Cascade Hydroponics as a Means to Increase the Sustainability of Cropping Systems: Evaluation of Functional, Growth, and Fruit Quality Traits of Melons

Abstract

The necessity of optimizing the nutrient and water efficiency in conventional hydroponics and enhancing their sustainability has given rise to the concept of cascade cropping systems. These achieve high water and resource use efficiencies, together with a lower environmental footprint, which is especially important for Mediterranean areas. However, scientific questions about the mechanisms that drive productivity in this system remain to be answered. This study aimed at a comprehensive evaluation of crop performance in cascade systems in terms of morphoanatomical and functional responses, also including product quality parameters, which influence the marketability of the fruit. In a three-month experiment, the dynamics of melon’s photosynthetic light use efficiency, pigment contents, growth parameters, and leaf compactness were assessed in a cascade system using drainage of tomato cultivation in comparison to classic hydroponic melon. The fruits’ chroma, hardness, total soluble solids, and pH were also measured. Comparable plant functional responses in the control and cascade melon plants resulted in similar growth and morphoanatomical traits. The fruit quality attributes were also found to be almost identical. It is proposed that the cascade system is both effective and sustainable in regions facing climatic and water scarcity pressures, such as those that are prevalent around the Mediterranean basin.

1. Introduction

Hydroponics has become a widespread cultivation technique due to its high productivity and reduced water and space requirements compared to soil cultivation [1]. The enhanced productivity potential is attributed to the ability to manipulate environmental conditions and optimize fertigation programs, ensuring that the precise nutritional requirements of each cultivated species are met, without the interference of the soil [2,3]. In this way, the absorption of nutrients by the roots is direct and it is not influenced by the usually unfavorable characteristics and physicochemical conditions of the soil [4].

Despite its potential to address the growing food needs of a rapidly expanding global population, hydroponics has several drawbacks that overshadow its benefits and cause skepticism about its widespread adoption. Gruda et al. [5] set the main goals of hydroponics (and soil-less cropping systems in general) towards increasing sustainability as (a) the optimization of nutrient and water efficiency and (b) the establishment of circular waste flows. The latter, in addition to serving as a solution for the former goal, is also a strategy to tackle one of the main environmental issues that are associated with the operational strategy of hydroponics, which is the discharge of the used nutrient solutions. In both open and closed hydroponic systems, the disposal of utilized nutrient solutions into the environment exerts further pressure on soil quality, resulting in substantial environmental degradation and wasting of resources [3,6]. Under this approach, the sustainability of this cropping system is seriously compromised, and the environmental footprint is increased due to the discharge of high quantities of chemicals into the environment [7].

Recent research has proposed cascade cropping systems as a promising way to implement the aforementioned circular waste flows [6,8]. These systems adopt a circular economy approach to managing the used fertigation solution; the drainage solution of a crop is driven to fertigate a second crop, with additional loops being added recently [4,9]. In this manner, the initial solution that is drained from the so-called primary crop is not discharged but is being exploited by the secondary crop, which absorbs the residual nutrients from the solution.

Following its introduction, cascade hydroponics was subjected to a series of trials with diverse crops, aimed at assessing its applicability and efficiency. Many relevant articles have emphasized the potential of cascade systems to reduce the environmental impact of greenhouse cultivation by achieving substantial water and nutrient conservation, along with a marked reduction in final nutrient leaching to the environment. Karatsivou et al. [9] substantiated the environmental advantages through improved water savings, and a remarkable 50% increase in nitrogen and phosphorus use efficiencies compared to controls was achieved. Naounoulis et al. [10] calculated leaching to the environment as a fraction of the initial irrigation volume to drainage volume in the cascade system compared to open hydroponic cultivation of the primary crop. They reported a 75% reduction in the proportion of leachate that was discharged in the cascade system. Similarly, the final leaching fraction of a cascade system incorporating leafy vegetables was reduced by 90% [9]. In another experiment where the nutrient solution from a semi-closed system of table tomatoes was reused for cherry tomatoes, yields similar to those of the control were observed [11]. Of particular note was the substantial 65% reduction in the leaching of harmful nutrients that was attained. Overall, this approach enhances the sustainability of the system, as the fertilizers are significantly depleted in the final solution to be discharged [12]. Furthermore, the inputs of fertilizer and water are significantly reduced, resulting in substantial economic benefits. This is achieved by utilizing the same volume of water and fertilizers to cultivate two crops, yielding two products in a single cycle.

