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Shade netting is being increasingly employed to mitigate the impact of excessive solar radiation on crops. However, the influence of this technique on tomato (Solanum lycopersicum L.) remains relatively unexplored. This research assessed the effect of two types of shade nets on the yield and antioxidant content of tomato under two different fertilization regimes. A randomized complete block design with a split-plot arrangement was used. The main plot factor was shade netting (black nets and white nets), while the sub-plot factor was fertilization management (conventional and organic). Significant differences were observed in yield between shade and fertilization management practices. Total yield was significantly higher (86.76 t ha-1) under white nets compared to that obtained under black nets. Interestingly, the response of the antioxidants evaluated varied. Conventionally fertilized plants yielded substantially higher than those organically fertilized, reaching 78.96 and 67.81 t ha-1, respectively. A markedly higher concentration of lycopene (P < 0.05), with 6.19 mg 100 g-1 fresh weight, was recorded under white netting. Conversely, the concentration of polyphenols exhibited nonsignificant differences (P > 0.05) between shade netting and full sun conditions. In terms of fertilization, the greatest lycopene concentration (5.65 mg 100 g-1 fresh weight) was observed in tomatoes under conventional fertilization. On the other hand, the highest concentrations of polyphenols (21.38 mg gallic acid equivalent 100 g-1 fresh weight) and vitamin C (24.30 mg 100 g-1 fresh weight) were associated with organic fertilization (P < 0.05). In conclusion, this study underscores the substantial positive impact of both white netting and conventional fertilization as effective strategies to augment total crop yield and lycopene concentration, thereby enhancing overall tomato fruit quality.
ABSTRACT
Shade netting is being increasingly employed to mitigate the impact of excessive solar radiation on crops. However, the influence of this technique on tomato (Solanum lycopersicum L.) remains relatively unexplored. This research assessed the effect of two types of shade nets on the yield and antioxidant content of tomato under two different fertilization regimes. A randomized complete block design with a split-plot arrangement was used. The main plot factor was shade netting (black nets and white nets), while the sub-plot factor was fertilization management (conventional and organic). Significant differences were observed in yield between shade and fertilization management practices. Total yield was significantly higher (86.76 t ha-1) under white nets compared to that obtained under black nets. Interestingly, the response of the antioxidants evaluated varied. Conventionally fertilized plants yielded substantially higher than those organically fertilized, reaching 78.96 and 67.81 t ha-1, respectively. A markedly higher concentration of lycopene (P < 0.05), with 6.19 mg 100 g-1 fresh weight, was recorded under white netting. Conversely, the concentration of polyphenols exhibited nonsignificant differences (P > 0.05) between shade netting and full sun conditions. In terms of fertilization, the greatest lycopene concentration (5.65 mg 100 g-1 fresh weight) was observed in tomatoes under conventional fertilization. On the other hand, the highest concentrations of polyphenols (21.38 mg gallic acid equivalent 100 g-1 fresh weight) and vitamin C (24.30 mg 100 g-1 fresh weight) were associated with organic fertilization (P < 0.05). In conclusion, this study underscores the substantial positive impact of both white netting and conventional fertilization as effective strategies to augment total crop yield and lycopene concentration, thereby enhancing overall tomato fruit quality.
Key words: Lycopene, polyphenols, shade nets, Solanum lycopersicum, type of fertilization, vitamin C.
INTRODUCTION
Tomato (Solanum lycopersicum L.) represents a significant reservoir of antioxidants such as lycopene, polyphenols, and vitamin C. At present, consumers have an increasing concern about the quality of food and health issues, seeking nutraceuticals that can enhance health and prevent age-related ailments (Martí et al., 2018). As a result, the food industry is actively exploring ecotypes with elevated content of beneficial compounds or adopting specialized crop management practices to bolster the nutraceutical profile. Within this context, factors like water stress (Sala et al., 2017), salinity (Fischer et al., 2017), solar radiation (Fuentealba-Sandoval et al., 2021), and nutrition have demonstrated significant influence on the nutraceutical content of crops such as quinoa, apples, and wheat. These abiotic influences are intrinsically intertwined with climate change, prominently marked by escalated radiation and temperature, thus imposing stress conditions for plants.
