a similar pattern, with the minimum value at 1495 and the maximum at 2490 mg L −1 . The leaching trends were similar to the NS applied ( Figure 2), but the leaching showed greater concentration. The total SO4 2− leachate was significantly higher in L100 treatment. No effect was found in relation to H2O2 supply (Table 5).
For Cl − concentrations over time, all the treatments presented a peak at 22 DAT, being higher in the L100 treatment, reaching 3100 mg L −1 of Cl − , decreasing from this moment on, and stabilizing until the end of the cultivation. In general, the concentration of Cl − in oxyfertigated treatments increased at the beginning and decreased at the end of the cycle for the non-oxyfertigated treatments. L100 presented the highest quantity of leachate, LWD intermediate value and LNSD the lowest level. Oxyfertigated treatments presented a lower Cl − concentration than non-oxyfertigated treatments (Table 5). Non-significant differences between oxyfertigation and non-oxyfertigation under LWD treatment were found ( Figure  4).
The behavior of cations concentration was similar: the concentrates were higher in the leachate than in the nutrient solution. There was a rise in all the treatments tested at 22 DAT. L100 presented the highest K + , Ca 2+ , and Na + concentration in leachate. Magnesium concentration was similar under the factors studied. Oxyfertigation led to higher K + , Na + , Ca 2+ concentrations than non-oxyfertigated treatments ( Table 6). Non-significant differences between oxyfertigation and non-oxyfertigation under LWD treatment for Na + were detected ( Figure 5).     standard deviation of 4 samples per treatment. Means with different letters within a column are significantly different at p < 0.05 (analysis of variance and least significant difference test). ns: indicates non-statistical differences. * : indicates statistical differences between treatments.

Figure 5.
Interactions between fertigation treatments and H2O2 supply on Na + leachate. L100 = raw leachate from C. lanatus, LWD: raw leachate from C. lanatus diluted with tap water until EC of 2.5 dS m −1 ; LNSD: raw leachate from C. lanatus diluted with standard nutrient solution until EC of 2.5 dS m −1 and H2O2 supply (S0 and = without H2O2 and SH2O2 = with 2% H2O2). Different lowercase letters indicate significant differences between treatments.

Water and Nutrient Use Efficiency
The dilution of leachates did not affect WUE. Nevertheless, the H2O2 supply significantly decreased this parameter. The highest NUE, PUE and KUE were detected under LWD (Table 7). Oxyfertigated treatments significantly decreased NUE but did not affect PUE and KUE. Non-significant differences between oxyfertigation and non-oxyfertigation under L100 treatment for NUE were found ( Figure 6). Table 7. Effects of fertigation treatments on water utilization efficiency (WUE) (g L −1 ), nutrient utilization efficiency (NUE) (g g −1 ), phosphorous utilization efficiency (PUE) (g g −1 ) and potassium utilization efficiency (KUE) in C. fruticosa at the end of the experimental period.

Nutrient Solution and Leachate Characterization
The nutrient solution coming from leachates had an average pH near to neutrality, but there was an acidification of the diluted and oxyfertigated nutrient solutions due to the acidity of the supply (water, standard nutrient solution or H2O2 supply) [29]. Peroxide oxygen is a weak acid that ionizes releasing protons into the medium [30]. Nevertheless, Soto-Bravo [31] did not find significant differences between the pH of nutrient solutions used in non-oxyfertigated and oxyfertigated tomato plants.
The desired pH range for most crops is between 5.0 and 6.2 [32,33]. Nevertheless, the application of H2O2 may require regulation of the pH because both alkaline pH (above 8.0) and highly acidic pH (below 4.0) have shown an inhibitory effect on the growth of water hyacinth plants [34]. Values above 7.0 induce deficiencies of P, Fe, Mn, and Cu and sometimes Zn. The pH values below 4.0 cause root damage because of the effects of H + . Below a 4.5 pH level, there are increases in the proportion of H2PO4 − concentration in the nutrient solution in undissociated forms not available to the plant. Several Mn and Al hydroxides are soluble at a pH level less than 5.0; below 5.5 pH levels, the growth of the plant may be affected because it may not absorb Ca, K and Mg [13,35].
