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INTRODUCTION
Light is a form of electromagnetic energy characterized by wave-particle duality and various physical properties that influence how it interacts with its surroundings. Energy and color are determined by frequency and wavelength while amplitude is related to its intensity. In a vacuum, light travels at a constant speed, and its interaction with matter gives rise to phenomena such as absorption, reflection, and transmission (Mamedov et al., 2015).
Light interacting with matter can manifest itself in many ways. Absorption involves the capture of energy by a material's molecules, reflection occurs when light bounces off a surface, and transmission allows light to pass through a material without being fully absorbed (Pribil et al., 2014). Some materials, when excited by light, emit energy at a different wavelength-a phenomenon known as fluorescence-which is observed in certain plant pigments.In plants, this range of electromagnetic waves spans a spectrum from 300 to 900 nm and includes the ultraviolet, visible, and infrared regions, all of which significantly impact plant growth and development (Fig. 1) (Cardona et al., 2018). Fluorescence takes place in the non-photochemical (radiative dissipation) pathway, where excited chlorophyll emits photons of longer wavelength (680-760 nm) and lower energy (Murchie and Lawson 2013). Most fluorescence that is recorded in a leaf at environment temperature is emitted by chlorophyll a of the PSII (Murchie and Lawson, 2013).
Figure 1 Electromagnetic spectrum and its main effects on plants. Prepared by the authors based on Azcón-Bieto and Talón (2008) and 11.
Light plays multiple roles in the plant environment, acting as the primary energy source through photosynthesis. For this process, plants specifically utilize visible light in the range of 400 to 700 nm, known as photosynthetically active radiation (PAR). In this process, light energy is primarily absorbed by two pigments: chlorophyll and carotenoids (Pribil et al., 2014).
In addition to being a source of energy, light acts as an environmental signal that plants use to regulate their physiological responses throughout their life cycle, influencing morphogenesis. Factors such as light quantity, direction, and daily duration (photoperiod) are crucial. Plants perceive these signals through photoreceptors that allow them to adapt to different environmental conditions and undergo key developmental transitions (Kami et al., 2010).
LIGHT RECEPTION IN PLANTS
When a chlorophyll molecule absorbs a photon, an electron becomes excited and transitions to a higher energy state. This state is unstable and can follow three paths: transferring the energy to another chlorophyll molecule until it reaches the reaction center of the photosystem, returning to its ground state by releasing heat, or emitting a photon of longer wavelength in a process known as fluorescence (Lambers et al., 1998; Stirbet and Govindjee, 2012).
In photosystem II, the absorbed energy initiates a series of chemical reactions, such as the photolysis of water, resulting in the release of oxygen and electrons. Excited electrons are transferred from chlorophyll P680 to the primary acceptor, pheophytin (Phe), and then to other electron transport chain molecules like plastoquinone (PQ) and cytochromes. During this process, oxygen (O₂) and its derivatives, such as superoxide (O₂-), can act as additional oxidants(Marcus and Sutin, 1985). The energy generated in this transport chain drives the synthesis of ATP and NADPH, which are essential for the Calvin cycle. In this cycle, carbon dioxide is converted into glucose and other sugars. This provides the plant with energy and contributes to the formation of organic matter necessary for its growth and development (Pribil et al., 2014;Mamedov et al., 2015).
In this phase of electron transfer and transport, photosystem II (PS II) pigment-protein complex is one of the two primary photosynthetic chain enzymes in thylakoid membranes of oxygenic organisms (Wydrzynski et al., 2005). Murchie and Lawson (2013) describe PS II as a water-plastoquinone oxidoreductase that catalyzes the light-induced stable charge separation, resulting in the formation of an ion-radical pair between the primary electron donor P680 and the first quinone acceptor QA: P680+QA-. The study by Allakhverdiev et al. (2010) showed that prior to P680+QA- formation, an electron is transferred from P* (P680*) to a pheophytin (Phe) molecule and then to QA. The formation of P680+QA- leads to two chemical processes: oxidation of water to molecular oxygen (after four PS II changes) and reduction of plastoquinone to plastohydroquinone (after two PS II changes). These reactions are spatially separated, occurring on different sides of the reaction center in the thylakoid membrane (Murchie and Lawson, 2013).
Carotenoids play a crucial role in protecting the photosynthetic system through energy dissipation and quenching mechanisms, in addition to their secondary function of absorbing energy in the blue and green wavelengths (Pribil et al., 2014; Mamedov et al., 2015; Landi et al., 2020). Additionally, photosynthetic activity and its efficiency increase with higher light intensity, temperature, and CO2 levels (Walters, 2005).