Studies have indicated that the concentration of essential antioxidants, such as vitamin C, lycopene, and polyphenols, are influenced by the microclimate around plants (Setyorini et al., 2018). When evaluating netting on tomatoes, Ilić and Fallik (2017) employed four different colored shade nets, founding that red and pearl nets not only augmented total yield and diminished the incidence of cracking and sunscald in tomatoes but also triggered a 64.9 µg g-1 increase in lycopene content, albeit with a concomitant reduction in β-carotene (48.1 µg g-1). Umanzor et al. (2017) found that red nets promoted the red color in apple fruits, suggesting that red light is key for the accumulation of pigments such as lycopene, which has also been observed in tomatoes. Furthermore, Montiel- Peralta et al. (2020) and De la Cruz-Ricardez et al. (2024) indicated that microclimatic conditions under shade nets can favor desirable characteristics, such as elevated lycopene levels, highlighting the importance of net type and environmental conditions to improve the quality and nutritional value of crops.
In the agricultural market, a diverse array of agricultural nets with distinct structural, radiometric, and physical attributes is available. Raschel looms are responsible for producing nets characterized by longitudinal 'chains' and transversal knitted threads. Furthermore, the importance of raschel nets in crop protection against adverse weather phenomena, such as strong winds and hailstorms, has been highlighted. These nets, characterized by their interconnected structure, act as a support system that prevents tearing and losses of structural integrity. Bosco et al. (2017) on apple orchards suggests that raschel nets act as physical barriers and create a more favorable environment for plant growth. The authors indicated that covering crops with hail nets can alter the microclimate, which positively influences the performance of fruit apple trees in subtropical climates.
The relationship between the use of shade nets and optimized fertilization management in tomato crops has been a subject of recent studies. For instance, Wu et al. (2022) found that the combination of organic and inorganic fertilization improves tomato yield and alters the physicochemical properties of the soil, resulting in a significant increase in fruit quality. Frías-Moreno et al. (2021) have mentioned that the quality of crops, including tomatoes, benefits from organic fertilization, which also results in greater antioxidant capacity and accumulation of bioactive compounds in the fruits. However, in the past decade, fertilizers have been linked to environmental concerns due to N leaching, encompassing issues such as eutrophication, soil acidification, and the release of nitrous oxide into the atmosphere.
It has been documented that fertilization practices, especially those regarding the form of available N, have a significant impact on the antioxidant composition and other qualitative attributes of tomatoes. Trandel et al. (2018) found that the N isotopic composition and the amount of N applied directly affect tomato fruit yield and quality. This is consistent with previous research showing that the use of inorganic N can enhance antioxidant production in tomatoes, contributing to their nutritional quality. On the other hand, a recent study conducted by Wang (2024) confirms that the proportions of ammonium and nitrate N impact tomato growth and quality. The author suggests that a proper balance between these N forms not only improves yield but also enhances the accumulation of bioactive compounds in the fruits. Additionally, González-Coria et al. (2022) highlight that organic fertilization can increase the content of phenolic compounds and antioxidants in tomatoes. This suggests that reduced N availability through organic fertilization practices may induce nutritional stress that stimulates antioxidant biosynthesis.
Variations in C/N ratios within plants, influenced by types of fertilizers, remain an active area of research. It has been described that variations in C/N ratios affect different plant organs and their adaptation to N-limited environments, which suggests that plants adjust N allocation to more active organs, with an impact on the synthesis of secondary metabolites, starch, and cellulose, by optimizing their resource use based on nutrient availability (Zhang et al., 2020). Furthermore, a study conducted by Li et al. (2022) revealed that adding organic materials with a high C/N ratio can improve soil health and nutrient availability, which affects the production of secondary metabolites in plants.
Oney-Montalvo et al. (2020) found a negative correlation between high N levels and the biosynthesis of polyphenols in Capsicum chinense, while Feduraev et al. (2020) highlighted the shared phenylalanine precursor for protein and polyphenol synthesis, in wheat.
Despite the efficacy of shade nets in mitigating stress induced by elevated radiation and temperature, limited research has explored their effects on tomato antioxidant content. Additionally, the accumulation of bioactive compounds within fruits is susceptible to fluctuations due to different fertilization strategies (Rasha et al., 2020). Consequently, the objective of this study was to assess the impacts of different types of shade nets and fertilization management on the yield, antioxidant content, and chemical quality indicators of tomato.