Electrical conductivity presented values between 3.4-4.1 dS m −1 in L100, higher than those found in LWD and LNSD, which were around 2.5 dS m −1 due to the higher concentrations of the ions, apart from H2PO4 − in pure leachate. Correspondingly, Soto-Bravo [31] found that oxyfertigation with H2O2 in tomatoes causes modifications in irrigation water EC, changing from 2.9 dS m −1 to 2.5 ds m −1 ; in L100 the values fell slightly to 0.7 dS m −1 . Nevertheless, under LWD and LNSD, only a slight decrease was found. These results could be due to the ion concentration effect on EC [36,37]. The values presented in LWD and LNSD treatments were in the same range as those recommended for the cultivation of C. fruticosa by other researchers [17]. For NO3 − concentration, the values found in L100 were higher than those detected in the LWD and LNSD treatments because the former consisted of pure leachate (higher EC), while treatments for the latter were obtained by dilution with tap water or standard nutrient solution (with lower EC). The NO3 − concentrations in the leachate from C. lanatus fluctuated throughout the experiment from 1264 to 870.2 mg L −1 , which may be related to the differential nitrogen uptake related to the growth of this species over time, as reported by Hershey and Paul [38]. In the L100 treatment, a significantly higher concentration of NO3 − was observed at the beginning of the cycle, which coincides with the highest nutrient demand of C. lanatus for vegetative growth [39]. The concentration of NO3 − increased significantly under the H2O2 treatments because of the mineralization process, increasing availability of NO3 − [40]. Under non-oxyfertigated treatments, H2PO4 − followed the same pattern throughout the cultivation cycle. Nevertheless, the supply of the standard nutrient solution enriched the leachates obtained from C. lanatus and improved their quality for ornamental plant production. Under an H2O2 supply, the concentration of H2PO4 − presented similar values. The concentration of H2PO4 − in our experiment exceeded the reference value (10 mg L −1 of P) proposed for the optimal growth of woody ornamental plants [41], but it was into the range recommended for the cultivation of ornamental plants [42]. Nevertheless, H2PO4 − concentration in LDSN treatments was higher from 22 DAT, and it must be considered that high levels can generate phytotoxicity problems [11,43] although in our experiment, C. fruticosa plants did not show any aesthetic damages.
For SO4 2− concentrations, L100, because it was pure leachate (with higher EC), presented higher concentrations than LWD and LNSD, and the other leachates employed for fertigation were diluted with tap water or standard nutrient solution (with lower EC). Sulphate is one of the most important anions in natural waters [44]. The treatments with H2O2 caused a decrease of SO4 2− concentration in the nutrient solution due to the precipitation of calcium sulfate [45].
Chloride concentration presented the same trend as SO4 2− concentration. At 29 DAT, Cl − concentration began to rise remarkably in the nutrient solution, reaching 800 mg L −1 .
In the cultivation of watermelon, there is an increase in the volume of leachate obtained in the phase of fruit fattening, causing a salts wash and a higher Cl − concentration in the leachate. The concentration is toxic for a large number of ornamental crops; however, a good tolerance is observed in rose cultivation with 30 mmol L −1 (1065 mg L −1 ) of NaCl [46]. In general, until 22 DAT, the addition of H2O2 increased the Cl − concentration in the nutrient solution but decreased afterwards. At low pH conditions, Cl − solubility increases; then, when the chlorides oxidate in chlorine oxides, they are eliminated as a gas and therefore Cl − concentration decreases at the end of the cultivation for oxyfertigated solutions [47].
Similar average concentrations of K + were observed in L100 and LNSD due to the dilution with standard nutrient solution. The treatments with H2O2 presented lower values, which can be related to the greater K + plant uptake. These concentrations are higher than those recommended by Yeager et al. [48], who propose a range of 30-50 mg L −1 .
Calcium and Mg 2+ concentrations in the nutrient solution presented similar trends, being higher in the L100 treatment. The H2O2 supply did not have a significant effect on the concentration of Ca 2+ in the nutrient solution. Our values were lower than the range established by Edwards and Norton [49] in peach plants (10-15 mg L −1 ).
It is relevant to mention that Na + concentration presented a peak at 43 DAT in all tested treatments, which could be due to the increase of transpiration and water uptake of C. lanatus plants, as suggested by Savvas et al. [50] in cucumber plants. The highest value was detected in L100. The H2O2 supply did not have a significant effect on the concentration of Na + in the nutrient solution.