According to Cardona et al. (2018), in the field, light intensity (the amount of light) and spectral profile (the quality of light) can change rapidly and unpredictably throughout the day, for example, due to passing clouds or wind-shaken leaves. Meanwhile, Vass (2012) reports that variations in light intensity, particularly during abiotic stresses that inhibit carbon fixation, can lead to overexcitation, forming excited chlorophyll triplet states that can react with O2 to produce damaging reactive oxygen species (ROS) (Fig. 2). To prevent or mitigate damage, many different protective mechanisms operate on different time scales and at different points in the excitation and electron transfer processes to maximize the efficient use of light and minimize damage (Takagi et al., 2016). These include modifying cofactor energetics in PSII to prevent dangerous reverse reactions (Brinkert et al., 2016) and activating non-photochemical quenching (NPQ) mechanisms to dissipate excess excitation in light-harvesting complexes as heat (Cardona et al., 2018). Therefore, to achieve good crop development and increased productivity, proper and adequate lighting must be ensured. Besides its role in photosynthesis, light also acts as a key environmental signal through specialized photoreceptors in the plant (Fig. 2). These photoreceptors enable the plant to perceive its surroundings, allowing it to direct adaptive responses to change environmental conditions. When activated, photoreceptors trigger specific signaling pathways that regulate various physiological processes (Folta and Carvalho, 2015).
There are three main types of photoreceptors identified in plants. Phytochromes absorb red and far-red (FR) light and play a crucial role in modulating germination. They are primarily involved in the synthesis and signaling of gibberellin (GA), as well as in processes such as de-etiolation and shade avoidance responses (Yang et al., 2014; Galvāo et al., 2019). Cryptochromes are sensitive to blue light and ultraviolet A (320-400 nm), influence hypocotyl growth, promote cotyledon development and opening, and are involved in the initiation of chloroplast development, leaf growth, and the photoperiodic control of floral initiation and circadian rhythms (Tóth et al., 2001; Wang et al., 2022). Phototropins respond to blue/UV-A light and play an important role in differential cell growth. They also optimize photosynthetic capacity by influencing processes such as stomatal opening, leaf flattening, and chloroplast movement to align with the direction of incident light (Folta and Carvalho, 2015).
These photoreceptors are widely distributed in plant tissues. While some physiological responses are triggered by a single photoreceptor, in many cases, multiple light sensors ensure a coordinated response (Kami et al., 2010; Sakamoto and Briggs, 2002). Plants absorb certain ranges of incident radiation and transmit the non-absorbed radiation to neighboring plants. The quality of light received by the plant in terms of intensity and the number of daily hours of exposure are determining factors that influence the plant's overall response to its light environment (Nguy-Robertson et al., 2015).
The time of exposure or the number of hours of light required for a crop to develop and reach flowering varies depending on the species. All plants need light, and below a certain threshold, very few can survive. Both excess light and insufficient light can have harmful effects on plants (Velez-Ramirez et al., 2011; Cardona et al., 2018).
PHOTO-SELECTIVE COVERS
The use of controlled conditions has advantages in mitigating abiotic factors that limit agricultural production, such as adverse weather conditions, including high precipitation, mechanical effects of hail, frost and strong winds (Flórez-Hernández et al., 2023). Ceballos-Aguirre et al. (2024) reported that macro tunnels minimize fertilizer leaching and reduce the volume and frequency of pesticide and fungicide applications. In addition, the system increases ambient temperature by 2-5°C, which accelerates the start of production and provides protection against mechanical damage from rain and frost (Yang et al., 2017 Ceballos-Aguirre et al., 2024). This system offers significant benefits, including increased productivity and fruit quality, as it allows for higher planting density and enables cultivation at any time of the year. Additionally, it enhances pest and disease management by reducing their incidence (Perilla et al., 2011).
The use of shade nets has increasingly become a viable alternative to protect crops and create a suitable microclimate by controlling humidity, shade, and temperature. This coverage has been shown to mitigate extreme climatic fluctuations, reduces stress and damage caused by heat, cold, and wind, and acts as a physical barrier against hail and birds (Shahak et al., 2009; Pandey et al., 2023).