The most important measure of the efficiency of cultivation systems is always the yield of the crops. Documented advantages of cascade systems in terms of productivity or yields that are comparable to those of classical hydroponics have been presented in works on basil, spinach, lettuce, and parsley from the Mediterranean region [6,9,13]. However, other studies have identified a decline in yield for certain secondary crops, attributed to two factors: (a) sub-optimal nutrient concentrations in the drainage solution when utilized without further fertilizer amendments and (b) increased salinity [14,15]. The latter entails risks for impaired plant function, which ultimately causes a reduction in biomass and yield in the secondary crops. Consequently, a pivotal aspect of implementing cascade hydroponics is the careful selection of secondary and tertiary crops in terms of salinity tolerance, as their limitations and/or adaptability are critical to the successful implementation of cascade hydroponics [16]. Nevertheless, relevant studies highlight that the observed moderate reduction in crop productivity could be partially compensated for by reduced production costs because of zero costs for fertilizers, as well as the considerable water savings [15,17]. It is worth noting that the measured electrical conductivity (EC) of the drainage solution is frequently well within the tolerance limits of major greenhouse crops, thereby enabling the successful cultivation of lettuce, spinach, parsley, and aromatic plants as secondary crops [6,9].

The yield aspect of cropping systems is crucial in their evaluation, as it is essential for their adoption at a large commercial scale. Nevertheless, a comprehensive evaluation of cascade systems should also include product quality parameters and crop performance in terms of morphoanatomical and physiological responses. Regarding the former, the acceptability of a product to consumers and thus the market is primarily based on visual stimuli (i.e., color), and subsequently on intrinsic attributes like palatability, which is based on the sugar content, hardness, acidity, etc. [18]. Their assessment is necessary to complete the picture of the product differences between hydroponics and cascade systems. Furthermore, functional and morphoanatomical characteristics are associated with the mechanisms that drive the growth and yield of crops. Therefore, assessing these parameters is crucial for understanding the processes that underpin crop productivity. The dynamics of these processes, influenced by the prevailing environmental conditions, directly impact the final produce and yield.

The aim of this study was to evaluate the impact of a cascade system comprising cucumber as the primary crop and melon as the secondary crop on the growth and physiological and fruit quality parameters of the latter. Additionally, morphoanatomical traits were investigated. These traits are usually overlooked in the relevant literature; however, their dynamics significantly influence energy and gas fluxes mediated by the crop in greenhouse cultivation. This comprehensive investigation of cascade hydroponics systems, with an emphasis on their potential for sustainable agriculture in the context of the Mediterranean region’s escalating water scarcity challenges, is deemed to be of substantial practical interest.

2. Material and Methods

2.1. Experimental Setup

The experiment was performed in the experimental farm of the University of Thessaly in Velestino (Central Greece) at an altitude of 85 m (39.22′, 44.85′). The cascade system consisted of two greenhouses, one with a size of 240 m², engaged for the cultivation of the primary crop, while the second was used for the cultivation of the secondary crop and occupied an area of 160 m². Both greenhouses have a north–south orientation, and their internal environmental conditions are automatically controlled (SERCOM, Automation SL, Lisse, the Netherlands and Argos Electronics, Athens, Greece). Since the experiment was performed during the spring–summer period, the temperature regime was set at 18–26 °C, with the heating system being activated when the daily temperature fell below 18 °C and the night temperature below 16 °C, while the cooling system (wet pad and fan) was initiated at air temperatures above 26 °C. The connection between the two greenhouses was through a plastic tube which transferred the drained solution from the primary crop into a 400 L collection tank located within the secondary crop greenhouse. The whole process was repeated automatically every two days. The fertigation head pumped solution from this collection tank to prepare the cascade irrigation solution.

