Light and shading modulate growth, antioxidant system and cut flower production of Zinnia elegans

Luz e sombreamento modulam o crescimento, o sistema antioxidante e a produção de flores de corte de Zinnia elegans

ABSTRACT

Zinnia elegans shows potential as a cut flower, however, its production in tropical regions can be limited by abiotic stresses. Controlling light quality with shading nets can minimize these effects. Thus, the objective of this study was to evaluate the effect of different shading nets on the vegetative growth, floral production, leaf anatomy, and antioxidant system of Z. elegans cv. ‘Luz da Lua’. The experiment was conducted during two seasons: spring-summer and autumn-winter, with plants grown under full sun, black nets (30% and 50%), and blue (50%) and red (50%) photoconversion nets. In the spring-summer period, plants exhibited greater vegetative growth and flower number. The 30% black and 50% red nets promoted the development of longer floral stems with higher biomass, while the highest number of flowers was obtained under the 30% black net. Cultivation under full sun resulted in lower flower production and shorter stems. Regarding the antioxidant system, higher enzymatic activity was observed in plants grown in spring-summer, notably higher catalase and ascorbate peroxidase activities under black nets, contributing to reduced oxidative damage. In contrast, the 50% blue net promoted higher H₂O₂ accumulation and lipid peroxidation, indicating the occurrence of oxidative stress. Plants grown under black nets also showed higher stomatal density. The use of shading nets favors the production of high-quality flowers, and the 30% black net is recommended for maximizing flower number, stem length, and the efficiency of the antioxidant system.

Index terms:
Floriculture; photomorphogenesis; oxidative metabolism; seasonal cultivation; stomatal plasticity

RESUMO

A Zinnia elegans apresenta potencial como flor de corte, porém sua produção em regiões tropicais pode ser limitada por estresses abióticos. O controle da qualidade da luz com telas de sombreamento pode minimizar esses efeitos. Assim, o objetivo deste estudo foi avaliar o efeito de diferentes telas de sombreamento no crescimento vegetativo, produção floral, anatomia foliar e sistema antioxidante de Z. elegans cv. Luz da Lua. O experimento foi conduzido em duas épocas: primavera-verão e outono-inverno sendo as plantas cultivadas a pleno sol e sob telas preta (30% e 50%) e fotoconversoras azul (50%) e vermelha (50%). No período de primavera-verão, as plantas apresentaram maiores crescimento vegetativo e número de flores. As telas preta 30% e vermelha 50% proporcionaram o desenvolvimento de hastes florais mais longas e com maiores valores de biomassa, enquanto que o maior número de flores foi obtido sob tela preta 30%. O cultivo a pleno sol resultou em menor produção de flores e com hastes mais curtas. Quanto ao sistema antioxidante, houve maior atividade enzimática nas plantas cultivadas na primavera-verão, com destaque para maior atividade de catalase e ascorbato peroxidase sob telas pretas, contribuindo para a redução dos danos oxidativos. Em contraste, a tela azul 50% promoveu maior acúmulo de H₂O₂ e peroxidação lipídica, indicando a ocorrência de estresse oxidativo. Plantas cultivadas sob telas pretas também apresentaram maior densidade estomática. O uso de telas de sombreamento favorece a produção de flores de qualidade, sendo a tela preta 30% indicada por maximizar o número de flores, o comprimento das hastes e a eficiência do sistema antioxidante.

Termos para indexação:
Floricultura; fotomorfogênese; metabolismo oxidativo; cultivo sazonal; plasticidade estomática

Introduction

The ornamental species Zinnia elegans Jacq. is traditionally cultivated in garden beds and stands out for its versatility and short cycle. It holds significant potential for the cut flower market due to its wide range of floral colors (Pêgo, Carvalho & Martins, 2021). For cut flowers, the market demands plants with longer stems, firm blooms, and good post-harvest durability qualities that are directly linked to the conditions under which the plants are grown (Martins et al., 2021).

However, cut flower production in regions with tropical climates faces substantial challenges. These include climatic variations, such as excessive solar radiation and high temperatures, which can limit floral development and quality. This occurs because, although Zinnia elegans can be produced year-round and performs better in spring and summer (Pinto et al., 2003; Lorenzi, 2015), the extreme conditions of the tropical environment can exceed its tolerance limit. Consequently, the use of technologies to mitigate these plant stresses has become essential. Shading screens emerge as an agricultural technique that allows for microclimate adjustment while simultaneously manipulating the quality of incident light to optimize plant development (Manja & Aoun, 2019).