Additionally, the use of covering materials or filters can alter both the quantity and distribution of incident radiation, providing certain advantages (Castilla, 2003). Casierra-Posada and Peña-Olmos (2015) suggest that passive light filters of different colors can offer economical alternatives for farmers by stimulating physiological responses that lead to a higher-quality product. Furthermore, polymeric materials can boost production and enhance product characteristics.
Plastic films
Plastic films alter the relative proportions of the radiation spectrum, acting as selective filters on the wavelengths that reach the crop. At the same time, they allow incoming light to be scattered and converted into diffuse light, which improves the penetration and distribution of the modified light spectrum within the plant canopy (Pandey et al., 2023).The use of plastic films has the potential to selectively modify the spectrum of incident light and trigger physiological responses in plants, resulting in higher yields, improved fruit quality and reduced pest and disease susceptibility (Pandey et al., 2023). As shown in table 1, the effects of plastic films depend on the species and stage of plant development.
Table 1. Physiological effects of photo-selective plastic films on different crops.
| Color | Cover specifications | Crop | Effects on crops | Authors |
|---|---|---|---|---|
| Red (R) 600-700 nm | PAR transmission: 78.2%. Opacity: 48% | Broccoli | During the seedling stage, greater dry weight (better performance in the field) | Casierra-Posada and Rojas, 2009 |
| PAR: 467.67 µmol m-² s-1. Opacity: 48.73% | Beets | Greater leaf area, total soluble solids, root diameter, and total dry weight | Casierra-Posada and Pinto-Correa, 2011 | |
| PAR: 86.95 µmol m-² s-1. Opacity: 71.01% | Strawberry | Greater leaf area and dry matter accumulation | Casierra-Posada et al., 2012b, 2014a | |
| R/FR: 1.51 | Golden shrimp | A 9% increase in height | Wilson and Rajapakse, 2001a | |
| Far red (RF) 700-800 nm | Transmission PAR: 84%. R/FR: 3.8 | Strawberry | Longer crop duration and more compact plants with shorter petiole length | Fletcher et al., 2004 |
| Transmittance: 61%. R/FR: 2.33. YCE Tints: #80, #75, #65 respectively: R/FR: 1.6, 2.3, 3.7. Transmittance: 0.75, 0.78, 0.81 | Cucumber. Tomato. Pepper Chrysanthemum | Height reduction | Murakami et al., 1997; Li et al., 2000 | |
| Transmittance: 61%. R/FR: 2.33 | Cucumber Tomato | Delay in flowering and harvest | Murakami et al., 1997 | |
| Estimated photostationary state (Pfr/P): 0.604 | Chrysanthemum Snapdragon Mouth | Increase in leaf area Reduction in height and internode length | Van Haeringen et al., 2008 | |
| R/FR: 0.65 | Golden shrimp Cat's whiskers. Persian shield | Decrease in leaf area and dry weight | Wilson and Rajapakse, 2001a | |
| R/FR Ratio: 0.65. Estimated photostationary state (Pfr/P): 0.66 | Golden shrimp | 10% reduction in height | Wilson and Rajapakse, 2001a | |
| R/FR Ratio: 0.65. Estimated photostationary state (Pfr/P): 0.66 | Cat's whiskers | 20% reduction in height | Wilson and Rajapakse, 2001a | |
| Unspecified | Sage | Reduction in: height, internode length, and stem dry weight | Wilson and Rajapakse, 2001b | |
| PAR Transmission: 71.2%. R/FR Ratio: 1.21. Phytochrome Photostationary state (φ): 0.73 | Snapdragon Chrysanthemum. Cucurbits. Solanaceae | Reduction in height | Brar et al., 2020; Khattak et al., 2004; Rajapakse et al., 1999; Li et al., 2000 | |
| Blue | Tints 1, 2, and 3%, respectively: PAR Transmission: 40, 33, and 25%. Phytochrome Photostationary state: 0.64, 0.61, 0.57 | Chrysanthemum | Reduction in: height, total dry weight, number of leaves, leaf area of the main stem, axillary shoots (related to increased pigment concentration) | Oyaert et al., 1999 |
| Green | PAR: 78.87 µmol m-² s-1. Opacity: 73.7% | Strawberry | Growth reduction. Increased accumulation of chlorophyll a | Casierra-Posada et al., 2011, 2014a |
| PAR: 78.87 µmol m-² s-1. Opacity: 73.7% | Calla lily | Greater accumulation of dry matter | Casierra-Posada et al., 2012a | |
| Shading: 42.7%. Light diffusion: 42.1% | Basil | Increase in height | Stagnari et al., 2018 | |
| Yellow | Shading: 28.3%. Light diffusion: 42.0% | Basil | Increase in biomass accumulation | Stagnari et al., 2018 |
| PAR: 234 µmol m-² s-1. Opacity: 52.4% | Swiss chard | Increase in relative growth rate (RGR) | Casierra-Posada et al., 2014b | |