Plant Morphological and Physiological Parameters
Plants fertigated with leachates were shorter than those fertigated with a standard nutrient solution, which may be explained by the decrease in the concentration of available nutrients in leachates and the positive effects of these nutrients in plant growth, as has been widely reported by Mengel and Kirkby [35]. However, this result disagrees with those obtained by Boyle et al. [51] and Westervelt [52], who reported an increase in plant height under low nutrient concentrations.
Flowers and Yeo [53] consider that the increase of salinity decreases vegetative growth. Parry et al. [54] consider that the promotion of growth might be attributed to an increase of carbon assimilation under H2O2 spraying, since even small increases in the leaf net photosynthesis rate can translate into large increases in biomass. Nevertheless, in our experiment, the morphological parameters studied did not show significant differences caused by the modifications in pH associated with dilution. This could be due to the combined effects of the potential growth increase caused by the higher NO3 − concentration in L100 [35] and the potential growth decreases under higher EC [55]. The H2O2 supply decreased LN (17%), LA (19%), TFW (19%), TDW (15%), and LWR (9%) and increased the RWR (20%) in plants growing in pots filled with peat moss and perlite. These results differ to those obtained by Niñirola [56], who did not find growth and development of watercress (Nasturtium officinale) under nutrient solution aeration treatments, while Soto-Bravo [31] did not find effects in tomato yield under chemical oxifertigation. On the other hand, Semida [57] found an increase of the number of leaves per plant and leaf area per plant in onion plants sprayed with H2O2 (1 mM) under salt stress. The total dry weight was lower under H2O2 treatments, which may be related to the lower number of leaves and leaf area and, therefore, to their lower capacity to generate bioassimilates. However, Li et al. [58] consider that the exogenous application of H2O2 participates in the synthesis of enzymes and antioxidant compounds that induce salinity tolerance in wheat seeds, with a positive effect on growth. It is necessary to highlight that the application of H2O2 increased RGB values in leaves of C. fruticosa plants, enhancing their aesthetic value and their market value.
The root weight ratio increased 1.2 times under H2O2 supply. These results are consistent with those obtained by Hammed et al. [59], who stated that exogenous H2O2 applications provide a more vigorous root system to wheat seedlings because the number of roots was higher, but root length was significantly shorter, indicating suppression of cell elongation, probably by premature initiation of the secondary wall formation. Leaves showed the opposite behavior to roots because the plants treated with H2O2 had a lower number of leaves and leaf area; nevertheless, García-Jiménez [60] found a higher stem DW in green bean and melon plants subjected to a chemical oxygenate application to the fertigation solution. Colunje et al. [61] also found a similar behavior in pepper crops supplied with ozonated fertigation. For this reason, this effect could be related to oxygenated nutrient solution more than to a specific effect of H2O2.
Relative water status significantly increased in treatments with dilutions (LWD and LNSD). Plants under stress often lose some water from their tissues, which can have rapid and major effects on cell expansion, cell division, stomatal opening, and abscisic acid (ABA) accumulation [62]. In our trial no significant effects were found in the H2O2 supply, which agrees with the results found by Lorenzen et al. [63], who did not detect any influence on the water content of Typha and Cladium plants under a high oxygen level in the culture solutions. Opposite results have been found by Ishibashi et al. [64] in soybean plants, in which there was an increase of relative water content in the leaves when H2O2 was applied. Moreover, Semida [57] found a significant rise of relative water content in onion varieties under H2O2 supply.
The concentration of photosynthetic pigments did not show significant differences among the treatments assessed. Nevertheless, Ahmad et al. [65] found a significant increase in Chla concentration with the foliar application of H2O2 in corn seedlings under low temperature conditions. Liu et al. [66] also observed that the exogenous application of H2O2 increased the antioxidant activity in cucumber leaves and decreased lipid peroxidation, protecting the ultrastructure of chloroplasts and mitochondria under osmotic stress conditions. Foliar sprays in two onion varieties grown under saline soil condition led to an increase in the chlorophyll and carotenoid contents. The low concentration of these parameters in onion plants under saline stress may be due to a decrease in intermediates of chlorophyll biosynthesis [67].