Unlike conventional black screens, which reduce irradiance in a neutral manner (Ilić & Fallik, 2017), colored screens selectively filter specific wavelengths, altering spectral ratios (such as red/far-red) that regulate plant morphophysiology. Studies on other ornamental species demonstrate that the influence of these screens varies depending on the species. The use of red screens can promote stem elongation and accelerate flowering, whereas blue screens can modulate plant height and increase leaf number (Cunha Neto et al., 2023; Oliveira et al., 2024). In the cultivation of Antirrhinum majus and Lisianthus, the use of red and blue screens improved commercial quality and altered the content of photosynthetic pigments (Li et al., 2017; Almeida, Calaboni & Rodrigues, 2021).

Furthermore, anatomical aspects can be altered in response to the conditions imposed by shading screens, such as variations in stomatal density (Nascimento et al., 2016).

Despite the benefits observed for several crops already evaluated, a knowledge gap remains regarding how Zinnia elegans responds to alterations in the light spectrum, specifically concerning its ability to adjust the antioxidant system to cope with seasonal cultivation variations. Understanding how distinct light environments affect plant physiology is fundamental to enabling management strategies for this species. Considering the increasing demand for diversification in cut floriculture and the lack of well-defined technical protocols for this crop, it was hypothesized that variations in shading levels and quality could affect, beyond growth and yield, the efficiency of the antioxidant system and the anatomical characteristics of Zinnia elegans cv. ‘Luz da Lua’, reflecting improved agronomic performance and ornamental potential. Thus, this study aimed to evaluate the vegetative development, floral production, antioxidant system activity, and anatomical alterations of Zinnia elegans cv. ‘Luz da Lua’ grown under different shading screens and full sun across two growing seasons, seeking to identify management conditions that optimize crop quality and yield.

Material and Methods

Growth conditions

The experiment was conducted in a region characterized by a Cwa climate (high-altitude tropical) according to the Köppen classification, located at 21°75’ S latitude and 45°00’ W longitude, at an altitude of 916.16 m. The area has an average annual precipitation of approximately 1,493 mm and an average annual temperature of 19.3°C. The study was carried out across two distinct seasons in 2024: autumn-winter, with an accumulated precipitation of 59.8 mm during the experimental period and average maximum and minimum temperatures of 29.2°C and 15.8°C, respectively; and spring-summer, with an accumulated precipitation of 168.03 mm during the experimental period and average maximum and minimum temperatures of 30.5°C and 22.1°C, respectively. Climatic conditions were monitored throughout the experimental period using data obtained from the official meteorological station of Lavras, MG, Brazil, maintained by the National Institute of Meteorology (INMET) (Figure 1).

Figure 1:
Maximum, average, and minimum temperatures (°C) and relative humidity (%) recorded monthly during the experimental period (2024-2025).

Measurements were performed at 12:00 h on clear, sunny days during the autumn-winter and spring-summer seasons. Data were collected on ten different days throughout the experimental period, and mean values were subsequently calculated for each growing environment. Light intensity was measured using a digital datalogger light meter (Model LDR-380, Instrutherm, São Paulo, Brazil). Photosynthetic photon flux density (PPFD, µmol m⁻² s⁻¹) was estimated from illuminance measurements (lux) using the solar radiation conversion factors proposed by Thimijan and Heins (1983). Mean PPFD values for each growing environment and season are presented in Table 1.

Table 1:
Light radiation values (µmol m^-2 s^-1) _, obtained by a lux meter, in cultivation environments during autumn-winter and spring-summer 2024-2025*.

Estimated photosynthetic photon flux density (PPFD, µmol m-2 s-1)
Growing environments	Autumn-winter	Spring-summer
Full sun	1478.0	1512.0
Black shade net 30%	1232.0	1458.0
Black shade net 50%	1086.4	1386.4
Red shade net 50%	1094.4	1296.4
Blue shade net 50%	1072.0	1303.7

• * Located at 21°45’ S latitude and 45°00’ W longitude, at an altitude of 916.16 m.

Plant material and treatments

Seeds of Zinnia elegans Jacq. cv. ‘Luz da Lua’ were used, featuring blooms characterized by yellow and red-streaked colors, obtained from the commercial company ISLA® (Isla sementes, Porto Alegre, Brazil). The experiments were conducted across two seasons: autumn-winter and spring-summer. In both seasons, five growing environments were evaluated, which constituted the treatments: full sun and four shading screens 30% black, 50% black, 50% blue and 50% red. The cultivation environments consisted of independent experimental screenhouses completely enclosed on the top and sides with the respective nets (30% black, 50% black, 50% blue, and 50% red). Due to the natural porosity of the mesh materials, continuous air and vapor exchange with the external environment was maintained.