| UV blocking | Spectrum > 400nm, PAR Transmission: 92%. PAR Transmission: 80%, Spectrum > 385nm | Tomato | Reduction in trips and whitefly attacks decreases Alternaria solani sporulation | Doukas and Payne, 2007; Kumar and Poehling, 2006; González et al., 2001; Vakalounakis, 1991 |
| Transmission: 250-400 nm: 7%. 400-1100nm: 54% | Peach | Reduction in aphid attacks | Chyzik et al., 2003 | |
| Transmission: PAR: 82%. UV-B: 36%. UV-A: 7% | Raspberry | Reduction in Japanese beetle attacks | Cramer et al., 2019 | |
| Spectrum: 500 - 700 nm. Transmittance: 83.16%. UV-A Radiation: 0.1. UV-B Radiation: 1.4 | Eggplant | Increase in: height, leaf area, 20% increase in production with larger and higher-quality fruits | Kittas et al., 2006 | |
| Spectrum: 400-700 nm | Strawberry | Decrease in flavonoid content, delayed fruit ripening but increased final productivity | Casal et al., 2009 | |
| Spectrum > 400 nm | Tomato Radish | Lower chlorophyll content | Tezuka et al., 1993 |
PAR:photosynthetically active radiation; R: red; FR:far-red; R/FR: ratio red/ far-red.
In horticultural production systems, red plastic covers have been shown positive effects on the ontogenesis of crops such as broccoli, beet, and strawberry, with significant increases in biomass accumulation, which reflects better photosynthetic efficiency (Casierra-Posada and Rojas, 2009; Casierra-Posada and Pinto-Correa, 2011; Casierra-Posada et al., 2012b, 2014a). On the other hand, far-red covers negatively impact the growth of various horticultural and ornamental species, reducing the total height of the plants; in some species, a decrease in flowering and crop yield has even been observed(Murakami et al., 1997; Li et al., 2000; Wilson and Rajapakse, 2001a). The behavior of phytochrome, a key molecule in light perception, may be responsible for this effect. Phytochrome exists in two photoconductive states: the inactive form (PR), absorbing red light, and the active form (PFR), absorbing long wavelength red light. Red-colored coatings promote the PR form of phytochrome, reducing auxin degradation and consequently promoting stem elongation (Schmitt et al., 1999; Francescangeli et al., 2007).
Yellow and green covers enhance dry weight, height, and chlorophyll a accumulation in certain vegetables. However, in the case of crops like chrysanthemums, blue light has not shown favorable effects on various growth and development variables(Casierra-Posada et al., 2014a; Stagnari et al., 2018). Casierra-Posada et al. (2014c) reported that plants grown under the yellow cover showed a higher water use efficiency compared to the other treatments (blue, red, green and control). Yellow-covered plants showed increased foliar dry matter accumulation. The same author reports that the different responses of plants to light quality can be explained by contrasts between different species, cultivars, or even the characteristics of the light sources used. The root to shoot ratio of all the plants grown under a cover (yellow, blue, red, green) showed lower average values than those found for the control plants grown without a canopy. This reduction ranged from 18.7 to 44.7% compared to controls. However, a significant difference was only found between control plants and the yellow, blue and green film treatments (Casierra-Posada et al., 2014c).
Colored plastic covers (yellow, blue, red, green) increase the chlorophyll content index (CCI) between 31.5 and 50.5% compared to uncovered (control) plants (Casierra-Posada et al., 2014c). Plant growth and development could be co-regulated by photoreceptors and other endogenous factors such as hormones and a temperature-sensing system (Facella et al. 2012; Brini et al. 2022). This suggests a complex relationship between different factors regulating plant morphology and physiology and underscores how plant species differ in their interactions with the environment.
Covers that block ultraviolet light are particularly important for the health of some fruit crops, as they help mitigate the damage caused by pests such as thrips, whiteflies, aphids, Japanese beetles, and even fungi like Alternaria solani(González et al., 2001; Kumar and Poehling, 2006; Doukas and Payne, 2007). In horticultural crops, the results regarding production physiology under these covers have been variable (Kittas et al., 2006; Casal et al., 2009). Blocking ultraviolet light is advantageous because it affects insects' vision, movement and ability to locate hosts (Meyer et al., 2021). In addition, some fungi depend on UV light to complete their multiplication and reproduction cycles and are therefore also affected by the exclusion of this radiation (Meyer et al., 2021).
