The main properties of diffusive covers are summarized in Figures 2 and 3 . Diffusive covers scatter the incoming sunlight, spreading it more evenly across the greenhouse space compared to traditional clear covers. Light diffusion is achieved through the cover material itself or a specific coating, that alters the angle of incident rays, reducing the direct light intensity while minimizing the shadow areas (Hemming et al., 2016). This mechanism ensures that light penetrates deeper into the crop canopy, reaching lower and inner leaves, which otherwise would be shaded from the direct exposure (Shi et al., 2024). The DCs can increase the light uniformity index up to 20% compared to traditional covers, supporting a more uniform assimilation in the canopy profile and leading to a more balanced growth and higher yield (Moulton et al., 2020). Besides, diffusive light reduces stress conditions due to excessive radiation and photoinhibition, hence the plant need for photoprotective responses, allowing plants to better adapt to variable sunlight intensity (Paradiso et al., 2024).

By reducing direct sunlight, DCs also enhance the greenhouse microclimate by lowering temperature and increasing air relative humidity (RH). Applications in arid climate (e.g., Saudi Arabia) demonstrated that DCs can gain 77% diffuse radiation in the greenhouse in the warmest hours of sunny days, while slightly reducing the air temperature and maintaining a more constant RH level (Al-Helal et al., 2020). These covers increase the light use efficiency at the whole crop level up to 30%, leading to higher production with lower energy input (Moreno-Teruel et al., 2022). Specifically, reflective-diffusive films (RDCs), which also reflect a portion of NIR radiation while diffusing the visible portion of solar radiation, increase diffuse light by 85% compared to the corresponding traditional covers (with the same PE), reaching a diffusion percentage of 33% of the transmitted radiation. In the tested experimental conditions, they performed similarly to diffusive film (DF) in terms of temperature reduction and RH fluctuation (Al-Helal et al., 2020).

Evidence about the impact of diffusive covers on various horticultural crops are shown in Table 1 and summarized hereinafter, based on data available in literature concerning leaf, sprout and fruit vegetables, as well as some ornamental crops.

In asparagus (Asparagus officinalis L.), the comparison between a NIR-reflective diffusive coating and a traditional PE film revealed that both covers increased the heat accumulation compared to outside but, additionally, the NIR-reflective diffusive cover provided a more uniform light distribution, improving photosynthetic efficiency in the lower canopy. This led to higher spears yield and quality, with increased calcium and magnesium content. The NIR-reflective diffusive coating also promoted a more vigorous shoot emergence after mother stalk removal, suggesting its potential in enhancing long-term stem vegetables productivity in greenhouse (Chen and Shen, 2022).

Diffusive covers enhanced the plant growth and fruit production of pepper (Capsicum annuum L.) in a Natural Ventilation Augmented Cooling (NVAC) greenhouse in Quebec (Canada) (McCartney, 2017). In this experiment, the improved light diffusion and moderated temperature led to a 28% increase in photosynthetic rate and a 31% reduction in transpiration, ultimately benefiting plant growth. Similarly, diffusive PE increased the leaf area, hence the photosynthetically active surface and assimilation capacity, which contributed to improved growth and productivity of pepper in a Mediterranean greenhouse in Southern Europe, leading to a higher marketable fruits yield (+3.1% compared to commercial PE) (Ávalos-Sánchez et al., 2023).

The use of diffusive PE films increased the leaf area and shoot elongation in greenhouse cucumber (Cucumis sativus L.) grown under arid conditions in middle east and north Africa, leading to higher biomass accumulation and fruit yield (+22% compared to conventional PE). Particularly, the enhanced light penetration into the canopy and the reduced heat stress resulted in a significant increase in fruit number and size (Al-Madani et al., 2024).