Zinnia seeds were sown in trays containing Mogifertil (Mogi das Cruzes, Brazil) commercial substrate, based on peat, charcoal, sand and fertilization with nitrogen, phosphorus and potassium. Twenty-one days after sowing, uniform and well-developed seedlings, approximately 10 cm in length, were selected for transplanting. The seedlings were transplanted into 1.7 L pots, with one plant per pot, filled with a substrate composed of soil, sand, and organic matter in a 1:1:1 ratio. The substrate mixture consisted of subsurface soil (a clayey Oxisol collected from a local slope at UFLA), sand, and organic matter (well-rotted bovine manure). The plants were supported with stakes and fertilized with a commercial mixed mineral fertilizer (NPK 10-10-10) supplemented with micronutrients (iron, manganese, zinc, copper, boron, and molybdenum) incorporated into the substrate; no top-dressing fertilization was applied. Irrigation was performed manually twice a day throughout the experiment. Plants were evaluated at 30, 60, and 90 days after sowing (DAS), considering growth, production, and flower stem quality, as well as the effects on chlorophyll content, the antioxidant system, and leaf anatomy. All evaluations and sampling were consistently performed during the morning period (between 7:00 AM and 11:00 AM).

Morphophysiological and yield analyses

Morphophysiological variables and flower stem production traits were evaluated at 90 days after sowing (DAS). Plant height, root length, and flower stem length were determined using a graduated ruler, while collar diameter was measured with a digital caliper, and the total number of leaves was manually counted. To determine fresh and dry mass of shoots and roots, plants were harvested and weighed on a precision balance. Dry mass was obtained after drying the plant material in Kraft paper bags in a forced-air circulation oven at 65°C for 48 h until constant weight. The total number of flower stems was recorded from the opening of the first flower until the end of the experiment in each growing season.

Chlorophyll content: The determination of chlorophyll a, b, and total chlorophyll contents was performed using a ClorofiLOG device (model CFT1030, Falker Automação Agrícola, Brazil). Readings were taken from three distinct leaves per plant, resulting in the chlorophyll index (FCI - Falker Chlorophyll Index).

Anatomical analyses

For the anatomical analyses, ten fully expanded Zinnia leaves were collected from the middle portion of the plants and fixed in a 70% FAA solution (formalin-aceto-alcohol) for 48 h (Johansen, 1940). Subsequently, they were transferred to 70% alcohol and stored until the time of sectioning. Paradermal sections were obtained by hand from the abaxial leaf surface using a razor blade. The sections were subjected to a 1% safranin staining process and then mounted onto permanent slides using synthetic resin (adapted from Bukatsch, 1972). The slides were observed under an optical microscope, and images were captured using a coupled camera. For the anatomical description, the presence of stomata was considered, determining the number of stomata, stomatal density (number of stomata per/mm²), polar diameter (length) (PD), and equatorial diameter (width) (ED) (Esau, 1977; Kraus & Arduin, 1997). To obtain the results, the images were processed using ImageJ software.

Antioxidant system activity

The enzymatic activities of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), along with the contents of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), were analyzed via spectrophotometry (Biemelt, Keetman & Albrecht, 1998). At 60 days after sowing, a sample of ten fully expanded Zinnia leaves was collected from each treatment and macerated in the presence of polyvinylpyrrolidone (PVP) and liquid nitrogen, and then stored in an ultra-low temperature freezer at -80°C.

CAT activity: 9 µL of the extract prepared from fresh Zinnia leaves were added to a 100 mM potassium phosphate solution previously incubated in a buffer at 30°C. To this mixture, 10 µL of 250 mM hydrogen peroxide (H₂O₂) were added. The activity was analyzed using an ELISA spectrophotometer by measuring the decrease in absorbance every 15 seconds for 3 minutes (Havir & McHale, 1990). CAT activity results were expressed in nmol H₂O₂ min^-1mg^-1 FM.

APX activity: 9 µL of the extract were added to a solution containing 100 mM potassium phosphate and 0.5 mM ascorbic acid. To this mixture, 9 µL of 250 mM hydrogen peroxide (H₂O₂) were added. The decrease in ascorbate absorbance was measured at 290 nm every 15 seconds for three minutes using an ELISA plate reader (Nakano & Asada, 1981). APX activity results were expressed in nmol AsA min^-1mg^-1 FM.

SOD activity: This was evaluated based on the extract’s ability to inhibit the photoreduction of nitro blue tetrazolium (NBT). A volume of 10 µL of extract was added to 195 µL of an incubation solution containing 100 µL of 100 mM potassium phosphate (pH 7.8), 40 µL of 70 mM methionine, 3 µL of 10 µM EDTA, 35 µL of distilled water, 15 µL of 1 mM NBT, and 2 µL of 0.2 mM riboflavin. The reaction was conducted in a reaction chamber (expanded polystyrene box) under fluorescent lamp illumination at 25°C for seven minutes. Subsequently, the absorbance was measured at 560 nm (Giannopolitis & Ries, 1977). SOD activity results were expressed in U SOD min^-1 mg^-1 FM.

H₂O₂ quantification: A buffer solution was used, containing 45 µL of 10 mM potassium phosphate (pH 7.0), 90 µL of 1 M potassium iodide, and 45 µL of the extract, totaling a final volume of 180 µL. Readings were performed in duplicate by measuring the absorbance at 390 nm. The H₂O₂ content was calculated using a standard curve previously prepared with 250 µM H₂O₂ (Velikova, Yordanov & Edreva, 2000). Results were expressed in µmol H₂O₂ mg^-1 FM.