Two treatments of the melon irrigation solution were employed for the purposes of this study:

(a)

Control: a freshly prepared standard hydroponic solution;

(b)

Cascade: the drainage solution from the primary crop (cucumber), corrected for pH.

Standard hydroponic solutions were prepared for the cucumber and melon controls according to Sonneveld and Donnan [19], as detailed in Naounoulis et al. [10], who reported results from the same experiment. The EC and pH target values were set at 2.35 and 5.8, respectively, in all treatments, the latter being adjusted with nitric acid. In accordance with Naounoulis et al. [10], the mean values of physicochemical parameters of the irrigation solutions that were provided to the two treatments during the experimental period are presented in

Table 1. Chroma parameters of melon fruits, measured at different time points throughout the experimental period; L* describes the lightness [0, black; 100 white], a* refers to the transition from green (negative values) to red (positive values), and b* refers to the transition from blue (negative values) to yellow (positive values). The different letters (in bold) indicate statistically significant differences between treatments for the specific parameter and measurement day.

Parameter	Measurement Day	Control	Cascade
L*	70	61.06 ± 0.94 a	64.53 ± 2.05 a
	77	63.83 ± 1.29 a	63.64 ±0.41 a
	90	65.19 ± 1.44 a	67.23 ± 0.55 a
a*	70	−3.82 ± 0.69 a	−2.57 ± 1.33 a
	77	−1.94 ± 0.38 b	−3.99 ±0.25 a
	90	1.01 ± 0.44 a	0.698 ± 0.47 a
b*	70	33.77 ± 2.23 a	41.07 ± 3.70 a
	77	38.07 ± 2.62 a	37.17 ± 0.60 a
	90	41.34 ± 2.49 b	48.29 ± 0.80 a

. An automatic programmable logic controller (PLC) (Argos Electronics, Athens, Greece) was employed to regulate the irrigation volume and frequency according to the prevailing temperature and radiation conditions; the range of daily irrigation events was 3 (in cloudy) to 10 times (in fully sunny days), with a flow of 0.5 L m⁻² being applied by drip irrigation in all cases.

The Cucumis melo cv. Masada was tested in this study, which is a cultivar originating from the well-known Galia variety, with soft, firm, and juicy flesh and a high tolerance to blights. The experiment lasted for 3 months (April to July 2023). In total, 192 plants, rooted in rockwool cubes (Grodan International, The Netherlands), were transferred to perlite bags of 33 L each (Nordiaagro, Athens, Greece). The perlite bags were placed on eight hydroponic channels (each 12.5 m long), with 12 bags per channel and two melon plants per bag (Figure 1). Each treatment had three replicates (channels) with a total of 75 plants, resulting in a planting density of 2.2 plants m⁻². This plant density is within the limits tested by Rodriguez et al. [20] that do not affect neither the growth nor the fruit quality of Galia cultivars. The two marginal channels, although planted and irrigated (one with control and the other with cascade solutions) were not included in the experiment, because they were not considered to be representative due to edge effects.

2.2. Measurements

2.2.1. Growth and Morphoanatomical Parameters

The total leaf area was measured at 3 time points during the experimental period, on day 30 (D30), D60, and D90, with the latter being the harvest day. At each measurement, 4 plants per treatment were randomly selected and cut at the stem–root interface. The leaves were then separated from the stems and placed on a white surface without overlapping. A photograph was then taken including all leaves, which was then analyzed using image analysis software (ImageJ v. 1.54h, open-source software hosted on GitHub, Inc., San Francisco, CA, USA) to estimate the total area of the leaves in m².

The dry biomass accumulated in the leaves, stems, and fruits was measured at the same time points as the leaf area measurements, plus an initial assessment on D1. After imaging, the separated plant parts were transferred to paper bags and placed at 70 °C to reach a constant dry weight.