Shade nets
Current nets are made of polypropylene or polyethylene threads with different fiber dimensions and holes, which allow for a mix of natural and spectrally modified light. Additionally, they transform direct light into scattered/diffuse light. The relative content of diffuse light, as well as the shading factor, are defined by the fabric design, density, and chromatic additives (Castellano et al., 2008; Shahak, 2014).
The red net has shown positive effects on the vegetative growth rate of various ornamental species and is also associated with an increase in the height of crops like lettuce and turmeric. In other crops, higher productivity has been observed, reflected in an increase in both the number and quality of fruits(Shahak et al., 2004; Ilić et al., 2012; Harish et al., 2022).
On the other hand, the blue net has led to a decrease in the height of several crops, and dwarfism has been observed in ornamental plants(Oren-Shamir et al., 2001). However, in crops like apples, an increase in photosynthesis and transpiration has been recorded(Bastías et al., 2012). Additionally, in other crops, increases in chlorophyll content and the concentration of phytochemicals and essential oils have been reported, suggesting that blue light could enhance the functional quality of some crops (Oliveira et al., 2016; Ilić et al., 2017).
The pearl net has been especially beneficial in crops like bell pepper, promoting a higher concentration of bioactive compounds and an increase in overall yield, in addition to reducing susceptibility to fungal infections. Positive results have also been reported in increased chlorophyll levels and enhanced antioxidant activity (Shahak et al., 2008; Selahle et al., 2015). As shown in table 2, the effects of photo-selective nets on different crops will vary depending on the species and stage of plant development.
Table 2. Physiological effects of photo-selective nets on different crops.
| Color | Nets specifications | Crop | Effects on crops | Authors |
|---|---|---|---|---|
| Shading: 20%. PAR: 678 µmol m-² s-1. R/FR: 0.90 | Avocado | Accumulation of phenols | Tinyane et al., 2018 | |
| Red | Shading: 25% UV Blocking | Turmeric | Greater height and number of leaves | Harish et al., 2022 |
| Shading: 50%. PAR: 641 µmol m-² s-1 | Marigold and violet | Greater leaf area | Abbasnia et al., 2019 | |
| Spectrum > 580 nm Shading: 30% | Ornamentals | Increase in vegetative growth rate. Shorter flowering time | Oren-Shamir et al., 2001; Shahak et al., 2009 | |
| Shading: 40%. PAR Transmission: 66.2% | Tomato | Increase in the number of fruits per plant, with larger size and higher lycopene content | Ilić et al., 2012 | |
| Shading: 50%. Transmittance: R + FR. Dispersion: ++ | Lettuce | Greater reflectance (lighter colors). Increase in height and fresh weight | Ilić and Fallik, 2017 | |
| Unspecified | Grape | Accumulation of dry matter in the fruits | Pallotti et al., 2023 | |
| Shading: 30% | Tomato. Pepper | Increase in fruit yield and improved the quality | Ben-Yakir et al., 2012 | |
| Shading: 30%. Dispersion PAR: 18%. UV: 35% | Apple tree | Higher photosynthetic rate | Shahak et al., 2004 | |
| Shading: 30%. Dispersion PAR: 18%. UV: 35% | Apple tree. Peach | Greater fruit set | Shahak et al., 2004 | |
| Shading: 40%. PAR: 744.13 µmol m-² s-1. Shading: 40%. R/FR Ratio: 0.85. PAR: 221.67 µmol m-² s-1 | Tomato. Cilantro | Greater synthesis of aromatic compounds | Tinyane et al., 2013; Buthelezi et al., 2016 | |
| Blue | Shading: 20%. PAR: 552 µmol m-² s-1. R/FR Ratio: 0.81 | Avocado | Increase in commercial yield and accelerates fruit ripening | Tinyane et al., 2018 |
| Shading: 50%. Transmittance: sunlight. Dispersion: 60% | Lettuce | Shorter stem length | Ilić and Fallik, 2017 | |
| Shading: 50%. PAR: 947 µmol m-² s-1 | Basil | Reduced branching, leaf size, fresh mass per plant | Milenković et al., 2019 | |
| PAR Shading: 59%. Indirect light: 47.8% | Ornamentals | Induces dwarfism | Oren-Shamir et al., 2001 | |