Lipid peroxidation: This was determined using the thiobarbituric acid reactive substances (TBARS) method. Aliquots of 125 µL of the extract were added to 250 µL of a reaction medium containing 0.5% (w/v) thiobarbituric acid (TBA) and 10% (w/v) trichloroacetic acid (TCA), followed by incubation at 95°C for 30 minutes. The reaction was stopped by rapid cooling on ice, and readings were performed in duplicate using a spectrophotometer at wavelengths of 535 nm and 600 nm (Buege & Aust, 1978). Results were expressed in nmol MDA g^-1 FM.

Experimental design and statistical analysis

The experiment was conducted in a completely randomized design (CRD) in a 2x5 factorial scheme, consisting of two growing seasons (autumn-winter and spring-summer) and five cultivation environments (full sun, 30% black, 50% black, 50% red, and 50% blue shading nets), with five replicates composed of eight plants per plot.

For growth variables (plant height, collar diameter, and leaf number), evaluations were performed at 30, 60, and 90 days after sowing (DAS) to characterize plant development over time. However, statistical comparisons among treatments were performed considering only the final evaluation at 90 DAS, which represented the endpoint of vegetative growth and flower production. For biochemical analyses, evaluations were performed at 60 DAS, and for anatomical and yield variables at 90 DAS.

The data were tested for normality of errors (Shapiro-Wilk), homogeneity of variances (Bartlett’s test), and independence of residuals (Durbin-Watson). Subsequently, data were subjected to analysis of variance (ANOVA). When significant interactions between growing season and cultivation environment were detected, means were compared by Tukey’s test at 5% probability. All analyses were performed using Sisvar software (Ferreira, 2011).

Results and Discussion

Plant growth and development

Significant interaction between growing season and cultivation environment was observed for plant height and collar diameter at 90 DAS (p ≤ 0.05). The growth of Zinnia plants was significantly influenced by the growing seasons, as expected. Plants grown during the spring-summer period exhibited greater plant height and collar diameter compared to those in the autumn-winter. During spring-summer, when higher temperatures and light incidence occurred, plant height was superior in plants grown under 50% red, 50% blue, and 30% black shade nets, while it was reduced in those grown under full sun. The collar diameter showed higher values in plants maintained under 50% red and 30% black nets (Figure 2).

Figure 2:
Plant height (A) and collar diameter (B) of Zinnia elegans cv. ‘Luz da Lua’, evaluated at 30, 60, and 90 days after sowing (DAS), cultivated in autumn-winter and spring-summer growing seasons. Uppercase letters compare growing seasons and lowercase letters compare cultivation environments exclusively at 90 DAS. Means followed by the same letter do not differ according to Tukey’s test (p ≤ 0.05).

Plant development is influenced by factors such as temperature, humidity, and light, which regulate physiological processes like photosynthesis and cell elongation (Taiz, Zeiger & Møller, 2024). In the spring and summer, the occurrence of higher temperatures and long photoperiods, combined with greater light availability, significantly promoted plant growth. Conversely, during the autumn-winter period, the shade nets did not influence plant development; thus, light quality was not a crucial factor, and plants exhibited similar heights regardless of the netting. It was hypothesized that the spectral quality alterations provided by the photoselective nets could triggers morphogenic responses to partially compensate for the growth limitations imposed by lower temperatures. However, under the winter conditions, the naturally lower solar radiation combined with temperature limitations overrode the spectrum qualitative effects, resulting in similar plant heights across all treatments. Indeed, Zinnia shows better development under warm conditions, which are typical of the spring-summer season (Martins et al., 2021), contributing to more vigorous vegetative growth. Furthermore, during the period with higher ambient temperatures, not only was greater plant growth observed, but also the specific effect of light quality provided by the different shade nets.

Variation in plant height as a function of light intensity and quality under photo-selective nets is related to photomorphogenesis, the process by which light regulates vegetative growth (Taiz, Zeiger & Møller, 2024). In this response, photoreceptors act according to the transmitted light spectrum. Under red and blue nets, phytochromes (which are sensitive to red and far-red light) and cryptochromes (which respond to blue light and UV-A radiation) actively participate in plant development. This differs from environments under full sun or black nets, where light quality is not selectively filtered (Tokutomi, Okajuma & Yoshihara, 2015).