Despite the short stature, also some leafy vegetables, like lettuce (Lactuca sativa L.) and spinach (Spinacia oleracea L.), benefit from diffusive light due to their dense canopy. Indeed, in these crops light scattering enhanced photosynthetic activity and biomass accumulation, increasing yield by 8-10% (Hemming et al., 2016). In contrast, some evidence highlighted a counterproductive effect of a PE diffusive cover on lettuce as it reduced PAR by 27% compared to control, without lowering air temperature but increasing the occurrence of tipburn (Bárcena et al., 2019). However, the increment in leaf number, avoided any negative effect on fresh and dry weight. Besides, a shade cover (lowering PAR by 76%) used as additional treatment prevented tipburn symptoms. Still in lettuce, the use of diffusive PE film showed to boost the chlorophyll synthesis, to promote a more uniform growth and to improve the health status, with a significant increase in the yield (+22% compared to conventional PE) and leaf content of total ascorbic acid (+9.4%) (Cozzolino et al., 2020).

Tomato (Solanum lycopersicum L.) is the most studied fruit vegetable under DCs and, in general, it shows a considerable improvement in photosynthetic efficiency and fruit yield when grown under a even light distribution. Indeed, this is particularly beneficial for those crops with erect habit in which the lower canopy usually faces light limitations while the upper one is exposed to light intensities above the saturation level. In these crops, diffusive covers alleviate shading of lower leaf layers and reduces photoinhibition in top leaves, while minimizing heat stress and photodamage, especially under strong sunlight conditions (Hemming et al., 2008). A PE cover with moderate diffusion properties enhanced photosynthetic activity (+21.5%) and increased tomato yield (3.2%) in Mediterranean climate, Almería, Spain (Moreno-Teruel et al., 2021). Studies conducted in the South of Italy also show that DCs boosted both early and total marketable yields of cherry tomato, with different increases depending on the rate of nitrogen fertilization (Paradiso et al., 2024). Additionally, combining DCs with a protein hydrolyzed biostimulant enhanced nutrient uptake and secondary metabolite production (i.e., phenols), improving the nutritional profile in cherry tomato (Paradiso et al., 2024). In tomato grown under a diffusive polyethylene (D-PE) film, a 15% increase in light intensity in the low canopy compared to conventional PE lead to a significantly higher photosynthetic efficiency (Moreno-Teruel et al., 2022). In tomato, highly diffusive covers can also enhanced the fruit quality and nutritional profiles, increasing lycopene, lutein, and phenolics content (+10.5%, +1.03%, and +14.5% respectively) (Ahmadi et al., 2018). Polycarbonate covers also resulted in an 8% increase in yield and improved photosynthetic efficiency in tomato (Moreno-Teruel et al., 2022).

Based on data from literature reported in Table 1 , the highest number of crops investigated under DCs belongs to the botanical family of Solanaceae, with 19 papers in total, with 9 studies focusing on tomato, 2 on pepper and 1 on eggplant (Solanum melongena L.; Cemek et al., 2005). In these crops, the use of DCs revealed a positive impact on plant growth and yield by improving light conditions and moderating temperature fluctuations. On fruit vegetables, covers like stabilized PE-UV and PE-IR enhance photosynthesis, and increase plant growth and fruit size and yield. These effects are presumably related to the plant protection from excessive UV radiation, reducing stress conditions, and the more stable microclimate.

The findings suggest that diffusive covers, such as D-PC and D-PE, have a broad impact also on plant nutritional and metabolic responses, particularly in terms of secondary metabolite production which enhances the overall product quality. For instance, an increase in the biosynthesis of carotenoids (i.e., lycopene and lutein) was found in tomato, lettuce, and melons, leading to healthier plants and a better product nutraceutical profile (Ahmadi et al., 2018; Baxevanou et al., 2018).

Referring to the mechanisms underlying the plant response to diffuse light, it has been hypothesized that it enhances the growth hormone efficiency, potentially influencing hormone-driven growth responses (He et al., 2021). These effects may include improved auxin distribution, supporting shoot elongation and balanced biomass allocation.

The effect of diffuse light has been also tested in floriculture. Specifically, Chrysanthemums (Chrysanthemum × morifolium L.) showed to benefit from scattered light, increasing CO₂ assimilation rate per leaf area unit (+5%), dry matter accumulation (+9.5%), number of lateral stems (+11%), and leaf area (+8%) compared with the control (Markvart et al., 2010). However, it is worth noting that the larger plant size observed under diffuse light led to a counterproductive increase in internal canopy shading compared to the direct light control.