| Shading: 54.2%. Light Dispersion: 62%. R/FR Ratio: 0.68 | Flowers | Delay in flowering, shorter stems, and inflorescences with smaller diameter and flower weight | Ovadia et al., 2009 | |
| Shading: 50%. Transmittance: sunlight. Dispersion: 60% | Lettuce | Increase in chlorophyll a and total chlorophyll | Ilić and Fallik, 2017 | |
| Shading: 25%. UV Blocking | Turmeric | Increase in chlorophyll content in terms of SPAD value, number of tillers, and curcuminoid content | Harish et al., 2022 | |
| Spectrum: 400 - 450 nm | Red amaranth | Increase in stimulation of antioxidant activity and total phenols | Khandaker et al., 2010 | |
| Shading: 47% PAR: 736.9 µmol m-² s-1 R/FR Ratio: 0.87 | Apple tree | Increase in transpiration and net carbon exchange rate Increase in net photosynthesis Increase in fruit size | Bastías et al., 2012 | |
| Shading: 50%. PAR: 238.33 µmol m-² s-1. Shading: 50%. Spectrum: 400 - 540 nm | Lemon balm. Basil | Increase in essential oil content | Oliveira et al., 2016; Martins et al., 2008 | |
| Pearl | Shading: 40%. PAR: 827.56 µmol m-² s-1 | Tomato | Increase in fruit weight, greater firmness, and an increase in bioactive compounds | Tinyane et al., 2013 |
| Shading: 35%. Dispersion PAR: 38%. UV: 48% | Apple tree | Increase in fruit size and overall yield | Shahak et al., 2004 | |
| Shading: 50%. PAR: 1100 µmol m-² s-1 | Basil | Greater leaf area index (LAI) and dry matter accumulation | Milenković et al., 2019 | |
| Shading: 50%. Transmittance: B + G + Y + R + FR. Dispersion: +++ | Lettuce | Higher contents of chlorophyll b, carotenoids, total phenols, and flavonoids | Ilić et al., 2017 | |
| Shading: 40%. PAR: 401.2 µmol m-² s-1. R/FR Ratio: 0.46. PAR transmission: 40%. Shading: 35% | Pepper | Post-harvest: Higher ascorbic acid and chlorophyll content increased antioxidant activity, fruit color, and firmness reduced weight loss during storage decreased fruit susceptibility to fungal infections | Selahle et al., 2015; Mashabela et al., 2015; Alkalia-Tuvia et al. 2014; Goren et al., 2011; Kong et al., 2013 | |
| Shading: 20%. PAR: 6 µmol m-² s-1 | Apple tree | Increased activity of the parasitoid Mastrus ridens, a biological control agent for Cydia pomonella | Yáñez et al., 2021 | |
| Shading: 18-35%. Mechanism: light reflection | Cucumber | 10% reduction in the incidence of cucumber mosaic virus (CMV) | Shahak et al., 2008 | |
| Shading: 18-35%. Mechanism: light reflection | Potato | Decrease in the incidence of potato virus Y (PVY) | Shahak et al., 2008 | |
| Green | Shading: 30% | Tomato | Greater fruit length, diameter, wall thickness, and number of locules | Zakher and Abdrabbo, 2014 |
| Yellow | Shading: 25%. UV blocking | Turmeric | Increase in leaf area, primary rhizome production, and dry matter accumulation | Harish et al., 2022 |
| Shading: 25%. UV blocking | Cucumber | Higher incidence of whiteflies (50%) and cucumber mosaic virus (CMV) | Harish et al., 2022 | |
| Shading: 48.6%. Dispersion: 44.1% | Eustoma. Widow's flower. Star of Bethlehem. Sunflower | Increase in stem length | Ovadia et al., 2009; Shahak et al., 2009 | |
| Shading: 50%. PAR: 705 µmol m-² s- | Marigold. Violet | Increased growth. Higher carotenoid, chlorophyll, and BRIX° content | Abbasnia et al., 2019 | |
| Shading: 40%. PAR: 851.81 µmol m-² s- | Tomato | Lower synthesis of aromatic compounds and lycopene | Tinyane et al., 2013 | |
| R/FR Ratio: 0.36. PAR transmission: 21% | Pepper | Greater synthesis of aromatic compounds, lycopene, and flavonoids. Reduction in photosynthetic rate and stomatal conductance | Mashabela et al., 2015 | |
| R/FR Ratio: 1.039. Shading PAR: 21.1% | Grape | Increased antioxidant concentration and reduces fungal diseases | Shahak et al., 2016 | |
| Shading: 30% | Vegetables | Lower incidence of aphids and whiteflies | Ben-Yakir et al., 2012 | |
| Grey | PAR: shading: 50.8%. Indirect light: 22.1% | Pittosporum | Increased branching and foliage density. Reduces leaf size and variegation | Oren-Shamir et al., 2001 |
PAR:photosynthetically active radiation; R: red; FR:far-red; R/FR: ratio red/ far-red; B: blue; G: green; Y: yellow.