Light in the red and far-red regions is perceived by phytochromes, which play a key role in regulating stem elongation and, consequently, plant height (Kim et al., 2003). The red photo-selective net modulated the incidence of these wavelengths, directly influencing plant height (Figure 1), which was greater compared to plants grown under other conditions. Furthermore, these plants exhibited higher collar diameter values, indicating that no etiolation occurred. In contrast, light in the blue region, absorbed by cryptochromes and phototropins, acts on stomatal opening and phototropism, in addition to affecting plant size and development (Takemiya, Inoue & Doi, 2005; Boccalandro et al., 2012; Christie et al., 2015), as previously observed in Hydrangea (Nesi et al., 2011), Dracaena fragrans (Gaurav et al., 2016), and Dieffenbachia amoena (Khomami et al., 2023).

Significant interaction between growing season and cultivation environment was observed for number of leaves, aerial part fresh massa and aerial part dry mass at 90 DAS (p ≤ 0.05) (Figure 3). The highest number of leaves was observed in plants maintained under the 30% black net. The aerial part fresh mass was higher for plants grown under the 30% black and red nets. The aerial part dry mass did not differ significantly among the cultivation environments (Figure 3).

Figure 3:
Number of leaves (A), evaluated at 30, 60, and 90 days after sowing (DAS), aerial part fresh mass (B), and aerial part dry mass (C), evaluated at 90 DAS, of Zinnia elegans cv. ‘Luz da Lua’ grown in autumn-winter and spring-summer growing seasons. Uppercase letters compare growing seasons and lowercase letters compare cultivation environments exclusively at 90 DAS. Means followed by the same letter do not differ according to Tukey’s test (p ≤ 0.05).

The control of light intensity provided by the 30% black shade net creates a balance in the cultivation environment, allowing the plant to perform its metabolic processes without undergoing thermal stress. In Zinnia, 30% shading in the cultivation environment favored leaf production, which was reflected in increased biomass (Zhang et al., 2022). The influence of the red photo-selective net on the increase in shoot fresh mass may be related to the modification of the transmitted light spectrum, which increases the incidence of wavelengths in the red and far-red range. This accelerates the plant development process, promoting a higher number of leaves and stem elongation, resulting in greater fresh mass (Paredes-Jácome et al., 2019).

Maximum root length was greater during the spring-summer season, with no significant differences observed among the cultivation environments (Figure 4). Root fresh mass was higher in the spring-summer for plants grown under the 50% red net, 30% black net, and full sun. Root dry mass differed between seasons; in the autumn-winter, no differences were found among environments. However, during the spring-summer, the highest values occurred under the 30% black, 50% red, and 50% blue nets. When comparing only the cultivation environments, differences in root dry mass were observed exclusively during the spring-summer period (Figure 3).

Figure 4:
Maximum root length (A), root fresh mass (B), and root dry mass (C) of Zinnia elegans cv. ‘Luz da Lua’, evaluated at 90 days after sowing (DAS), grown in the autumn-winter and spring-summer growing seasons. Uppercase letters compare growing seasons and lowercase letters compare cultivation environments exclusively at 90 DAS. Means followed by the same letter do not differ according to Tukey’s test (p ≤ 0.05).

In some species, root growth may experience less impact from different cultivation environments because they are protected underground and not directly exposed to the environmental variations that affect the shoot (Miotto et al., 2021). The use of photo-selective or shade nets primarily alters the light conditions incident on the shoot. These effects are perceived by the roots through integrated physiological signals, such as the allocation of photoassimilated carbon. Under shading, plants often prioritize shoot growth over root development to maximize light interception, which can alter the shoot-to-root ratio even if overall photosynthetic rates are affected. This behavior may be related to higher photosynthetic activity during the spring-summer, when conditions such as higher light intensity and temperature favor the production of photoassimilates. The increased availability of carbohydrates can contribute to root growth, even if the roots are not directly exposed to variations in light quality (Pasternak et al., 2023). Thus, although the nets modify the light environment of the shoot, their effects may be indirectly reflected in the root system through the allocation of assimilates.

The flower production and flower stem length of Zinnia were significantly influenced by the growing season. The spring-summer period provided a higher total number of flowers and longer stems compared to the autumn-winter season (Figure 5).

Figure 5:
Total number of flower stems (A) flower stem length (B) of Zinnia elegans cv. Luz da Lua after 90 days grown in the autumn-winter and spring-summer growing seasons. Uppercase letters compare growing seasons and lowercase letters compare cultivation environments at 90 DAS. Means followed by the same letter do not differ according to Tukey’s test (p ≤ 0.05).

Among the cultivation environments, the highest total number of flowers was observed under the 30% black shade net, which differed significantly from the other treatments. For flower stem length, the best results were consistently obtained under the 50% red, 50% blue, and 30% black nets. The full sun environment resulted in the lowest number of flowers and shorter stems in both the autumn-winter and spring-summer seasons (Figure 5).

The superior performance of Zinnia during the spring-summer for flower production and stem elongation is directly associated with the more favorable climatic conditions of this period. Higher temperatures and greater solar radiation availability intensify photosynthetic activity, promote cell expansion, and stimulate the elongation of flower stems (Singh et al., 2023).