MATERIALS AND METHODS
The experiment was conducted during the 2015-2016 and 2016-2017 growing seasons at El Nogal Experimental Station (36°35'56" S, 72°04'51" W), located in the Universidad de Concepción, Chillán, Ñuble Region, Chile. 'Mykonos' tomato (Solanum lycopersicum L.) plants, characterized by determinate growth, were utilized for the study. A randomized complete block design with a split-plot arrangement was employed. The primary plot factor involved crop netting using either black or white shade nets, while the secondary plot factor pertained to the type of fertilization, either organic or conventional. Transplantation of tomato plants spacing of 1 m × 0.6 m between plants. Fertilization was carried out according to soil chemical analysis. Accordingly, the following nutrient doses were applied per hectare: 275 kg N, 45 kg P2O5, 300 kg K2O, and 72 kg CaO. In the case of conventional fertilization, these nutrients were supplied via the application of 527 kg urea (46% N), 202 kg calcium nitrate (16% N and 26% CaO), 98 kg triple superphosphate (46% P2O5, 20% CaO), and 500 kg potassium chloride (60% K2O) per hectare. On the other hand, organic fertilization consisted of 2363 kg lupine (Lupinus albus L.) meal (7.9% N), 500 kg granulated N sourced from dried blood (14.5% N), 237 kg phosphate rock (19% P2O5, 30% CaO), 250 kg NK mineral fertilizer (7% N, 21% K2O), and 476 kg potassium sulphate (52% K2O). Upon completion of pollination (fruit set), the experimental plots were enveloped with shade nets, either white or black in color. High-density polyethylene nets (Raschel nets, Marienberg Industries, Santiago, Chile) characterized by 35% shading were installed at a height of 1.9 m above the ground (Figure 1). Non-covered plants served as the control group.
Environmental conditions
The temperature (°C) and relative air humidity (% RH) were recorded at 15 min intervals with a Hobo data logger device (HOBO Pro HR/Temp, Onset Computer Corporation, Bourne, Massachusetts, USA). The variation of the total photosynthetically active radiation (PAR) intensity (total PAR) and diffuse photosynthetically active radiation (diffuse PAR), expressed in active photosynthetic photon flux density (PPFD, µmol m-2 s-1) were determined on sunny days without cloudiness, with the quantum sensor LI-191SA connected to a data logger LI-1400 (LI-COR, Lincoln, Nebraska, USA).
Light conditions
Sunlight spectral transmission through the netting materials was evaluated following the methodology proposed by Olivares-Soto et al. (2020). For this evaluation, a 1 × 1 m sample of the material was positioned at a height of 1.5 m above the ground, and spectral light transmission was measured under full sunlight at solar noon (12:30-13:30 h), across a wavelength range of 340-1600 nm. Measurements were conducted using UVVIS-IR spectrophotometers connected to a CR2 cosine receptor (StellarNet, Tampa, Florida, USA). Variations in light conditions were quantified in terms of total photosynthetically active radiation (PARtotal) and diffuse photosynthetically active radiation (PARdiffuse) using a LI-191SA quantum sensor connected to a LI-1400 datalogger (LI-COR). Total photosynthetic photon flux density (PPFDtotal, µmol m-2 s-1) was measured at 09:00, 11:00, 13:00, 15:00, and 17:00 h on a clear sunny day. The diffuse proportion (PPFDdiffuse, µmol m-2 s-1) was estimated using the method adapted from Umanzor et al. (2017), by placing an opaque disk 50 cm from the PAR sensor and covering it completely with the shade of the disk.
Leaf area index measurement
Leaf area index (LAI) was determined under field conditions using an AccuPAR LP-80 LAI ceptometer (Decagon Devices, Pullman, Washington, USA) equipped with 80 quantum sensors. Three measurements were performed for each plant, both above and below the canopy, on a completely clear day.
Yield and productive parameters determination
Tomato fruits, at the red-ripe stage, were randomly harvested from the central rows and the midsection of each plant. Fruits were quantified in terms of number, and their fresh weight was measured using a digital scale (Bel, L1002, Bel Engineering, Monza, Italy). Equatorial and polar diameters were gauged using a digital caliper (Mitutoyo Corporation, Kanagawa, Japan). Subsequently, total fresh yield per hectare was calculated (t ha-1), and marketable yield, which estimates total fresh yield minus losses due to sunburn, blossom and cracking, was also estimated.
Analysis of chemical quality indicators
A composite sample of undiluted fresh juice (0.1 mL) was subjected to triplicate analysis to determine soluble solids (SS) content, quantified in °Brix, employing a portable digital refractometer (Atago Co. Ltd., Tokyo, Japan). Titratable acidity (TA) was determined in triplicate (AOAC, 2010) by homogenizing 10 g sample with 50 mL distilled water. From this solution, a 10 mL aliquot was extracted and titrated with 0.1 N sodium hydroxide along with 0.25 mL phenolphthalein (1%) as an indicator, until achieving a pH 8.2. A professional PH90 pH meter (WTW, Darmstadt, Germany) was employed for this procedure.