Regarding green and gray nets, although they have been less studied, an improvement in fruit quality and plant branching has been noted, with greater compactness of the latter(Oren-Shamir et al., 2001). Finally, the yellow net has shown contrasting effects on the synthesis of aromatic compounds and chlorophyll content, with increases reported in ornamentals like marigold and violet and decreases in crops like pepper. Additionally, this net has promoted greater growth in ornamental plants, highlighting the differential response of various crops to the light spectrum (Shahak et al., 2009; Abbasnia et al., 2019).
PHOTOELECTRONIC COVERS
The spectrum of incident light is modified through fluorescence towards longer wavelengths, facilitated by the presence of light-emitting fluorescent compounds embedded in a matrix. The most used photo-selective covers with spectral shifting are those containing dyes (Pandey et al., 2023).
Photoelectronic covers have improved flower yield and fruit number, and positive effects have also been reported on the growth, antioxidant capacity, and biomass of lettuce and arugula (Tab. 3). It is important to note that this review included only a limited amount of information on photoelectronic technology, as our focus was on plastic films and shade nets. Photoelectronic technology is less commonly used by small farmers; however, we acknowledge its potential to optimize light spectrum use and suggest that future research could further explore its applications.
Table 3. Physiological effects of photoelectronic films on different crops.
| Color | Specifications | Crop | Effects on crops | Authors |
|---|---|---|---|---|
| Red | Unspecified | Carnation | Increase in stem yield | Magnani et al., 2008 |
| Unspecified | Rose | Longer and thicker stems, larger shoots | Mascarini et al., 2013 | |
| Fluorescence: 600-690 nm, Transmittance PAR: 57.3%, R/FR ratio: 1.17 | Strawberry | Slightly brighter fruits | Hemming et al., 2006 | |
| Blue | Fluorescence: 410-490 nm, Transmittance PAR: 83.1%, R/FR Ratio: 1.08 | Strawberry | Increase in the accumulated number of fruits by 8 to 12% | Hemming et al., 2006 |
| Orange Magenta | Unspecified | Cucumber | Greater vegetative growth | González et al., 2003 |
| Magenta | Unspecified | Strawberry Cucumber | Reduction in the incidence of thrips | González et al., 2003 |
| Yellow | Transmittance PAR: 76.6% | Lettuce Arugula | Greater antioxidant capacity and increased dry matter percentage | Magnani et al., 2008 |
PAR:photosynthetically active radiation; R: red; FR:far-red; R/FR: ratio red/ far-red.
LIGHT, INSECTS, VOLATILE COMPOUNDS, AND THEIR RELATIONSHIP
According to Shimoda et al. (2013), insects can see ultraviolet (UV) radiation. Nocturnal insects are often attracted to light sources that emit large amounts of UV radiation, leading to the development of devices that exploit this behavior, such as light traps for monitoring pest incidence and electric insect killers. On the other hand, some diurnal species are attracted to yellow; yellow traps are used for studying pest incidence, and yellow adhesive plates are employed for pest control. Lamps that emit yellow light have been effectively used to control the activity of nocturnal moths, thereby reducing damage to fruits, vegetables, and flowers. Covering cultivation facilities with films that filter radiation near UV rays reduces the invasion of pests like whiteflies and thrips in the facilities, thereby reducing damage. Reflective material placed on cultivated soil can control the approach of flying insects such as aphids. It is anticipated that future developments in light sources, such as light-emitting diodes, will be used to promote integrated pest management.
The authors observed that the use of colored filters and their interaction between tomato plants (Solanum lycopersicum) and a key pest, like Tuta absoluta (Lepidoptera: Gelechiidae), results in effects that influence light quality. This interaction between light and plants generates an abiotic stress that induces the production of chemical substances in the tomato fruits. These substances, in turn, lead to differentiated behaviors in the larva's attack on the fruit.
