Shading screens to protect ‘Hass’ avocado trees from afternoon solar radiationAbstract:
The main objectives of this study were to evaluate the effect of shading screens on fruit set, development and quality of ‘Hass’ avocado trees exposed the afternoon solar radiation. The experiment was conducted in a commercial ‘Hass’ avocado orchard, using a randomized block design with five treatments and five replicates, with two plants per experimental plot. Control treatment (noscreen), 42.5% Luxinet screen, 8% white screen, 30% red screen and 50% black screen were used to protect plants from the afternoon sun. Chlorophyll a, b and total contents were determined; fruit set; yield (t ha-1), with fruits being classified as small, medium and large. The results have shown that the 8% white screen favors higher productivity and greater number of medium and large ‘Hass’ avocado fruits. The side of the canopy under morning sun favored greater number of‘Hass’ avocado fruits. Further studies are needed using red and black shading screens, which at 30% and 50%, respectively, result in higher percentage of medium and large ‘Hass’ avocado fruits.
Index terms
Persea americana Mill.;; abiotic stress;; sun protection
Resumo:
Os principais objetivos deste trabalho foram avaliar a interferência de telas de sombreamento na frutificação, no desenvolvimento e na qualidade do abacate ‘Hass’ exposto à radiação solar da tarde. O experimento foi conduzido em um pomar comercial de abacateiro ‘Hass’, utilizando um delineamento em blocos casualizados, com cinco tratamentos e cinco repetições, com duas plantas por parcela experimental. Um tratamento de controle (sem tela), 42,5% de tela Luxinet, 8% de tela branca, 30% de tela vermelha e 50% de tela preta foram utilizados para proteger as plantas do sol da tarde. Foram determinados os teores de clorofila a, b e total; a frutificação; a produtividade (t ha-1), e os frutos foram classificados em pequenos, médios e grandes. Os resultados mostraram que a tela branca de 8% favorece maior produtividade e maior número de frutos médios e grandes no abacateiro ‘Hass’. O lado da copa sob o sol da manhã favoreceu maior número de frutos do abacateiro ‘Hass’. São necessários mais estudos para as telas desombreamento vermelha e preta, que a 30% e 50%, respetivamente, que resultam em alta porcentagem de frutos médios e grandes de abacateiro ‘Hass’.
Termos para indexação
Persea americana Mill.;; estresse abiótico;; proteção solar
Introduction
Avocado crops (Persea americana Mill.) have shown significant growth in planted area in recent years, driven by favorable prices in both domestic and international markets.
This trend has positioned this fruit as an attractive option for fruit producers to diversify investments in new orchards. According to IBGE (2021), the cumulative avocado production increased from 300.874 tons in 2021 to 338.238 tons in 2022, with the states of São Paulo, Minas Gerais, and Paraná standing out as leading producers.
In Brazil, tropical avocado varieties account for the largest share of production and cultivated area. However, the ‘Hass’ (subtropical) avocado variety has shown a significant increase in planted area and production, mainly in response to rising international demand (GARCIA et al., 2021). This growth scenario is largely due to its health benefits, as it is rich in monounsaturated fatty acids and antioxidants (WANG et al., 2019; FORD et al., 2023). This growing popularity has made the cultivation of the ‘Hass’ avocado variety a promising investment strategy.
Estimates suggest that by 2030, avocados will solidify their position as one of the most globally traded tropical fruits, with exports exceeding four million tons (FAO, 2024).
However, despite this outlook, Brazil’s share in the global avocado trade remains modest, accounting for only 3% of the total production volume, highlighting significant untapped potential. Recently, the opening of new markets for Brazilian avocados indicates a growing trend in the country’s fruit exports (CNA, 2024).
However, avocado orchard management faces challenges that limit production in terms of both volume and quality. Among the main limiting factors are the low fruit set rate, which is essential for increasing orchard productivity, and sunburn damage to leaves and fruits, caused by excessive solar radiation (BHORE et al., 2021).
In studies conducted with ‘Hass’ avocado trees, Pereira et al. (2022) attributed the low fruit set rate to the high temperatures recorded between July and August, the flowering period of plants, particularly on the canopy side exposed to afternoon sunlight.