Analysis of antioxidants
Lycopene content was assessed following the procedure outlined by Vinha et al. (2014), with modifications. A 0.5 g fresh weight (FW) sample was homogenized with a T18 digital ultra turrax (IKA, Wilmington, North Carolina, USA) in a 2.5 mL hexane-acetone solution (1:1). The resultant mixture was centrifuged at 3500 rpm for 15 min. The supernatant was sequentially transferred to a separatory funnel containing 2.5 mL petroleum ether, with intermittent addition of distilled water. After phase separation, the aqueous phase was discarded. The petroleum ether phase, containing pigments, was concentrated at 35 °C for 1 h using a Branson 5800 ultrasonic bath (Emerson Electronic Co., Wallingford, Connecticut, USA). Lycopene content, in mg 100 g-1 FW, was determined using a 2.5 mL aliquot of the petroleum ether extract. Absorbance was recorded at 470 nm with an Orion AquaMate 8000 UV-VIS spectrophotometer (Thermo Fisher Scientific, Waltham, Massachusetts, USA).
Total polyphenol content was evaluated according to Riahi et al. (2009), with adjustments. Prior to extraction, a calibration curve was constructed using a 5 g L-1 gallic acid (GA) solution. A 0.5 g sample was treated with a 5 mL water, methanol, and formic acid solution (24:25:1 ratio). After ultrasonic treatment and a 24 h rest period, samples were centrifuged at 3500 rpm for 15 min, and the supernatant was collected. A vial containing 120 µL Folin Ciocalteau reagent (1 N) received 25 µL sample and 1.62 mL water. Subsequently, 340 µL 20% sodium carbonate solution was added, followed by a 2 h incubation in darkness at room temperature. Absorbance was measured at 725 nm using an Orion AquaMate 8000 UV-VIS spectrophotometer (Thermo Fisher Scientific). The results were expressed as mg gallic acid equivalent (GAE) per 100 g FW.
Vitamin C content was analyzed in triplicate, following the method of Ciancaglini et al. (2001), with modifications. A 10 g FW sample was mixed with 5 mL distilled water and filtered through a funnel and laboratory mesh. A 10 mL aliquot from the obtained volume was titrated with 0.005 mol L-1 iodine solution, using 5 mL 0.5% starch solution as an indicator. The end point was determined by the first formation of a dark blue permanent trace due to the starch-I complex. The amount of ascorbic acid was calculated based on the volume of solution used for titration (mL), moles of I from the reaction, and moles of ascorbic acid from the reaction, utilizing the titration equation: Ascorbic acid + I2 → 2 I- + Dehydroascorbic acid. The results were expressed as mg 100 g-1 FW.
Statistical analysis
The mean comparison was conducted utilizing the least significant difference test (LSD) (p < 0.05, p < 0.01). To compare the total and diffuse radiation levels beneath the nets, an ANOVA was used, employing the PROC ANOVA procedure. For all crop-related variables, the ANOVA was executed via the PROC MIXED procedure (SAS version 9.1, SAS Institute, Cary, North Carolina, USA).
RESULTS
Environmental conditions
During both seasons, light intensity was significantly higher (p < 0.01) in the control treatment (non-covered plants) compared to that in the white and black net treatments. Figure 1 illustrates the daily fluctuations in total PAR and diffuse PAR. Both black and white nets collectively reduced total PAR by an average of 23% and 33% in the 2015-2016 season, while this reduction extended to 24% and 34% in the 2016-2017 season, respectively. Notably, the white net consistently exhibited a greater quantity of diffuse light compared to the black net (p < 0.01), with increases of 142% and 138% for the first and second seasons, respectively. Furthermore, the white net registered significantly higher levels of diffuse light in contrast to the control (p < 0.01), being 51% and 52% higher in the respective seasons.
Minimum, maximum, and average temperatures recorded in the shade net and control treatments for the two seasons are depicted in Figure 2. During the 2015-2016 season, the use of the white net reduced the average temperature by 0.8 °C and the maximum temperature by 1 °C, while it increased the minimum temperature by 1.7 °C compared to the control. Conversely, the black net yielded an average temperature reduction of 1.7 °C, a maximum temperature reduction of 1.9 °C, and a minimum temperature increase of 0.9 °C in comparison to the control. In the 2016-2017 season, the white net led to an average temperature reduction of 1 °C, a maximum temperature reduction of 1.1 °C, and a minimum temperature increase of 1.6 °C with respect to the control. Likewise, the black net contributed to an average temperature decrease of 1.8 °C, a maximum temperature decrease of 2 °C, and a minimum temperature increase of 0.9 °C compared to the control.