The difference between the two seasons demonstrates the sensitivity of Zinnia to thermal variations and reinforces temperature as a determining factor for the cultivation of this species. Similar results were reported by Pêgo, Carvalho and Martins (2021) in Z. elegans cv. ‘Gigante da Califórnia Branca’, where the full sun environment, 50% black net, and 20% black net significantly influenced the number of flowers and plant height.

The higher flower production under the 30% black net indicates that moderate shading creates more balanced conditions between light intensity and temperature, favoring the reproductive development of Zinnia. This result is consistent with the radiation values observed (Table 1), in which this environment presented intermediate light levels relative to full sun and the other shade nets. While full sun cultivation was characterized by higher light intensities (up to 1512 µmol m⁻² s⁻¹), potentially associated with radiation excess stress, the 50% shade nets more sharply reduced light availability, which may limit photosynthetic activity.

The 30% black net provided an intermediate condition, sufficient to sustain high photosynthetic rates without inducing significant light stress, thus favoring the allocation of photoassimilates toward flowering. Zhang et al. (2022) associated this behavior with the reduction of excess radiation stress, which can compromise photosynthetic capacity and limit investment in reproductive structures.

The flower stem length was favored not only by the 30% black net but also by the 50% red and 50% blue photo-selective nets. This result highlights the phenotypic plasticity of the species, which maintains its capacity for stem elongation even under moderate light intensity, provided the incident spectrum is suitable. As previously discussed, the spectral modulation promoted by the red and blue nets activates specific photoreceptors that regulate cell elongation, a response that extends to the flower stems. Nonetheless, these results are not conclusive, as other growth factors in Zinnia plants were not affected by spectral quality.

The full sun environment, characterized by high radiation and elevated temperatures, imposes stress conditions that reduce flower production and stem length. This behavior is corroborated by the radiation data (Table 1), in which full sun cultivation presented the highest light intensity values, reaching up to 1512 µmol m⁻² s⁻¹, while the shaded environments showed a reduction in these levels. This high radiation availability can induce photoinhibition and compromise photosynthetic efficiency, resulting in lower availability of photoassimilates for reproductive development. This effect is consistent with the findings of Bruner et al. (2025), who attributed the reduced flowering of Zinnia cv. ‘Gigante da Califórnia Branca’ under high light conditions to photoinhibition and hormonal imbalance.

Chlorophyll content

The chlorophyll a, b, and total indices were significantly higher in plants grown during the spring-summer compared to those from the autumn-winter season. However, there was no significant difference in the chlorophyll a:b ratio between the growing seasons or among the cultivation environments (Figure 6).

Figure 6:
Chlorophyll a (A), chlorophyll b (B), total chlorophyll and chlorophyll a/b ratio (C, D) of Zinnia elegans cv. Luz da Lua, evaluated at 30, 60, and 90 days after sowing, grown in the autumn-winter and spring-summer growing seasons. Uppercase letters compare growing seasons and lowercase letters compare cultivation environments exclusively at 90 DAS. Means followed by the same letter do not differ according to Tukey’s test (p ≤ 0.05).

In studies conducted with Zinnia (cv. ‘Red California Giant’), Begonia ‘Megawatt’, and Gladiolus under different light conditions, no variations were observed in the accumulation of chlorophyll a, b, and total either (Oliveira et al., 2024; Martins et al., 2021; Paulus et al., 2024).

The results of the present study suggest that spring-summer conditions provide a more favorable environment for chlorophyll biosynthesis, possibly due to the higher light availability and temperature. On the other hand, the absence of differences among the cultivation environments indicates that Zinnia exhibits stability in photosynthetic pigments even under variations in light intensity and spectral quality, evidencing a certain physiological resilience. However, distinct results were reported for species such as Antirrhinum majus and Lisianthus, in which the use of photo-selective nets promoted changes in the content of photosynthetic pigments (Li et al., 2017; Almeida, Calaboni & Rodrigues, 2021), indicating that the response to light quality varies from one species to another.

Anatomical Analyses

Plants grown during the spring-summer exhibited higher values for stomatal density, polar diameter, and equatorial diameter compared to those from the autumn-winter (Figure 7). Stomata on Zinnia leaves were found on the abaxial surface, characterizing the species as hypostomatous (Zoulias et al., 2018). Among the cultivation environments, the highest stomatal density values were observed in plants grown under 30% and 50% black nets, while full sun showed the lowest values, and the red and blue photo-selective nets displayed an intermediate behavior. Regarding stomatal size, there was a trend toward higher polar diameter values in plants grown under full sun and black nets, whereas the lowest values occurred under red and blue nets. For the equatorial diameter, the 30% black net showed the highest values, followed by full sun and the 50% black net. In general, the photo-selective nets resulted in smaller stomata.