The leaf specific mass (LSM) was calculated from the dry weight and area measurements of leaf disks. Specifically, leaf disks were taken from mature leaves positioned in the center of the plant at 6 time points during the cultivation period, with 15 replicates per treatment. Leaf disks were subsequently dried at 70 °C for 24 h to reach a constant weight; then, the LSM was calculated and expressed as g dm⁻².

2.2.2. Physiological Parameters

The concentrations of photosynthetic pigments were determined photometrically on D9, D30, D60, and D90. Disks were removed from mature, healthy leaves with a cork borer and extracted in 80% acetone with a mortar and pestle. After the recording of absorbance at 720, 663, 646, and 470 nm with a double-beam spectrophotometer (UV-1900, Shimadzu, Kyoto, Japan), the equations of Lichtenthaler and Welburn [21] were used to estimate the concentrations of chlorophyll a (Chl a), Chl b, and carotenoids (Car) as μg cm⁻².

The Photochemical Reflectance Index (PRI) was measured at 6 time points from D9 to D85, covering the entire experimental period, with 15-day intervals on 15 mature leaves per treatment, selected from the middle height of the plant. Care was taken to perform the measurements at the same time (10:00–11:00) on clear, sunny days. The PlantPen PRI120 (Photon Systems Instruments, Drásov, Czech Republic) was used, which non-destructively measures leaf reflectance at 531 and 570 nm and produces the PRI.

2.2.3. Fruit Quality Determination

Fruit quality parameters were measured on D70, D80, and D90 on 10 mature melons per treatment, which were of a commercial size.

The fruit hardness was assessed as the peel hardness and edible part (flesh) hardness with a digital penetrometer (Turoni 53205, T.R. Turoni srl, Forli, Italy) using a 6 mm plunger. For each fruit, both parameters were assessed at two different spots of the equatorial part of the fruit with the peel (for peel hardness) and after removing it (flesh hardness).

For the measurement of total soluble solids, each melon was cut into pieces, and then the necessary amount of juice was collected in plastic beakers with a hand squeezer. The measurement was conducted with the refractometer PAL-1 (ATAGO, Tokyo, Japan), and the results were expressed as Brix %. The pH was measured in the above-described juice with a pH meter (HΙ 9024, HANNA instruments).

The color was measured at three points along the equatorial line of the intact fruit with a portable colorimeter CR-400 (Konica Minolta Optics Sensing Inc., Osaka, Japan). The CIE L*a*b* color space was then utilized to classify the color components, employing the following parameters [22]:

L*, where 0 is black and 100 is white;

a*, with negative values denoting a green color and positive values a red color and their grading;

b*, with negative values denoting a blue color and positive values a yellow color and their grading.

2.3. Statistical Analysis

A one-way analysis of variance (ANOVA) was conducted on the data following the examination of their normality (Shapiro–Wilk test) and homogeneity of variances (Levene’s test) (p ≤ 0.05). In cases where the criteria for ANOVA were not met, the non-parametric Kruskal–Wallis test was carried out. JASP (version 0.14, JASP Team, 2021) was used for all statistical analyses.

3. Results

The pattern of dry mass accumulation in different plant parts was followed throughout the experiment, and the results are presented in Figure 2. Both the leaves and stems of the melon plants did not show any significant difference on all the measurement days (Figure 2A,B). A remarkably high growth rate was observed in both treatments. Indicatively, during the first month of cultivation, the leaf mass exhibited a 16-fold and 17-fold increase in the control and cascade plants, respectively. In the second month, this increase was 3-fold in both treatments. With respect to the dry weight of the fruit, only on D60 was there a significant decrease of 25% in the cascade treatment, which was subsequently balanced at the final measurement on D90 (Figure 2C).