There were significant differences in proline concentration for the two studied factors. L100 presented the higher value, which was linked to their higher salinity. Cordyline fruticosa plants synthesize proline as a major compatible solute to adjust the osmotic pressure when Na + and Cl − accumulate in their tissues, and these results coincide with the mechanism to maintain cell homeostasis under salt-stress conditions observed in Casuarina equisetifolia plants [68]. Leaf proline concentration decreased significantly under the H2O2 supply in our experiment, a finding that contradicts the results obtained by Ishibashi et al. [64], who noted an increase in proline in soybean plants under water scarcity and foliar application of H2O2. The reduction of free proline by H2O2 application in onion plants may be attributed to the crucial role of H2O2 in mitigating the negative salt effects [57].
In general, the leachate volumes obtained from C. fruticosa in the different treatments are considered normal for the handling of irrigation in crops without soil with regularquality water [69]. There was a significant decrease in plant growth in the treatments fertigated with leachates that were diluted with standard nutrient solution and without H2O2 supply. This is because plants fertigated with the same treatment but supplied with H2O2 had a lower leaf area index. The relatively high evapotranspiration is probably caused by the high LAI (Leaf Area Index) [70,71]. Inversely, drainage rises when plant transpiration rate is low [72].
The evolution of leachate pH and fertigation solutions during the trial were similar. The oxyfertigated nutrient solutions have more acidic values (4.70-4.90) than leachates (6.26-7.23). Blond peat is acidic, but limestone and dolomite, normally incorporated to neutralize its acidity, increase the buffer capacity and provide Ca 2+ and Mg 2+ [73,74], causing their solution from the substrate not to reach such low pH values [75]. Chen et al. [76] have also demonstrated that substrates containing pine bark, sand, and peat buffer alkaline pH of irrigation water to a measured substrate pH of about 6.5. At 35 DAT, a significant decrease in the leachate pH was observed, probably due to the loss of the buffering capacity of the substrate, because carbonates are depleted. None of the pH values sampled in the leachate fell outside the critical ranges (pH of 6.5-9.5) established by the EEA (2014).
A decline in leachate pH occurs throughout the cultivation cycle. The evolution of leachates pH and nutrient solutions pH was similar in the treatments without H2O2; however, in the treatment LWD + H2O2, pH is higher (with values from 7.7 at 15 DAT to 7.4 at 50 DAT) than in L100 + H2O2 and LNSD + H2O2 (with values from 7.8 at 15 DAT to 5.0 at 50 DAT and from 7.6 at 15 DAT to 4.9 at 50 DAT, respectively). Opposite results were obtained by Betancur and Garcia-Caparros et al. [77,78], who found that when the concentration of H2O2 increases, the pH tends to alkalinization.
In drainage water, EC values were slightly lower for H2O2 treatments than for nonoxygenated treatments during most of the cultivation cycle of watermelon grown in perlite bags, possibly due to the slightly higher percentage of drainage water [79]. Nevertheless, Soto-Bravo [31] did not obtain significant differences in the EC when injecting H2O2 continuously into the irrigation water for the tomato crop.
L100 presented the highest NO3 − concentration in the fertigation water, and this was reflected in the leachate. Leachates contain many organic forms of nitrogen; when applying H2O2, these forms are oxidized and remain in the form of NH4 + which is nitrified and converted to NO3 − , making it more available for plants. This fact must be considered before the application of fertilizer to avoid excessive amounts of NO3 − and the toxicity generated, defects in production of cultivation, and unnecessary expense [76]. The leachates generated exceeded by 20 times the limit imposed by the European Union for NO3 − in groundwater [80].
The elevated H2PO4 − concentration found in LNSD and LNSD +H2O2 treatments could be due to the dissolution of precipitated forms of calcium phosphates, associated with low pH. In general, lower concentrations of H2PO4 − were found in drainage solutions, with concentrations between 50% and 100% of the input in soilless culture [81]. From 35 DDT, the supply of H2PO4 − exceeded C. fruticosa requirements, which were especially high in the oxygenated treatments, so it was leached in large quantities [82].
Although the polluting element par excellence is NO3 − , SO4 2− may be taken as a critical parameter in the assessment of groundwater quality [45]. The standard for SO4 2− is 250 mg L −1 in waters intended for human consumption, a quantity 8 times less than the concentration in the leachates of our experiment.