Figure 7:
Stomatal density (A), polar diameter (B), and equatorial diameter (C) of Zinnia elegans cv. ‘Luz da Lua’ evaluated at 30, 60, and 90 days after sowing (DAS) grown in autumn-winter and spring-summer growing seasons. Uppercase letters compare growing seasons and lowercase letters compare cultivation environments exclusively at 90 DAS. Means followed by the same letter do not differ according to Tukey’s test (p ≤ 0.05).

These results indicate that both the season and cultivation conditions influence stomatal characteristics, reflecting morphophysiological adjustments of the plants to environmental variations. Some species demonstrate seasonal anatomical plasticity, adjusting the quantity and size of stomata as a strategy for physiological optimization under different light and temperature conditions (Driesen et al., 2020). This plasticity is also evident in the response to different cultivation environments.

The higher stomatal density under 30% and 50% black nets indicates an adaptive response to lower light intensity. Under moderate shading without spectral changes, the plant increases the number of stomata to maximize CO₂ uptake and maintain photosynthetic efficiency (Lau et al., 2018; Toscano & Romano, 2021). In contrast, the lower stomatal density under full sun reflects a water conservation strategy, reducing transpiration in an environment with high irradiance and evaporative demand (Rossatto, Hoffmann & Franco, 2009; Driesen et al., 2020).

It is also noted that this reduction in stomatal density was accompanied by an increase in stomatal size, expressed by the higher polar and equatorial diameters. Such morphological compensation allows for greater conductance per stoma, mitigating the limitation imposed by the lower number of structures and ensuring CO₂ assimilation (Franks et al., 2009). In plants grown under red and blue photo-selective nets, on the other hand, smaller stomata may provide an advantage in environments with light fluctuations, as smaller-sized stomata exhibit faster opening and closing responses, optimizing water use and CO₂ uptake (Drake et al., 2013).

Beyond the effect of intensity, spectral quality also modulates leaf anatomy. The lower stomatal density observed under red and blue nets, associated with smaller stomata, suggests that red and blue light, by activating specific photoreceptors, can influence guard cell differentiation and the stomatal distribution pattern (Zheng & Van Labeke, 2017). Results contrasting with this study were observed in ornamental sunflower, in which the use of photo-selective nets promoted higher stomatal density on the abaxial surface of the leaves compared to full sun cultivation (Nascimento et al., 2016).

It is important to highlight that all this variation in stomatal structure occurred without compromising chlorophyll indices, which indicates the robustness of the photosynthetic apparatus in Zinnia. Furthermore, anatomical plasticity is observed through the adjustment of stomatal density and size as a function of light intensity and quality. This allows the species to maintain photosynthetic efficiency under different cultivation conditions, either by maximizing CO₂ uptake in shaded environments or by water conservation in environments with high irradiance (Poorter, Pons & Reichgelt, 2025).

Figure 8 shows microscopic images of the paradermal sections, highlighting the variation in stomatal density among the treatments.

Figure 8:
Microscopic images showing hypostomatic stomata of Zinnia elegans cv. ‘Luz da Lua’ leaves, 90 days after sowing. Legend: Paradermal sections of Zinnia elegans leaves. A - 30% black shading net (16.25 stomata mm^-²); B - 50% black shading net (15.75 stomata mm^-²); C - 50% red photo-selective net (13.75 stomata mm^-²); D - 50% blue photo-selective net (12.75 stomata mm^-²); and E - Full sun (9.25 stomata mm^-²). Scale bar = 50 μm.

Biochemical Analyses

Enzymatic activity also varied between growing seasons, with higher values observed during the spring-summer (Figure 9). Regarding the effect of light conditions, superoxide dismutase (SOD) activity showed no significant difference. In contrast, catalase (CAT) exhibited higher activity in plants grown under 30% and 50% black nets. Ascorbate peroxidase (APX) showed superior activity under the 50% black net, differing from the other environments. Hydrogen peroxide (H₂O₂) levels were higher under the 50% red, 50% black, and 50% blue nets. Malondialdehyde (MDA) reached its highest concentration in plants grown under the 50% blue net, differing significantly from the other treatments (Figure 9).

Figure 9:
Activity of superoxide dismutase (SOD) (A), catalase (CAT) (B), ascorbate peroxidase (APX) (C), hydrogen peroxide (H₂O₂) (D), and malondialdehyde (MDA) (E) in Zinnia elegans cv. ‘Luz da Lua’ evaluated at 60 days after sowing (DAS) grown in autumn-winter and spring-summer growing seasons. Uppercase letters compare growing seasons and lowercase letters compare cultivation environments exclusively at 60 DAS; means followed by the same letter do not differ according to Tukey’s test (p ≤ 0.05).

The higher values for all biochemical variables observed during the spring-summer reflect the greater demand imposed by radiation and temperatures, conditions that favor the generation of reactive oxygen species (ROS) and, consequently, the activation of the antioxidant system (Pospíšil, 2016; Sharma et al., 2024). The differences among the cultivation environments indicate that the spectral quality of light, modified by the photo-selective and shading nets, can also modulate the antioxidant response, influencing the intensity of oxidative stress and the plants’ ROS scavenging mechanisms.