The total leaf area completes the picture of plant growth (Figure 3A). The control slightly outperformed the cascade treatment in the first and last measurements; however, no significant differences existed in any of the cases. The LSM shown in Figure 3B exhibited some fluctuation during the experimental period, with the highest values being recorded at the beginning, but the two treatments showed identical profile and values.

The concentration of the photosynthetic pigments, their relationships, and the temporal dynamics are presented in Figure 4. The Chl a, Chl b, and Car contents showed the same profile during the experimental period, with the highest values being reached in the third month, which was especially obvious for Chls. In all cases, both treatments exhibited similar levels, and, consequently, the Chl a/Chl b ratio was also similar. Nevertheless, in terms of the Chls/Car ratio, the control plants showed a slight but statistically significant decrease compared to the cascade treatment on D9 and D60.

The PRI showed remarkable fluctuations during the experimental period, with the highest values appearing in the first measurement, followed by the last two dates (Figure 5). Both treatments followed the same profile of change throughout the experiment. The only statistically significant difference appeared on D70, where the control plants exhibited slight, but significantly lower values compared to the cascade plants.

The fruit quality parameters illustrated in Figure 6 showed non-significant differences between treatments and non-remarkable variations among the measurement dates. The hardness of the peel remained stable around 2–2.5 N in all measured fruits during the last 20 days of cultivation (Figure 6A). The flesh hardness (Figure 6B) exhibited slightly lower values in the control at the first two measurements. However, in the last one, the picture was reversed, with the control fruits exhibiting a statistically significant 72.4% increase in flesh hardness compared to the cascade plants. In contrast to this, the cascade fruit values remained almost stable during the last two measurements. Similarly, the levels of total soluble solids were stable (Figure 6C), with a slight increase on D90, in both treatments. This profile was also followed by the pH values of the fruits, which never exceeded 6.5 in any of the cases during the experimental period (Figure 6D).

The color components were measured at three time points during the last month of the experiments and are depicted in Table 1. The L* parameter showed high values, which corresponds to bright colors ranging between 61.06 and 65.19 in the control fruits and 63.64 and 67.23 in the cascade fruits. The a* measurements ranged from −3.8 to 1 for the control and −3.99 to 0.7 for the cascade treatment. A statistically significant decrease in a* was observed at the second measurement in the cascade fruits, indicating a greener color. A wider range of b* parameter values was recorded in the cascade fruits (37.2–48.3), which was also shifted to higher values compared to the 33.8–41.3 corresponding to the control fruits. This was significant at the last measurement of D90, where the cascade fruits were more yellow, showing a statistically significant increase in b* compared to the control.

4. Discussion

Mediterranean regions are confronted with the challenge of water scarcity, a problem that is becoming increasingly pressing [23]. Cascade soil-less systems have been shown to achieve high water use efficiencies, with increases ranging from 22% [10] to 50% in lettuce [9] in comparison with conventional monoculture hydroponics. Moreover, numerous studies have documented high nitrogen, phosphorus, and potassium use efficiencies [9,10]. In the same experiment as ours, Naounoulis et al. [10] measured the concentrations of nutrients in the final discharge solution. They found that the increased resource efficiency was accompanied by a significant reduction in N and P leaching into the environment, at levels of 70% and 86%, respectively, compared with monoculture. This reduction is attributed to the extensive reuse of these nutrients by secondary crops, i.e., melon. In addition to the resource and environmentally relevant issues, the outcome of the cropping system, i.e., the yield, has a significant impact on its adoption by farmers and commercial greenhouses.