This condition affects the visitation of pollinators, such as Apis mellifera, which, even when influenced by food attractants, visited this side of plants less frequently compared to the side receiving morning sunlight. As reported by Garner and Lovart (2016), the primary cause of flower and fruit abortion in ‘Hass’ avocado trees is related to fertilization issues. These issues are linked to pollen grain germination and pollen tube growth, both of which are adversely affected by high temperatures.
The cultivation of ‘Hass’ avocados under shading nets of different colors has been identified as a tool for improving the marketable yield of fruits, also influencing their ripening patterns during storage (PETER et al., 2018). In addition to the direct benefits to fruit quality, protective nets also help reduce wind speed, canopy temperature, and maintain relative humidity, thereby promoting shoot growth and enhancing the plants’ photosynthetic response (MUPAMBI et al., 2018).
Investing in technologies that improve avocado quality is even more crucial when considering its export potential, as this marketing channel offers significantly higher returns of investment to producers.
Therefore, adopting strategies to mitigate sunburn damage (DOMINGUES NETO et al., 2024), wind-induced injuries, and smallsized fruits (BLAKEY et al., 2016) is essential, particularly for ‘Hass’ avocados, which must meet export standards.
In this context, the objective of this study was to evaluate the effect of shading nets in the protection of ‘Hass’ avocado trees from afternoon solar radiation on fruit set and development.
Material and Methods
The experiment was conducted in a commercial orchard during the 2022/2023 growing season, using four-year-old ‘Hass’ avocado trees grafted onto ‘Margarida’ rootstock.
The orchard was planted in an East- West row orientation with spacing of 8 x 5 m, totaling 250 plants per hectare. The orchard has a drip irrigation system with two lines and is located in the municipality of Arealva, state of São Paulo, Brazil (20°04’51” S, 49°54’52” W, 500 m a.s.l.).
The climate of the region where the experiment was conducted is classified, according to the Köppen-Geiger system as Aw, which represents tropical climate with dry winter. The average annual temperature and precipitation are 22.3 °C and 1,267 mm,respectively.
The experimental design used was a randomized block design with five treatments and five replicates, using two plants per experimental plot. Treatments consisted of different types of shading nets used to protect the canopy of ‘Hass’ avocado plants from afternoon sunlight. Treatments were as follows: control (no net), 42.5% Luxinet, 8% white net, 30% red net, and 50% black net, with each treatment covering a total length of 50 meters.
The shading nets were installed in the experimental area during the first week of August 2022, prior to flowering. They were supported at both ends by 5 meters height eucalyptus poles and buried 1 meter deep, totaling 6 meters. Additionally, two smooth wire strands were fixed to support the nets at heights of 1.65 meters and 4 meters above the ground, respectively. The structure was assembled at a distance of 0.80 meters from the canopy on the side exposed to afternoon sunlight. To enhance fruit set, two Apis mellifera beehives per hectare were placed in the orchard for pollination during the flowering period (August and September).
To evaluate the effect of treatments on ‘Hass’ avocado trees, the foliar chlorophyll content was determined using samples of mature leaves collected from the middle third of plants on the side exposed to afternoon sunlight, which were protected by the different shading nets. From this analysis, the a, b, and total chlorophyll concentrations (mg cm-3) in the reading solutions were determined using methodology proposed by Arnon (1949).
The total number of fruits per plant was determined by counting the number of fruits divided between fruits produced on the canopy side exposed to the afternoon sun (protected by shading nets) and those produced on the side exposed to the morning sun. This assessment was conducted 30 days after anthesis, when fruit set was observed across the different treatments.
The productive characteristics were measured when fruits from each treatments reached average dry matter content of 23%, the standard used for harvesting ‘Hass’ avocados intended for export (ABPA, 2019).
All fruits from the analyzed plants in each treatment were harvested, counted, and weighed in the field to estimate the production per plant and yield (t ha-1), considering a population of 250 plants per hectare.