Superoxide dismutase (SOD) activity did not differ among the environments, indicating that the production of the superoxide radical (O₂⁻) was similar across all treatments. This stability suggests that the variations in light intensity, temperature, and spectral quality were not severe enough to induce primary oxidative stress, which would typically occur only under extreme conditions, such as water deficit, salinity stress, or threshold temperatures (Guo, Zhao & Guan, 2024; Khamis et al., 2025).

Unlike SOD, which showed no significant variation among treatments, CAT showed higher activity in plants grown under 30% and 50% black nets, while APX exhibited higher activity in plants kept under the 50% black net. These results indicate an adaptive response related to the scavenging of hydrogen peroxide, which is naturally formed in cellular metabolism, including as a byproduct of SOD activity. CAT and APX act in the decomposition of H₂O₂, preventing the formation of highly toxic hydroxyl radicals (Anjum et al., 2016). However, the lower CAT activity under the 50% blue net suggests a specific effect of blue light on oxidative metabolism. Radiation in the 400-500 nm range can stimulate photosynthetic processes and ROS generation; however, the reduction in CAT activity may have limited H₂O₂ removal, contributing to its accumulation and the lipid peroxidation observed under these conditions.

The higher H₂O₂ contents observed in plants grown under 50% red, black, and blue nets reflect an imbalance between ROS production and scavenging, especially during the spring-summer. In moderate concentrations, however, H₂O₂ acts as a signaling molecule, inducing the activation of the antioxidant system (Mittler, 2017; Xia et al., 2017).

In the autumn-winter, the accumulation of H₂O₂ under the 50% red net occurred even under lower light intensity, which may be related to the efficiency of radiation in the red spectrum (600-700 nm) in stimulating photochemical processes. However, this stimulation did not translate into greater plant growth, possibly due to increased ROS generation and the lower efficiency of scavenging mechanisms, such as reduced CAT activity at low temperatures (Dreyer & Dietz, 2018).

As a consequence of this imbalance, the increase in MDA levels under the 50% blue net evidences the occurrence of lipid peroxidation and damage to cellular membranes (Su et al., 2019). This result can be explained by the harmful interaction between light intensity and spectral quality in this environment, which favors photochemical excitation and ROS generation. Associated with this, the lower CAT activity suggests a limited capacity for H₂O₂ scavenging, contributing to its accumulation (Hogewoning et al., 2010). Furthermore, radiation in the blue range may intensify this process by modulating responses associated with oxidative stress, acting synergistically to increase oxidative damage to the membranes.

Thus, considering the effect of the growing seasons, it is observed that Zinnia shows better development under higher temperature conditions. In addition to inefficient development, the quality of the floral stems produced during the autumn-winter period is inferior (Figure 5A), failing to meet the standards for the cut flower market. The modification of spectral conditions was not effective in overcoming temperature limitations. In this context, the biochemical parameters indicate lower activation of the antioxidant system and potential accumulation of reactive oxygen species, reflecting a less efficient metabolism under these conditions (Figure 9).

In contrast, during the spring-summer season, the stems reached the quality required for commercialization as cut flowers (Figure 5). Analyzing the growing conditions during this period, the spectral modification yielded different results, indicating that plants exhibit superior quality, with better height development (Figure 2) and floral stem height (Figure 5B), when grown under the 50% red net and 30% black net, which also resulted in higher flower production. This is supported by the stomatal density observed in plants under the 30% black net, which presented approximately 159 stomata mm⁻², favoring gas exchange. Furthermore, the adjustment of the antioxidant system showed greater efficiency in controlling reactive oxygen species, limiting cellular damage even under conditions of higher metabolic demand.

Zinnia shows potential and viability for cut flower production; however, the cultivation environment is crucial to support plant development and ensure high-quality floral stems.

Conclusions

Spring-summer cultivation is more efficient due to the more favorable climatic conditions for the development of Zinnia elegans cv. ‘Luz da Lua’. The use of shading nets is an effective strategy to optimize the cultivation of this species when intended for cut flower production. Photoconversion nets improve stem quality by promoting greater length. Specifically, the 30% black shading net provides higher flower production and offers a balanced environment for the crop.

Acknowledgments

This study was supported by the Coordination for the Improvement of Higher Education Personnel - Brazil (CAPES) - Funding Code 001 and National Council for Scientific and Technological Development (CNPq).

Data Availability Statement

Data available upon request to authors.

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• Editor de seção:
  Renato Paiva http://orcid.org/0000-0001-5107-0285

Publication Dates

• Publication in this collection
  17 Aug 2026
• Date of issue
  2026

History

• Received
  12 May 2026
• Accepted
  16 July 2026