4.1. Growth and Morphoanatomical Responses

Yield is the outcome of crop growth and function in response to environmental factors. In the present study, the melon growth, in terms of biomass accumulation in leaves, stems, and fruits, was similar in both the cascade and control treatments. Another aspect of plant growth, the total leaf area, which indicates the assimilative potential of the crop, was also found to reach the same levels in the control and cascade systems. The important morphoanatomical feature of the LSM, which represents the allocation of biomass per unit area in the leaves and indicates the compactness of the leaf tissue, showed no differences between treatments. The response of crop growth and yield is strongly influenced by the crop species and the design of the cascade system [8,13,14]. Regarding the latter, the main concerns are the adequacy of the drainage solution to meet the nutrient requirements of the secondary crop, as well as the dimensioning of both the donor and recipient crop areas. In studies where the requirements were fulfilled, the area dimensioning considerably affected the outcome of the cascade system. The 3:1 ratio of the primary to the secondary crop area (m²), as proposed by Naounoulis et al. [10] for the cucumber–melon combination, resulted in a similar melon yield to the melon monoculture (control). Elvanidi et al. [6] reported that a 1:1 to 1:1.5 ratio resulted in species-specific differences in dry mass accumulation, with basil showing similar levels to the control, although cascade rosemary and peppermint exhibited statistically significant reductions compared to the controls. In contrast, Karatsivou et al. [9] found increased biomass in cascade spinach and lettuce and a slight reduction in parsley cultivated as a secondary crop, with tomato as the primary one. In the latter experiment, a 1:1 ratio of tomato to lettuce and parsley was used, while the 1:3 ratio for tomato to spinach resulted in positive results in terms of spinach growth. A 22% decreased yield was also reported for rosemary that was fertigated with sequential reuse of the drainage solution of melon in the work of García-Caparrós et al. [8]. However, the authors state that the water and nutrient savings in this treatment were significant, which led them to propose this cropping system as an effective means of addressing water scarcity in the Mediterranean region. Similarly, Avdouli et al. [15] reported that the performance of basil was significantly reduced in their cascade system, while the total amino acid and ascorbate contents increased, thus compensating for the reduction in fresh produce through the enhancement of bioactive compounds. Santos et al. [24] explored the potential of growing lettuce under different fertigation solutions with 0–100% drainage. They found that 100% drainage caused a significant effect, i.e., a 41% reduction in leaf area and a 20% reduction in head diameter. Puccinelli et al. [17], working with wild species that were cultivated with tomato effluent, found no effect on the leaf succulence in a cascade system, corroborating our results on the LSM.

The growth response is mainly mediated by the high salinity of the effluents, with a minor contribution from nutrient deprivation or phototoxic root exudates, as proposed by the robust experimental design of Puccinelli et al. [17]. In the same experiment as ours, Naounoulis et al. [10] measured the elemental profile of melon leaves in relation to the yield response and found that the cascade treatment plants had similar nutrient levels, although significantly lower levels of PO4³⁻ and NH4⁺ were found in their fertigation solution. Therefore, the total yield of the cascade melons was similar to that of the control. Regarding the EC of the fertigation solution, they reported an average of 2.67 dS m⁻¹ in the cascade system, which was similar to the 2.5 dS m⁻¹ of the control fertigation solution. Higher ECs, above the limit of 3.2 dS m⁻¹, have been reported to decrease the yield of melon plants [25]. Additionally, studies of salt stress effects on melon growth document a decrease in photosynthetic performance and yield at ECs above 5 dS m⁻¹ [26]; an increase in the intercellular electrolyte leakage in the leaves at above 4.3 dS m⁻¹, with lower values not significantly affecting the physiology [27]; and the deterioration of fruit quality at ECs of 8 dS m⁻¹ [28]. According to the aforementioned evidence, salinity may not be considered a parameter of influence in the results obtained in the present study.

4.2. Functional Responses

The dynamics of the functional parameters of melon that were assessed in the present study correspond to similar behaviors between control and cascade plants. The PRI is correlated with the photosynthetic light use efficiency; therefore, it provides crucial indications of the photosynthetic activity and the variability of rates in response to diurnal and seasonal environmental fluctuations or possible plant stresses [29,30]. Although the PRI showed strong fluctuations throughout the experimental period, possibly attributed to the leaf age and environmental reasons, the profile exhibited by both treatments was the same. This study is the first to evaluate the response of the PRI in cascade systems. However, Karatsivou et al. [9]—measuring gas exchange—confirmed the indifferent photosynthetic response of lettuce, spinach, and parsley that were irrigated using tomato drainage.