The behavior of Cl − concentration could be due to the joined effects of the low pH, related to the higher solubility of Cl − in the leached solution, and the oxidation of Cl − in chlorine oxides, eliminated in the form of gas, decreasing the concentration of Cl − at the last period of the trial [47]. However, this ion does not cause a problem in crops that have a short growth period and a low transpiration rate, e.g., lettuce (Lactuca sativa) [83] as well as some ornamental plants such as Cordyline fruticose.
The rise of cation concentration in the nutrient solution for the fertigation of Citrullus lanatus can be related to the enrichment in K + , Ca 2+ and Mg 2+ , for it contributes to induce flowering [84,85] and fructification [86,87]. Nevertheless, the requirements of C. fruticosa plants were lower. Baixauli and Aguilar [81] recommend leachate concentrations in the range of 50-100% for K + , 100-150% for Ca 2+ and 100-300% for Mg 2+ , as the inputs in soilless culture.
Sodium concentration increased progressively in treatment with pure leachates; the significant increase at the end of the trial may be due to an imbalance between the transpiration rate and the absorption of this ion, causing a lower leachate volume and an increase in Na + concentration [8,86]. Calcium concentration followed a similar evolution; these two parameters share some of the same pathways in the cell since they are similar ions [88].

Water and Nutrient UtilizationEfficiency
Water utilization efficiency was unaffected for the fertigation treatments. Although the EC affected the relative water content, there was no effect on TDW. These results were dissimilar to those obtained by Lee and Van Iersel [89], who reported a reduction of WUE in Chrysantemum morifolium under salt stress. The decline in WUE under H2O2 supply could be related to the biomass decrease.
As far as nutrient utilization efficiencies were concerned, NUE, PUE and PUE increased in plants fertigated with LWD. The decrease in NUE, PUE and KUE in L100 and LNSD could be related with an excess of these nutrient requirements by plants, as Ristvey et al. [90] observed in azalea. In contrast, KUE was higher under saline conditions in Atriplex [91], which could occur since it is a halophyte plant, while C. fruticosa is a species semitolerant of salt. The decline in NUE under the H2O2 supply could be related to the biomass decrease. Nevertheless, the interaction found between factors indicates that there was no effect of H2O2 supply under the L100 treatment. Lorenzen et al. [63] found different responses in utilizing oxygenated culture solutions: Typha showed a lower NUE than plants in low oxygen culture solutions, but NUE in Cladium was not affected by the oxygen level. With PUE, similar results were found by Lorenzen et al. [63], who observed that oxygen applications in the rhizosphere had only a limited effect in PUE in Typha and Cladium.

Conclusions
The dilution (with water or standard nutrient solution) of the leachates obtained from the cultivation of Citrullus lanatus for their reuse as fertigation solutions for the cultivation of Cordyline fruticosa causes important modifications in the nutrient parameters studied. The addition of a commercial oxidant with H2O2 causes the acidification of the nutrient solution, while EC is not affected, although it decreases in the leachate obtained in treatment with pure leachates. There is an increase of the NO3 − concentration in the nutrient solutions but a decrease in the leachate when watermelon leachates were diluted with tap water. Phosphate concentration increases significantly in the nutrient solutions and in the leachates. Sulphate concentration decreases in the nutrient solutions but increases in the leachate diluted with tap water. Chloride increases in the nutrient solution constituted by leachates; it seems that there is a positive effect in most saline treatment. Cations follow a similar evolution in all the nutrient solutions tested, with no significant differences in the leachates, except for the Na + concentration, which rises. Hydrogen peroxide causes a reduction of the number of leaves per plant, leaf area, water status and total plant DW, also modifying biomass distribution, with an increase in root weight ratio and a decrease in DW percentage in leaves. Nevertheless, H2O2 application enhanced the values of RGB. The concentrations of photosynthetic pigments are not affected. The application of H2O2 does not generate stress in the plants, valued by the concentration of proline. Nutrient utilization efficiency is higher under LWD, and the efficiency in the utilization of water and NO3 − decrease under an H2O2 supply. In conclusion, it can be stated that the reuse of leachate diluted with water is recommended in nutritionally undemanding crops such as C. fruticosa. Furthermore, the H2O2 supply improves tolerance to salinity and enhances root growth.