The main properties of photoselective covers are summarized in Figures 2 and 3 . Photoselective covers, available as colored or clear polymers, reduce the radiation reaching the crop by selectively blocking some wavelengths, altering light quality for desired physiological, phenological and morphological responses (Pandey et al., 2023). Some PhCs are designed to allow the passage of specific wavelengths, such as R to modulate the R-B-FR ratio to regulate the plant growth rate and architecture, and UV-B to stimulate the secondary metabolite production (Tafoya et al., 2018; Wong et al., 2020). Rai, 2020 showed that the increased exposure to UV (both A and B) radiation strongly induced gene expression changes in Arabidopsis and, in addition to promoting the plant growth, it modifies the response and interaction of various photoreceptors and alter RNA transcription. However, photoselective films experience a 2-4% reduction in light transmittance in their life span, due to weathering, and condensation can lower transmittance by an additional 5% (Abdel-Galil, 2014). Photo-induced pigment degradation within plastic matrixes can impair the cover performance, though recent materials are more durable, with some lasting up to 15 years (Blanke, 2008).

The modified light spectrum provided by photoselective films influences growth hormone regulation, promoting stem elongation and biomass accumulation (He et al., 2021). Table 2 shows data about the effects of PhCs on various horticultural crops and how targeted growth responses, including flowering and increase of fruit size, can be attained.

Leafy vegetables, such as lettuce, show strong adaptability to spectral modifications, that enhance photosynthesis and biomass accumulation while improving water use efficiency and secondary metabolite production, hence they are suitable as advanced light manipulation technologies. In lettuce, polypropylene covers increasing R and reducing B and Y-G light significantly improved the assimilation rate and reduced stomatal conductance and transpiration, leading to a higher water use efficiency and biomass compared to non-modified cover (Khramov et al., 2022). Additionally, photoselective films enhanced both stress tolerance and produce quality in red lettuce by decreasing temperature (up to 1.9 °C), boosting leaf area, chlorophyll content, fresh and dry weight, as well as secondary metabolites, including flavonoids and anthocyanins (Amaro de Sales et al., 2021).

Wild rocket (cultivar ‘Reset’) showed a 30% improvement in chlorophyll content and consequently a higher photosynthetic efficiency and leaf growth and yield due to UV-to-R/B spectrum conversion (Paradiso et al., 2023). By converting harmful UV radiation, these films can improve crop resilience to sunlight-induced stress playing a pivotal role in mitigating abiotic stress, as reported by Pandey et al. (2023).

Members of Cucurbitaceae family exhibit notable responses to light manipulation strategies. Melon (Cucumis melo L.) showed a strong benefit from spectral changes in terms of productivity. Precisely, ethylene-vinyl acetate (EVA) films enhancing B and R transmission boosted yield by 52% (through both fruit size and weight) and sugar content, compared to standard film (Lemarié et al., 2018). Furthermore, NIR-absorbing PE films alleviate heat stress in melon, maintaining photosynthesis and enhancing soluble solids (Murakami et al., 2017).

These findings highlight the potential of targeted light management for optimizing fruit quality and yield. Cucumber plants showed a better growth under UV-transmitting covers, which increased yield by 21-25%, and enhanced leaf chlorophyll and phosphorus content. Furthermore, visible light diffusion through these covers mitigated environmental stress symptoms, leading to better plant health and reducing Aphis gossypii infestations (Abd El-Aal et al., 2018).

Members of the Solanaceae family, such as tomato, eggplant, and potato, show varying responses to PhCs materials and light manipulation strategies. Tomato grown under PE films with high UVA transmission (90-100%) and anti-NIR properties achieved higher yields and better fruit quality compared to the corresponding conventional cover. However, in some environments, anti-NIR films reduced PAR and raised temperatures, which negatively impacted yields. The PE covers with 3-5% UV transmission boosted eggplant yield by 20%, along with improvements in height, leaf production, and fruit quantity (Kittas et al., 2006). Potato (Solanum tuberosum L.) under EVA covers enhancing B and R transmission showed a yield increase of 11-13% (through more numerous smaller tubers) and enabled an earlier harvest (-8 days) compared to the standard EVA film without photoselective additives (Lemarié et al., 2018).