The contents of photosynthetic pigments and their ratios were similar between the control and cascade plants. Furthermore, their variation during the cultivation period was limited. These results are in line with those reported by Karatsivou et al. [9] on lettuce, spinach, and parsley that were irrigated with different drainage percentages up to 100%. Analogous results for a cascade system with no effect on photosynthetic pigment levels were found by Pucinelli et al. [17] in Picris hieracioides, as were slight but statistically significant increases in chlorophylls and carotenoids in Plantago coronopus. In an experiment where spearmint, dill, parsley, and celery were grown on a 10% tomato drainage solution diluted with water, the chlorophyll content showed a species-specific response, with spearmint showing an increase of 15% and all the others a significant decrease, reaching 40% in celery [6].

4.3. Fruit Quality

Fruit quality is an important consideration in an agricultural context, as it affects the overall consumer acceptability and thus the marketability of the product. Drivers of acceptability include intrinsic fruit traits such as color, flesh firmness, and, notably, sweetness [31]. Sweetness, typically measured as °Brix, serves as an indicator of both the taste and ripeness of the fruit and therefore strongly influences consumer preferences [32]. The cascade treatment in the present study did not influence the Brix% value of the harvested melon, since similar values to those of the control were recorded in all three harvests during the experimental period. Ben and Kafkafi [33] stated that the stem of melon stores P, which is subsequently mobilized towards the leaves to facilitate the transfer of sugar to the growing fruits. They found a positive linear correlation between the P content and total soluble solids in fruits. In our case, the soluble solids measured in fruits were identical, although according to Naounoulis et al. [10], the cascade melon showed lower P concentration than the control. According to Senesi et al. [34], considerable variations in °Brix are to be expected between successive ripening stages of melons, but in our case, only minimal differences were observed, with only a slight increase of 15% in the last measurement (D90) in both treatments.

The peel and flesh hardness showed only minor changes during the last 20 days of the experiment, when the measurements were taken. The only significant difference recorded in this experiment was the increased flesh hardness of the control fruits at the final assessment on D90. The typical process of fruit ripening includes the depolymerization of cell walls and an increase in water contents, resulting in flesh softening [35]. Fertilizer treatments are also responsible for differences in flesh hardness, as reported by Martuscelli et al. [36], who found that it increased at high P doses. The pulp’s pH also affects the flavor, as it is inversely correlated with organic acids, so it decreases during fruit ripening due to acid hydrolysis, a process that largely contributes to the increase in sweetness and decrease in sourness [37]. In the present experiment, the pH fluctuated within the narrow range of 6.3–6.5, showing similar levels in both treatments, with these values being in line with other reported cultivars that were measured immediately after their harvest [37].

The fruit color is the first visual cue that attracts the consumer and is therefore an important attribute that influences marketability. The combination of the L*, a*, and b* measurements at three time points of melon harvest showed insignificant differences between treatments. However, the cascade fruits showed significantly greener peel on D77, which became more yellow than that of the control on D90. Although the higher b* values on D90 may be an advantage for this yellowish variety, the overall assessment of treatment effects on color components may be considered rather minimal.

5. Conclusions

Comparable functional and biochemical plant characteristics, such as photosynthetic light use efficiency and photosynthetic pigment content, in both control and cascade melon plants resulted in similar growth and morphoanatomical responses. The reuse of a drainage solution proved effective in achieving almost identical biomass accumulation in melon plants and fruits, as well as fruit quality attributes, as indicated by color, sweetness, and hardness descriptors, which ultimately influence the marketability of the final product. In light of these results, together with the reported advantages in terms of water and nutrient use efficiency and the improved environmental impact, we propose the cascade system as an effective and sustainable cropping system for areas experiencing climatic and water scarcity pressures, such as those around the Mediterranean basin. Further studies are needed to evaluate the cascade system for other greenhouse crops that are more nutrient-demanding and/or have higher sensitivity to climate-imposed stresses.