Minimum, maximum, and average relative humidity (RH) data recorded in the shade net and control treatments for the two seasons are presented in Figure 3. In the 2015-2016 season, and with respect to the control, the white net exhibited increases of 1.5%, 1.7%, and 1.6% in average, maximum and minimum RH, whereas the black net recorded increases of 2.9%, 3.2%, and 3.2%, respectively. In the 2016-2017 season, the white net led to increases of 1.5%, 1.8%, 1.9% for the same parameters, while the black net resulted in respective increases of 3%, 3.4%, and 3.6% compared to the control.
Light conditions
The black shade net reduced UV-A light transmission (350-400 nm) by 56%, while the white net reduced it by 30%. The white net allows approximately 98% of radiation in the visible light spectrum (PAR, 400-700 nm), whereas this value is 70% under the black net (Figure 4). Differences were also observed in near-infrared (NIR) transmission; the black net reduced NIR transmission by 40%, while the white net did not alter transmission in this spectrum. All nets had no effect on the red/far-red ratio associated with phytochrome activity (Figure 4).
Regarding photosynthetically active light transmission, the use of the white net significantly reduced total photosynthetic photon flux density (PPFDtotal) by 24% compared to that in the non-covered condition (control), whereas the black net resulted in a 35% reduction (Figure 2). The white net increased the proportion of diffuse PPFD (PPFDdiffuse) by 51%, while this value was 38% lower under black net when compared to the control (Figure 2).
Leaf area index (LAI)
Nonsignificant interaction between type of shade net and fertilization was observed (p > 0.05). Leaf area index (LAI) exhibited significant responses to both factors during the study period (p < 0.05) (Figure 5). In the 2015-2016 season, plants grown under black nets surpassed significantly (p < 0.05) their counterparts under white nets in terms of LAI. Plants cultivated under full sun conditions recorded lower LAI values of 1.23 and 1.22 m2 compared to plants grown under protective covers in the 2015-2016 and 2016-2017 seasons, respectively. Regarding fertilization treatments, plants subject to conventional management displayed a higher LAI than those under organic management. For conventional fertilization, LAI values were 13.4% and 8.8% higher compared to those of organic fertilization in the first and second seasons, respectively.
Yield determination and productive parameters
There was no interaction between the type of shade net and fertilization management. Nevertheless, significant variations were observed in yield and marketable yield in response to both factors (p < 0.05). Plants cultivated under white net yielded the highest total harvest of 83.9 and 89.6 t ha-1 during the 2015-2016 and 2016-2017 seasons, respectively. However, nonsignificant differences were noted in yield between plants grown under black net cover (64.6 and 70.4 t ha-1 and those exposed to full sun conditions (62.8 and 69.0 t ha-1) during both seasons (Figures 6A, 6B).
Tomatoes grown under white shade netting displayed significantly greater fresh weight per fruit, reaching 220.0 and 233.8 g in 2015-2016 and 2016-2017 seasons, respectively. Conversely, nonsignificant differences were observed in the fresh weight of tomatoes from plants grown under black nets or in full sun, with values of 179.4 and 173.8 g for the first and second seasons, respectively (Figures 7A, 7B). Conventionally fertilized plants yielded a significantly higher total harvest of 75.5 and 82.4 t ha-1, during the first and subsequent season, while organically fertilized plants produced a lower yield of 65.4 and 70.2 t ha-1, respectively (Figures 6A, 6B). The number of fruits exhibited nonsignificant response to netting or fertilization management in both seasons (Figures 7C, 7D). Therefore, the higher yield registered under white shade netting and conventional fertilization is primarily attributed to the greater fresh weight of the fruit associated with these treatments rather than the number of fruits.
Proportionally, plants grown without any protective cover (control), significant fruit losses (p ≤ 0.05) were observed due to sunburn damage in both seasons. Similarly, it was noted that organic fertilized crops experienced higher losses from sunburn compared to those conventionally fertilized, with losses amounting to 20.16 and 14.40 t ha-1 in the 2015-2016 season and 17.2 and 11.8 t ha-1 in the 2016-2017 season, respectively (Figures 8A, 8B). Losses caused by blossom (Figures 8C, 8D) and cracking (Figures 8E, 8F) were lower than those resulting from sunburn. Similarly, when adjusted for total yield, the proportion of all losses (expressed as a percentage) were lower in the conventionally fertilized plants, reaching 9.1% and 7.0% vs. 15.6% and 12.0% in fertilization and season, respectively.