EVA copolymer-enhanced covers applied to watermelon (Citrullus lanatus L.) and potato improved fruit size and weight, sugar content, and yield (+10-12%), while anticipating the harvest (Lemarié et al., 2018). In tomato, plants under 90-100% UV-A transmission films achieved superior fruit yield and quality compared to other light-modifying technologies such as LDe (light diffusing energy) and anti-NIR covers (Lopez Marin et al., 2019). These covers can enhance secondary metabolite production, enriching the nutritional value of crops. For example, UV-B blocking films increased the phenolic acids and quercetin content in rocket (Eruca vesicaria), bolstering the plant stress resilience (Mormile et al., 2019), and high R/FR ratios enhanced the antioxidant activity in strawberries (Fragaria × ananassa L.) (Fletcher et al., 2002).

Beside the plant growth and metabolism, specific spectrum manipulation can alleviate biotic stress by controlling pathogen development and improving the plant reaction. For instance, a negative impact on fungi can be due to the higher UV-A transmission, such as in certain diffusive PEs, that may create unfavorable conditions for fungal growth and reproduction, inhibiting spore germination and mycelium development. Besides, a positive influence on the plant can depend on the enhanced light availability, improving photosynthetic efficiency, leading to stronger plant vigor, and strengthening the natural defense mechanisms and tolerance to pathogen infections. Consistently, UV-blocking films helped reducing biotic stresses, significantly lowering the occurrence of fungal diseases such as powdery mildew and early blight in tomato and pepper (Avalos-Sánchez et al., 2023).

In a trial for a new NIR-reflective film, Alsadon et al. (2016) measured a lowering of the average temperature by 9 °C compared to outside the greenhouse. The experiment also compared other commercial covers (no details on the types), which also showed a temperature decrease but of smaller magnitude (7 and 6 °C). Gas exchange parameters (photosynthetic and transpiration rate, and stomatal conductance) were associated with temperature variations, showing an inverse correlation with temperatures. The improvement in photosynthetic response resulted in a higher yield in cucumber (Cucumis sativus L.).

Red light enriched spectra stimulated tiller production and shoot growth in wheat (Triticum aestivum L.) (De Salvador et al., 2008).

Various hail nets (red, blue, grey, and white) with photoselective properties were tested on kiwifruit (Actinidia chinensis var. deliciosa A. Chev.), with a shading effect ranging from 20.4% to 27.3% of PAR (Basile et al., 2012). All net treatments determined an increment in light scattering and changes in light spectrum and influenced productive traits: red and blue nets increased fruit weight and dry matter content, while grey and white nets improved fruit firmness and reduced fruit drop, indicating that net color can be used to modulate both fruit yield and quality. However, different response to shading and spectral changes were observed in the two years of observation, highlighting the occurrence of the interaction of treatments with climate conditions.

On plants of the genus Prunus (P. avium L. and P. persica L.) grown in pots, various plastic photoselective and photoluminescent films were tested, revealing that altering the spectrum (mainly R and FR) affected the vegetative activity and photomorphogenesis of cherry and peach shoot, and highlighting how the use of B photoselective films is useful in containing the plant size while R and G covers to increase it (Schettini and Vox, 2010).

In lemon (Citrus jambhiri L.), red nets help mitigate thermal stress by diffusing solar radiation, which reduces excessive heat buildup and enhances nutrient content and growth (Brar et al., 2020).

In the ornamental plant Anthurium andraeanum , PE, NIR filters increase the flower stem yields by 3-10% (Romero et al., 2018). In chrysanthemum (Dendranthema grandiflorum L.), R-FR spectrum enhanced plant height and flower quality (Li et al., 2000).