This concept is validated by Miresmailli et al. (2014), who highlight that plants can convey information about attacking herbivores to the plant itself and the natural enemies of those insects through the emission of specific chemical signals. Plants can even respond chemically to herbivore oviposition before feeding damage occurs. They produce a wide spectrum of chemical substances in various aerial and underground tissues, which they use to defend themselves against biotic stressors (such as pathogen infection, insect feeding, and viruses) and abiotic stressors (such as drought, salinity, flooding, heavy metals, heat, ozone, among others).
According to Miresmailli et al. (2014), some chemical substances are induced after an attack. This author also reports that in some cases, the compounds directly affect the herbivore, while in others, they attract organisms from different trophic levels. Examples include the scent of flowers, phytotoxic root exudates, and stem latex, all of which modify interactions with other species.
Some of these volatile substances in tomatoes were studied by Pizzo et al.(2024), who found that they play a fundamental role in plant defense. These compounds, known as terpenes and terpenoids, represent a large family of hydrocarbon biomolecules and are present in many species, including plants, animals, and microorganisms. Recognized for their distinctive odors, these compounds play a crucial role in the chemical defense strategies of plants, acting as repellents or toxins against herbivores and pathogens, while simultaneously attracting natural predators. Terpenoids differ from terpenes in the rearrangement of oxidation states. In tomato plants, a wide range of terpenoids are produced, synthesized, and stored within specialized glands called trichomes located on the plant's surface. These trichomes serve as reservoirs for a variety of terpene compounds that can deter herbivores, attract predators, and influence interactions between plants and pollinators. The composition of terpenoids within tomato trichomes varies among different cultivated varieties and accessions. In particular, the terpene α-zingiberene is known for its repellency against whiteflies.
Meanwhile, Gong et al. (2023) describe the location and the characteristics where volatile compounds are released, specifically through stomata, which regulate the release of volatile organic compounds (VOCs) in addition to controlling the flow of CO2 and H2O. The production of VOCs in plants is also induced by mechanical damage. Although photosynthesis provides critical molecules to produce defense-related chemicals, inhibiting growth often leads to an increase in defenses, generating natural herbivore repellents as the primary anti-herbivore defense. These VOCs also function as informative compounds for plant-to-plant communication, influenced by changes in stomatal opening and closing. Some oxidized lipids are antimicrobial, and volatiles containing α, and β-unsaturated carbonyls are collectively referred to as reactive electrophilic antibacterial substances. Additionally, these authors determined that potassium is the second most abundant mineral nutrient in plants, regulating the activity of metabolic enzymes. This element is essential for maintaining cell turgor, stomatal movements, and tropisms. The loss of K+ from tissues affected by stress is caused by major abiotic and biotic phenomena, highlighting its role as an essential macronutrient and in the rectification of potassium channels in guard cells.
There are different characterization techniques to evaluate and quantify volatile chemical substances, as suggested by Katna et al. (2024), such as high-performance liquid chromatography (HPLC), liquid chromatography-tandem mass spectrometry (LC-MS/MS or LC/TQ), and gas chromatography-mass spectrometry (GC-MS/MS). Additionally, gas chromatography paired with an electron capture detector (GC-ECD) is detailed for tomato in this reference.
In general, agricultural production faces various challenges, with adverse climatic conditions resulting from climate change standing out. This phenomenon is altering environmental conditions, including light, which is a key factor in agriculture as it directly impacts the physiological processes of plants. To meet the growing demand for food, it is crucial to implement alternatives that optimize production systems. Understanding how different light spectra and intensities influence the growth, development, and yield of crops, as well as the production of bioactive compounds such as vitamins, antioxidants, and phytochemicals, will help develop more efficient and sustainable cultivation systems. These systems will not only be capable of meeting the food needs of a growing population but will also play a fundamental role in adapting agriculture to current challenges, while also adding value to production chains.
CONCLUSION
The above considerations lead to the conclusion that the photosynthetic efficiency of conventional cultivation with the use of photoselective cover can be indirectly improved. Additionally, the use of photoselective cover creates an interaction between plants and insect pests, where the plants produce chemicals that affect insect attack behavior. New light sources, such as LEDs, are expected to be developed and used in the future to support integrated pest management.
Understanding the specific responses of each crop could lead to the development of agronomic strategies that enhance efficiency and productivity.
Light quality also enhances the functional quality of agricultural products by improving the synthesis of organic compounds. This results in competitive market advantages and greater nutritional benefits for consumers.