Regarding blossom-end rot damage, nonsignificant differences were noted between black and white shade nets in both seasons. However, significant losses of 1.8 t ha-1 were observed in the control treatment during the 2015-2016 season, but no differences were detected in the second season. For fertilization management, the same trend was observed during the study period, with organically fertilized plants recording greater losses. Conversely, losses observed due to fruit cracking did not show significant differences for the evaluated factors in both seasons (Figure 8).
Analysis of chemical quality indicators
Chemical quality had nonsignificant interaction between the examined factors (p > 0.05). The soluble solids and titratable acidity levels were 4.96 and 4.98 °Brix and 0.49 and 0.52 for the 2015-2016 and 2016-2017 seasons, respectively, being significantly higher (p ≤ 0.05) in tomatoes plants grown under white shade netting. Regarding the effect of fertilization, both soluble solids and titratable acidity values were significantly higher in conventionally fertilized plants in the first season (4.85 °Brix and 0.51%, respectively). However, in the second season, nonsignificant differences (p > 0.05) were observed between organically and conventionally fertilized plants for the evaluated factors (Figure 9).
In the context of the present study, evaluating the environmental conditions under the white net yielded similar results. The alterations induced by this type of net in the assessed parameters probably contributed to improved photosynthetic efficiency and, consequently, enhanced overall fruit quality (weight, soluble solids, and titratable acidity). For most C3 species, maximum net CO2 uptake is achieved at relatively low irradiance (600-900 µmol m-2 s-1), representing approximately 30%-40% of total sunlight (1500-2000 µmol m-2 s-1) (Ilić and Fallik, 2017).
Analysis of antioxidants
In terms of contents of vitamin C, carotenoids and total polyphenols, nonsignificant interaction was observed between type of net and fertilization management (p > 0.05). The plants covered with black shade net had a significantly lower performance regarding vitamin C and carotenoid contents, while all treatments behaved similarly in terms of polyphenol content. Higher concentrations of carotenoids were observed under white shade netting, with values of 6.33 and 6.06 mg 100 g-1 FW in the 2015-2016 and 2016-2017 seasons, respectively (Figures 10A, 10B). In relation to fertilization, a significant difference was observed (p ≤ 0.05) in the contents of vitamin C, carotenoids and total polyphenols during the first season. Organically fertilized tomatoes recorded higher concentrations of vitamin C, with values of 24.95 and 23.65 mg 100 g-1 FW for the respective seasons (Figures 10A, 10B). In addition, conventionally fertilized tomatoes reached higher levels of carotenoids compared to organically fertilized tomatoes, with values of 5.86 and 5.44 mg 100 g-1 FW in the 2015-2016 and values of 4.04 and 4.84 mg 100 g-1 FW in the 2016-2017 season, respectively (Figures 10C, 10D).
The concentration of polyphenols responded significantly only to the type of fertilization (p < 0.05) (Figures 10E, 10F), with higher values of 19.18 and 23.57 mg GAE 100 g-1 FW in tomatoes from organically fertilized plants in the first and second seasons, respectively.
DISCUSSION
The influence of shade net color on light diffusion has been the subject of various studies. According to Oliveira et al. (2016), light-colored shade nets (e.g., white nets) favor a higher proportion of diffuse light compared to black nets, which primarily transmit direct light through their openings. This suggests that light-colored netting may increase light diffusion, resulting in a more favorable environment for plant growth. Additionally, the findings of Ramphinwa et al. (2022) confirm that black nets significantly reduce both the light intensity and the spectral composition of light reaching the plants, which can negatively affect their growth and development. The authors found that as the percentage of shade provided by black nets increases, the light reaching the plants is considerably reduced, potentially limiting photosynthesis and, therefore, crop yield. On the other hand, the same study also revealed that plants grown under white shade nets showed greater growth compared to those under black nets, suggesting that light quality is a determining factor in plant development and reinforcing the idea that lighter-colored shade nets not only allow greater light transmission but also improve the quality of the growing environment.
The influence of shade nets on temperature reduction and increased relative humidity has been confirmed by recent research. Momeni (2022) found that the use of shade nets in orchards results in a significant decrease in temperature and an increase in RH, corroborating previous findings that 30% shading factor can create a favorable microclimate for plant growth by increasing humidity, which is crucial under water stress conditions. This agrees with Morales et al. (2018), who reported that, in the absence of nets, the average temperature was 2 °C higher and RH was 4% lower. The characteristics of the covering material influence the effectiveness of the nets in modifying the microclimate.