The main properties of switchable covers are summarized in Figures 2 and 3 . The optimal characteristics of a greenhouse cover vary according to the geographic location and crop type, as no single cover material is universally suited to all crop-climate combinations. Typically, greenhouse covers have fixed optical properties that regulate the amount of sunlight entering the structure. As a result, the intensity and quality of sunlight (including spectrum and balance between direct and diffused light) may not be ideal for the crop throughout its growth cycle, since light requirements change in the developmental stages (Baeza et al., 2019). To address these limitations, new materials with switchable optical properties are being developed, allowing nearly instant adjustments to light conditions inside the greenhouse or employing supplementary methods like temporary coatings, mobile/fixed screens, and heating/cooling systems (Baeza et al., 2019).

Switchable covers (SCs) are dynamic materials that adjust their features in response to environmental changes, such as temperature or light intensity, offering greater flexibility in regulating light and thermal parameters during the different plant developmental stages. These covers dynamically filter PAR and NIR light, optimizing light quality to improve photosynthesis. For instance, electrochromic and thermochromic covers adjust transparency in response to environmental triggers, optimizing the growth conditions in greenhouse. This technology reduces the heat stress while enhancing photosynthetic efficiency, ensuring consistent growth even in extreme conditions (Baeza et al., 2019, 2020). Many PhCs and SCs are engineered with UV stabilizers and weather-resistant polymers to extend lifespan up to 10–15 years (e.g., Blanke, 2008; Baeza et al., 2020).

Effects of SCs on plant growth, photosynthesis and secondary metabolism in vegetables and ornamentals are shown in Table 3 .

In Mediterranean climates, these covers can significantly improve light and temperature conditions, especially in regions with fluctuating climate (Romero et al., 2018; Soussi et al., 2022). Indeed, SCs stabilize physiological processes by preventing excessive transpiration and reducing stress-induced metabolic changes. It is modelled that this regulation promotes crop health in high-value vegetables like tomato and ornamentals like Anthurium andraeanum , ensuring stable growth even under challenging environmental conditions (Romero et al., 2018; Soussi et al., 2022). It is estimated that under SCs tomato yield can increase up to 20% due to the improved environmental conditions (Romero et al., 2018; Baeza et al., 2019). Additionally, by stabilizing temperature and minimizing stress responses, SCs have an indirect positive influence on plant hormonal balance. This is particularly beneficial for sensitive crops like tomato, where hormonal disruptions can negatively impact plant productivity (Romero et al., 2018).

For fruit crops, in melons, SCs promoted early flowering and increased sugar content while mitigating heat stress during high temperature periods (Pandey et al., 2023). Dynamic shading improved fruit quality and sugar accumulation in raspberry (Rubus idaeus), though excessive shading remains a potential drawback (Pandey et al., 2023).

In tuber crops like potato (Solanum tuberosum L.), thermochromic covers help regulate temperature, reducing extreme heat exposure and promoting growth and earlier harvest (Pandey et al., 2023).

Overall, the efficiency of SCs varies across the crop types: to summarize, high-value ornamentals benefit from stabilized physiological processes, root and tuber crops experience enhanced early growth, and fruit vegetables show improvements in yield, fruit quality, and stress tolerance. However, SCs application is influenced by several other factors, such as initial and maintenance costs, crop productivity goals, local climate, and technological support (Ahmadi et al., 2018).

Passive dynamic control technologies represent a significant advancement in greenhouse management, as they automatically adapt their properties in response to environmental changes, without requiring external energy input. These technologies allow materials to react to varying climate parameters such as temperature, sunlight, and humidity, thereby enhancing the energy efficiency while maintaining better growth conditions in greenhouse (Soussi et al., 2022; Zhang et al., 2022; Ghiasi et al., 2023). For instance, thermochromic materials alter their properties based on temperature fluctuations, effectively reducing heat gain and providing thermal comfort for the plants (Padilla et al., 2021). Similarly, photochromic materials adjust their transparency in response to light levels, minimizing the glare (Nikolaou et al., 2018; Lin et al., 2022). Additionally, photoelectric materials, including crystalline and inorganic thin-film glazing, not only provide shading to protect plants from excessive sunlight but also generate energy, further contributing to the sustainability of greenhouse operations (Timmermans et al., 2020; Maraveas et al., 2021, 2023a).

to modify their properties, facilitating real- By integrating these innovative materials, greenhouse managers can optimize growing conditions while saving resources.