To classify fruits as small (<120 g), medium (120–180 g), and large (>180 g), all fruits from each evaluated plant were individually weighed and categorized. Additionally, fruits from the morning sun-exposed side and the afternoon sun-exposed side of plants were separated. This classification aligns with the international standard used for ‘Hass’ avocado marketing, which is based on fruit size: large fruits correspond to CAT1 (>180 g), CAT2 (120–180 g), and CAT3 (<120 g) (WHO, 2013). In addition to fruit weight, the international classification also considers visible defects on the fruit peel.
Data on chlorophyll content were submitted to analysis of variance, and the means were compared using the Tukey’s test at 5% significance level. For fruit set rate and yield data, a 2x5 factorial arrangement was considered, with the first factor represented by the position of the plant canopy (exposed to the morning and afternoon sun) and the second factor corresponding to the types of shading screens (control, 42.5% luxinet, 8% white net, 30% red net and 50% black net). All analyses were performed using the SISVAR software 5.8 version (System for Analysis of Variance).
Results and Discussion
The shading nets significantly affected (p<0.05) the a, b, and total chlorophyll levels in the leaves of ‘Hass’ avocado trees, indicating that the use of this technology may directly influence the plants’ photosynthetic response (Table 1). When analyzing fruit set and plant productivity, it was observed that there was no significant interaction (p>0.05) between shading screens and the canopy position of the ‘Hass’ avocado plants exposed to morning and afternoon solar radiation.
However, it is worth mentioning that a significant isolated effect (p<0.05) was observed for the number of fruits harvested 30 days after anthesis and the final production as a function of the different shading nets and the position of the plant canopy (Table 1).
It was observed that canopy protection with red and black shading nets resulted in higher levels of photosynthetic pigments compared to the other treatments, particularly in terms of total chlorophyll content (Figure 1C).
Chlorophyll concentration in leaf tissues serves as an indicator of a plant’s susceptibility to light intensity. Due to its chemically unstable structure, chlorophyll is easily degraded, especially under high light intensities (WANG et al., 2022). Considering that under field conditions, ‘Hass’ avocado trees are exposed to various biotic and abiotic factors, and that leaves are the primary photosynthetic organs of plants, responsible for significant dry matter accumulation, chlorophyll degradation can potentially restrict plant growth and development, limit production, or directly or indirectly affect the quality of harvested fruits (RONDON et al., 2024).
Light plays a crucial role in driving carbon metabolism in plants, with quality and intensity significantly impacting photosynthesis as well as other morphological, physiological, and biochemical parameters (SHAFIQ et al., 2021). As reported by Rodriguez et al. (2017), in avocado trees, chlorophyll content varies significantly depending on the age of plants and the position of leaves within the canopy, being further directly influenced by climatic conditions.
Chlorophyll a (a), chlorophyll b (b) and total chlorophyll (c) contents of ‘Hass’ avocado leaves under different shading screens used to protect from afternoon solar radiation.The values of parameters presented in each figure are indicated as mean and standard deviation (n=5). Different letters denote statistically differences between different treatments (Tukey test, P = 0.05).
Shading alters environmental factors, including light intensity and quality (FIORUCCI; FANKHAUSER, 2017). Under shaded conditions, increases in total chlorophyll, chlorophyll a, and chlorophyll b levels are observed (ILIĆ et al., 2017; CHEN et al., 2021), resulting in a more intense green coloration of leaves. High chlorophyll levels are essential for normal photosynthesis in plants. Studies with different shading levels (0%, 30% and 75%) revealed that chlorophyll, carotenoid, and soluble sugar contents followed the pattern: S75% > S30% > S0%. This increase in chlorophyll content is attributed to the activation of protochlorophyllide oxidoreductase (HAN et al., 2023).
Studies using electron microscopy have demonstrated that shading induces the development of plastids in tea leaves, leading to an increase in the number of chloroplasts and the compaction of thylakoids (LIU et al., 2020). Chlorophyll a is typically associated with the reaction centers involved in photosynthesis, while chlorophyll b plays a role in light capture. The chlorophyll a/b ratio is reduced under shading conditions (LIU et al., 2020; CHEN et al., 2021).