Peavey et al. (2022) reported that radiation transmission through a net can exhibit variability attributed to factors like pigment stability deterioration under high UV radiation, alterations in thread tension due to net opening and closing, temperature fluctuations, and accumulation of dust, affecting shading properties. However, the outcomes of this study indicate comparable light transmission capacities across seasons and net types (Figure 2). These findings might be attributed to inherent manufacturing characteristics of the nets. Raschel nets used in agriculture, especially in tomato cultivation, are made from high-density polyethylene raffia that includes UV radiation stabilizers, being a determining factor in the degradation of plastics. Prolonged exposure to UV radiation can cause significant damage to materials, which in turn affects the durability and effectiveness of the nets in their protective function. The UV radiation stabilizers are essential to mitigate these adverse effects. The incorporation of protective additives in polyethylene films can significantly reduce degradation rates and thus allow maintaining their mechanical and functional properties for longer periods (Weizman et al., 2022). Additionally, UV also impacts plant growth and development. Exposure to UV radiation can induce oxidative stress in plants, affecting their growth and productivity. However, UV radiation has also been documented to stimulate the production of secondary metabolites in plants, which could be beneficial for their resistance to pests and diseases (Vanhaelewyn et al., 2020). Additionally, the nets were installed in a stationary position for a relatively short period of time (60 d), without exposure to adverse climatic conditions like strong winds or hail. The presence of an antistatic additive within the net could further contribute to these outcomes by preventing dust accumulation. In both seasons, the white net consistently displayed significantly higher levels of diffuse light (p < 0.01) with respect to both the black net and the control (Figure 2). The impact of shading nets on light diffusion and microclimatic conditions has been the subject of study in recent years. In terms of light transmission, the color of shade netting plays a crucial role as described by Ayala-Tafoya et al. (2018), who reported that colored shade nets can modify the spectral quality of light, which influences photosynthesis and plant growth. For example, protective covers that allow more diffuse light, such as lighter-colored nets, can enhance photosynthetic efficiency and biomass production. In contrast, black nets mainly transmit light through their openings, leading to lower overall light levels beneath them, as they absorb a significant portion of incoming solar radiation. Additionally, shade nets can reduce temperatures in the root zone, creating a more favorable environment for plant development.
The observed elevation in leaf area index (LAI) among conventionally fertilized plants (Figure 5) is consistent with the findings of Ronga et al. (2015), who reported a substantially higher LAI in conventionally fertilized plants. The gradual release of nutrients, which are characteristics of organic fertilizers used in this study, might have led to a limited nutrient availability at the root level, potentially hindering leaf development. Conversely, the utilization of nets led to an enhancement in LAI values compared to the control (Figure 5). This aligns with observations made by Ilić et al. (2015), who noted elevated LAI values in tomato plants covered by nets with a shading factor of 40%.
The higher LAI (Figure 5) observed in conventionally fertilized plants coincides with that reported by Ronga et al. (2015). The gradual release of nutrients, which is characteristic of organic fertilizers used in this study, might have led to a limited nutrient availability at the root level, potentially hindering leaf development. In the present study, the nets increased the LAI with respect to control (Figure 5). This agrees with Ilić et al. (2015), who observed a higher value in tomato plants covered by nets with a shading factor of 40%. It has been described that, under low light conditions, plants undergo morphological changes to maximize the use of light, and thus plants grown under shade tend to have a larger leaf area because their cells expand more in order to receive the necessary light for the photosynthetic process.
The higher yield observed in tomatoes grown under white net may be attributed to the increased availability of photosynthetically active light observed under this protective cover (Figure 2). While the use of black net reduced the proportion of diffuse light, white net increased it by approximately 50%, which suggests that the light microclimate under this type of net is much more favorable for photosynthesis by improving light distribution within the plant canopy and leaf gas exchange, as recently reported for greenhouse tomatoes with different levels of diffuse light transmission (Zheng et al., 2020). Another benefit of increased diffuse light is its effect on reducing leaf and fruit temperatures. The use of nets with a higher capacity to increase diffuse light transmission is more effective in lowering fruit temperatures (Umanzor et al., 2017), which would be advantageous in reducing the incidence of quality issues, such as sunscald, sunburn, and other problems associated with excessive radiation and temperatures, thereby positively impacting the commercial yield of tomatoes under white netting.