Active dynamic control technologies require an external energy source time adjustments tailored to specific environmental conditions (Baeza et al., 2020). These systems provide a precise control over light transmission and diffusion, which can significantly enhance plant growth and energy efficiency. For instance, electrochromic materials can change their opacity or color in response to an applied electric current, allowing for meticulous control over both light and heat transmission (Baeza et al., 2020; Maraveas et al., 2023a). On the other hand, mechano-chromic materials adjust their properties through mechanical inputs, such as deformation, which further enhances their adaptation to varying environmental scenarios (Guo et al., 2024). Additionally, polarized particle devices, including Suspended Particle Devices (SPDs) and Polymer Dispersed Liquid Crystals (PDLCs), are controlled via electrical signals to effectively modulate light and heat in the greenhouse (Ghosh et al., 2024). By incorporating these advanced technologies, greenhouse operators can optimize growing conditions dynamically, to improve crop yield and resource management.

To improve plastic properties, like temperature resistance, heat dissipation, droplet formation and dust deposition prevention, some functional additives, fillers, air bubbles, reinforcements (e.g., glass or carbon fiber), and colorants are often incorporated Castilla (2013). Examples are UV absorbers and stabilizers that help protect plants in greenhouse while blocking UV-B radiation over 40 kJ/m², controlling the UV transmission rate, typically 70-90% in common materials (Zhang et al., 2019). Anti-fog and IR-blocking additives further prevent fogging and restrict harmful IR radiation (Kittas et al., 2006).

To protect plastic films and panels from UV damage and photodegradation, stabilizers and additives like black carbon modify the optical properties of cladding materials (Aldaftari et al., 2019). The UV absorbers and additives also shield plants from harmful radiation and help limit the spread of pests and pathogens (Antón et al., 2014).

Infrared light absorbers minimize heat loss and short-wave emission, typically having wavelengths between 700 and 2500 nm. Long-wave absorbers (2500-40,000 nm) reduce heat loss from plants in greenhouse by absorbing these wavelengths. Surfactants and antistatic agents lower surface tension to reduce the dust accumulation on plastic films (Maraveas et al., 2023b).

Red-emitting plastic greenhouse extensions provide stunning and vivid patterns and colors. As the pigment volume fraction increases, the efficiency of greenhouse gases rises. For optimal radiation control, HVAC (heating, ventilation, and air conditioning) systems are generally more effective than pigments, as titanium dioxide (TiO₂) and diamond particles in pigments can selectively reflect near-IR (800–2500 nm) while transmitting visible light at shorter wavelengths (Aldaftari et al., 2019).

Diamond-based pigments are advanced materials used in greenhouse cladding to optimize light management and thermal control. They effectively reduce heat buildup while allowing sufficient light for photosynthesis, offering a cost-efficient alternative to traditional cooling systems. Their unique optical properties enable them to reflect near-IR radiation while transmitting visible light, surpassing conventional pigments like TiO₂ in performance. Additionally, TiO₂ particles are less effective in reflecting IR radiation, and diamond particle-based pigments have been shown to reduce radiation (Aldaftari et al., 2019). Transparent Solar Distillers (TSD) combine TiO₂ nanoparticles in multi-purpose greenhouse coverings to use solar energy for water desalination (Rabhy et al., 2019). These TSD pigments also enhance greenhouse cladding by managing incoming and outgoing energy wavelengths. By filtering high-energy wavelengths during the day, greenhouse covers prevent overheating while ensuring sufficient light for photosynthesis. At night, they retain heat by limiting long-wavelength energy loss, supporting stable internal temperatures for crop growth and development.

One of the more recent innovations involves the use of antimony tin oxide (ATO) nanoparticles in plastic films, which help manage solar radiation while maintaining adequate light levels for photosynthesis. These nanoparticles reduce the amount of heat absorbed by the greenhouse while allowing enough PAR to pass through, ensuring both energy efficiency and healthy crop growth (Zhang et al., 2024a).