This reduction enhances light energy capture due to the increased accumulation of chlorophyll b, an accessory pigment in photosynthesis that broadens the spectrum of light used in the process. Thus, the higher chlorophyll b content represents an adaptation to shading, enabling the plant to absorb a wider range of light wavelengths and maintain more efficient photochemical reactions. As a result, there is greater carbohydrate production under shaded conditions (ILIĆ et al., 2018).
Different types of shading nets reduce the solar radiation reaching plants and generally increase chlorophyll concentration (ILIĆ et al., 2017).
The total chlorophyll content depends on the type of shading and the plant genotype (ILIĆ et al., 2019). Studies have found that blue and black nets induce a more significant increase in chlorophyll concentration compared to red and Luxinet “pearl” nets, with the increase being more pronounced in chlorophyll b than in chlorophyll a (ILIĆ et al., 2017).
On the other hand,other findings showed higher chlorophyll b and total chlorophyll values in red photoselective nets, regardless of cultivar used (ALMEIDA et al., 2021). In another study, yellow nets promoted the highest increase in chlorophyll content in Ficus carica (JOKAR et al., 2021). In the present study, red and black nets increased the total chlorophyll, chlorophyll a, and chlorophyll b levels compared to other treatments (Figure 1). These increases contributed to higher production of medium and large-sized fruits (Figures 4 and 5).
The white net resulted in the highest number of fruits set, differing significantly only from the Luxinet shade net, which showed the lowest number of fruits set (Figure 2A).
Final plant productivity was more affected by the treatments, particularly with the use of the red net and the Luxinet net, which led to less productive plants (Figure 2B).
Fruit set rate (a) and final yield (t ha-1) (b) of ‘Hass’ avocado plants under different shading screens used to protect against afternoon solar radiation. The values of parameters presented in each figure are indicated as mean and standard deviation (n=5). Different letters denote statistically differences between different treatments (Tukey test, p = 0.05).
The use of shading nets is already widespread in protecting numerous crops from excessive heat, cold, hail, wind, and solar radiation (SHAHAK, 2014; ZAIT et al., 2020).
Alon et al. (2022) suggest that 60% silver shading nets can potentially reduce thermal stress in ‘Hass’ avocado plants during extreme heat events. However, they emphasize the need for further studies in different cultivation regions to evaluate their effects on flowering, fruit set, fruit size, and quality.
El-Naby et al. (2020) evaluated the mitigation of the effects of thermal stress using shading nets on ‘Washington Navel’ orange trees grown in the Al-Nubaria region of Egypt, and found that white nets reduce the effects of stress by up to 25%.
In similar studies with ‘Honeycrisp’ apple trees grown in pots and submitted to varying thermal stress intensities and high light exposure, blue shading nets with 22% retention reduced the effects of solar radiation and improved the efficiency of photosynthetic light use at the leaf level (MUPAMBI et al., 2018). However, investigations about the ability of shading nets to mitigate the adverse effects of extreme climatic events on avocado trees remain limited.
Analyzing the productive aspects in isolation based on the position of the plant canopy exposed to morning and afternoon sunlight, it was observed that afternoon solar radiation negatively affected the fruit set rate and the final crop productivity (Figure 3A and B). These findings are consistent with the results reported by Pereira et al. (2022), which indicate that the production of ‘Hass’ avocados, even under subtropical conditions, is not uniform across both canopy positions and is more susceptible to the negative effects of afternoon solar radiation.
Therefore, the application of cultural practices that mitigate these adverse effects is essential for achieving higher yields, particularly when applying technologies that do not harm agroecosystem agents.
Fruit set rate (a) and final yield (t ha-1) (b) of ‘Hass’ avocado plants in the portions of the canopy with morning sun (without shading screens) and afternoon sun (with shading screens). The values of parameters presented in each figure are indicated as mean and standard deviation (n=5). Different letters denote statistically differences between different treatments (Tukey test, p = 0.05).
‘Hass’ avocados produced in Brazil are primarily sold on the international market, where they achieve higher added value. Avocados without major defects, such as skin blemishes, spots caused by scab or anthracnose, are classified as CAT1 and CAT2, which correspond to larger weights and higher commercial value.
In contrast, those classified as CAT3, weighing less than 120 grams or showing defects, are used for oil extraction, frozen pulp production, or discarded. This context is important because, in addition to the number of fruits harvested after pollination (Figure 2A), which directly influences orchard productivity, fruit quality directly impacts the profitability of the activity.