Regarding the analysis of chemical quality indicators, the elevated levels of soluble solids and titratable acidity (Figure 9) align with the findings of Youssef and Eissa (2017), who reported similar outcomes in conventionally fertilized tomatoes. In the present study, restricted N availability in organically fertilized plants were likely to contribute to the development of tomatoes with reduced weight and size. This limitation in N availability may have also contributed to the reduced soluble solids content and lower titratable acidity in these tomatoes. Such effects can be attributed to the constrained photosynthetic process and subsequent carbohydrate synthesis. Notably, N serves as a fundamental element in chlorophyll composition and C cycle enzymes, playing a pivotal role in photosynthesis. Enhanced N availability supports higher leaf N content and promotes an increased photosynthetic rate (Li et al., 2013).
According to the antioxidant analysis, our results agree with those of Hallmann (2012), who found a higher content of these antioxidants in fruits of plants fertilized with organic sources. Likewise, the highest concentration of vitamin C in tomatoes from organically fertilized coincides with the results obtained by Oliveira et al. (2016), who observed a higher content in fruits grown with organic fertilizers. The controlled release of N from the fertilizers used in the present study, possibly affected the synthesis of both antioxidants. Relative differences in the release of nutrients from various fertilizers could result in different C/N ratios in the plant, and in turn lead to a difference in the synthesis of secondary metabolites. According to the theory of equilibrium C/N, if N is readily available, plants would mainly produce compounds with a high content of N (proteins for growth), whereas if the availability of N were limited for growth, the metabolism of the plant would change towards the elaboration of compounds containing C, such as polyphenols and vitamin C. On the other hand, the same precursor, phenylalanine, is shared in the synthesis of proteins and polyphenols (Feduraev et al., 2020). Therefore, any favorable condition for protein biosynthesis, such as increased N availability, would result in a lower biosynthesis of polyphenols. The characteristics of the fertilizers used in the present study could have affected the C/N ratio by limiting the availability of N for the plant. However, photosynthesis may have assigned additional C to the synthesis of compounds for defense, in this case polyphenols and vitamin C. The greater proportion of diffuse light registered under white netting (Figure 2) could have allowed the arrival of light to fruits that generally grow under shade; consequently, the production of polyphenols and vitamin C could have been stimulated as a photoprotection mechanism. Diffuse light penetrates deeper into the canopy, scattering in many directions, allowing a more efficient use by plants than direct light as it creates a more homogeneous light profile in the canopy.
Lycopene accounts for approximately 80%-90% of the total carotenoids present in ripe tomatoes and their concentration of lycopene depends on multiple factors, including N. According to Vardanian et al. (2025), N is the main element that forms acetyl-CoA, the enzyme who plays a fundamental role in the synthesis of carotenoids and causes the conversion of β-carotene to lycopene. However, the reduction in radiation and temperature (Figures 1 and 2) due to the implementation of white netting could have helped enhance antioxidant synthesis. Furthermore, the higher lycopene content found in tomatoes grown under white net may also be attributed to differences in light conditions between the nets. Previous studies have demonstrated that increased solar light supplementation, particularly in the UV and red-light ranges, significantly increased lycopene content in tomatoes (Liu et al., 2009). The present study found that white shade nets increased light transmission in the UV and red spectra, as well as in the visible light (PAR) spectrum, compared to the black nets (Figure 2). This partially explains its positive effect on the higher lycopene content in tomatoes grown under this type of net, which is further enhanced by the increased diffuse light that benefits plant photosynthesis (Zheng et al., 2020). The greater proportion of diffuse light generated by the white net could have increased the concentration of lycopene by allowing the arrival of moderate radiation to the fruits. In addition, an indirect effect could have exerted the greater leaf development achieved by conventionally fertilized plants grown under white netting (Figure 10).
CONCLUSIONS
This study concludes that the use of white netting and conventional fertilization are effective strategies for enhancing both total crop yield and concentration of lycopene in tomatoes. Separately, each technique improves overall fruit quality, including fresh weight, size, soluble solids, and titratable acidity. The various antioxidants assessed in this study exhibited diverse responses to the different fertilization methods. Conventional fertilization was found to enhance the accumulation of lycopene, while organic fertilization resulted in higher concentrations of polyphenols and vitamin C. Additionally, the color of the shade net was found to create distinct environmental conditions for the crop, thereby influencing the antioxidant content within the tomato fruit.
Author contributions
Conceptualization: A.P., S.F., A.U-H. Methodology: S.F., R.B., R.W. Software: A.P. Validation: S.F. Formal analysis: S.F., A.P., A.U-H. Investigation: S.F., R.B., A.P. Resources: R.W. Data curation: R.W. Writing-original draft: S.F., A.P. Writing-review & editing: R.W. Visualization: S.F Supervision: A.P. Project administration: S.F. Funding acquisition: S.F. All co-authors reviewed the final version and approved the manuscript before submission.
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