The use of shading nets to protect against afternoon solar radiation influenced the percentage of fruits classified as large, medium, and small, corresponding to CAT1, CAT2, and CAT3 categories, respectively.
Analyzing the fruits harvested from the canopy side exposed to morning sunlight, higher production of medium-sized fruits was observed, regardless of treatment (Figure 4A). Furthermore, plants under red shading nets produced higher percentage of CAT1 and CAT2 fruits, which show greater market value. This trend became even more evident when analyzing the canopy side exposed to afternoon solar radiation (Figure 4B), where red nets, along with white and black nets, stood out by yielding higher percentage of CAT1 and CAT2 fruits (Figure 4A).
Percentage of small (S), medium (M) and large (L) ‘Hass’ avocados produced in the portions of the canopy with morning sun (a) and afternoon sun (b), as a function of the use of different shading screens.
The absence of a shading net or the use of the Luxinet net, particularly on the canopy side exposed to afternoon sunlight, significantly favored the production of small fruits.
As previously mentioned, these treatments showed lower chlorophyll levels, which impacted the plants’ photosynthetic activity and, consequently, the dry matter accumulation.
This explains the high percentage of smaller fruits observed under these types of solar protection (DOMINGUES NETO et al., 2024).
The adoption of white (8%), red (30%), and black (50%) shading nets can be considered effective strategies to mitigate abiotic stress, such as that caused by excessive solar radiation, and to promote the production of larger fruits. Additionally, there are reports that the use of these shading nets may also help reduce wind damage (BLAKEY et al., 2016), a critical factor for the commercialization of ‘Hass’ avocados, as it ensures production that meets export standards.
Analyzing the overall fruit classification from both canopy positions of ‘Hass’ avocado trees, it was observed that the red shading net resulted in the highest percentage of medium and large fruits, with corresponding decrease in the number of small fruits (Figure 5). Additionally, it was observed that the control treatment and the use of the Luxinet shading net were the only cases where the percentage of small, non-commercial fruits exceeded that of medium and large fruits.
These commercially valuable categories yield higher market prices and consequently provide greater profitability to the producer.
Total percentage of small (S), medium (M) and large (L) ‘Hass’ avocado fruits under different shading screens used to protect against afternoon solar radiation.
The high percentage of low-standard commercial fruits under the white shading net may be associated with its low retention of just 8%, suggesting the need for further studies to validate its effectiveness in mitigating the harmful effects of high solar radiation and/or temperature.
The use of shading nets with either low or high density can have negative effects on the growth and development of perennial trees, which have long cycles, potentially reducing productivity by either inducing or inhibiting vegetative growth at the expense of yield (DOVJEK et al., 2020).
Considering the projections of the Intergovernmental Panel on Climate Change (IPCC), which estimate global warming of 1.2 to 3.0°C by 2050 depending on greenhouse gas emission pathways (CHUNG et al., 2022), there is growing concern regarding production areas for various fruit crops, particularly avocados. These changes in temperature patterns, combined with intense solar radiation in cultivation regions, may restrict the growth of commercial avocado cultivars (ALON et al., 2022; DOMINGUES NETO et al., 2024; RONDON et al., 2024).
Conclusion
Canopy protection using red and black shading nets resulted in higher total chlorophyll content. The red shading net (30%) promoted the highest number of medium and large ‘Hass’ avocado fruits, while the white shading net (8%) resulted in the lowest number of small and low-commercial-value fruits. The canopy side exposed to morning sunlight produced higher number of ‘Hass’ avocado fruits. Further studies on the use of red and black shading nets should be carried out, as these, at 30% and 50% coverage, respectively, led to higher percentage of medium and large ‘Hass’ avocado fruits.
Data Availability
The data that support the findings of this study are available from the corresponding author, Paula, M.R.S., upon reasonable request.
Acknowledgments
We thank ROMA and CITROPACK screen industries for providing the shading nets for this research.
References
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Edited by
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Scientific Editor
Alexandre Pio Viana
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Associate Editor
Juliana Domingues Lima
