Effect of shading on quizalofop-p-ethyl selectivity in Provisia riceABSTRACT:
Herbicides that inhibit the acetyl coenzyme-A carboxylase enzyme are used as an alternative for controlling imidazolinone-resistant weedy rice (Oryza sativa f. spontanea). Selective control can only be achieved in resistant cultivars, and environmental conditions can interfere with this selectivity. The objective of this study was to evaluate the effect of shading on quizalofop-p-ethyl (QPE) selectivity in resistant hybrid rice 132 PV. The field experiment was conducted in a randomized block design with four replications. Treatments consisted of 24-h shading periods relative to the timing of two QPE applications: before the first application; after the first application; before and after the first application; before the second application; after the second application; before and after the second application; before the first and second applications; after the first and second applications; normal light conditions; and an untreated control. The following variables were evaluated: chlorophyll index, phytotoxicity, plant canopy cover fraction, yield components, and grain yield. Shading before and after the second application resulted in the highest phytotoxicity symptoms at 21 days after application. The plant canopy cover fraction did not differ from the untreated control in any of the treatments. The chlorophyll index was higher 3 days after the second application in treatments with shading after as well as before and after that application. The yield components and grain yield were not affected. Shading, especially for prolonged periods during the second QPE application, has the potential to increase phytotoxicity symptoms in Provisia™ rice but does not interfere with yield components or the grain yield. These results contribute to understanding the light influence on QPE selectivity in Provisia™ rice and may help optimize application timing, especially in El Niño years with reduced solar radiation availability.
Key words:
phytotoxicity; 132 PV; ACCase inhibitors; weedy rice; Oryza sativa L
RESUMO:
Uma alternativa para o controle de arroz-daninho (Oryza sativa f. spontanea) resistente à imidazolinonas é o uso de herbicidas inibidores da enzima acetil coenzima-A carboxilase. O controle seletivo só pode ser realizado em cultivares com resistência, cujas condições ambientais podem interferir na seletividade. Neste sentido, o objetivo deste estudo foi avaliar o efeito do sombreamento na seletividade de quizalofope-p-etílico (QPE) ao arroz híbrido com resistência 132 PV. Para isso, o experimento foi conduzido a campo em delineamento de blocos casualizados com quatro repetições. Os tratamentos foram períodos de sombreamento durante 24 horas em relação as datas das duas aplicações de QPE: antes da primeira aplicação; após a primeira aplicação; antes e após a primeira aplicação; antes da segunda aplicação; após a segunda aplicação; antes e após a segunda aplicação; antes da primeira e segunda aplicação; após a primeira e segunda aplicação; condições normais e testemunha sem aplicação. Além disso, as variáveis analisadas foram índice de clorofila, fitotoxidade, fração de cobertura das plantas, componentes produtivos e rendimento de grãos. Os resultados revelam que o sombreamento antes e após a segunda aplicação resultou nos maiores sintomas de fitotoxidade aos 21 dias após a aplicação. A fração de cobertura das plantas não se diferenciou da testemunha sem aplicação em nenhum dos tratamentos. Os índices de clorofila foram maiores três dias após a segunda aplicação nos tratamentos com sombreamento após, e antes e após essa aplicação. Ademais, Os componentes produtivos e o rendimento de grãos não foram afetados. Portanto, o sombreamento, principalmente por período prolongado na segunda aplicação de QPE, possui potencial para aumentar os sintomas de fitotoxidade em arroz Provisia™, mas não de causar interferência nos componentes produtivos e rendimento de grãos. Esses resultados contribuem para a compreensão da influência da luminosidade na seletividade do herbicida QPE em arroz Provisia™ e podem auxiliar no posicionamento das aplicações, principalmente em anos de El Ninõ com menor disponibilidade de radiação solar.
Palavras-chave:
fitotoxidade; 132 PV; inibidores de ACCase; arroz-daninho; Oryza sativa L
INTRODUCTION
Rice (Oryza sativa L.), the second most widely cultivated cereal and one of the most consumed worldwide, plays a strategic role in food security. In addition to being an important energy source, it provides vitamins, minerals, and essential amino acids, serving populations with different socioeconomic levels, especially those with lower purchasing power, due to its relatively lower cost compared to other cereals and contribution of approximately 20% of caloric intake (FUKAGAWA & ZISKA, 2019; SOSBAI, 2025). In the 2022/23 growing season, global rice production was approximately 525.6 million tons with a per capita consumption of 52.6 kg per year (FAO, 2024). Brazil stands out among the world’s largest producers with annual paddy rice production ranging from 10 to 12 million tons in recent seasons. Approximately 70% of national production comes from the state of Rio Grande do Sul, where the minimum tillage system with direct seeding predominates, accounting for 51.7% of the total irrigated rice area (IRGA, 2025; SOSBAI, 2025).
In this production context, weed interference is one of the main factors limiting yield, as grasses, sedges, and broadleaf species compete with rice for water, light, and nutrients (OLAJUMOKE et al., 2016). Among weeds, weedy rice (Oryza sativa f. spontanea) stands out due to its origin from the de-domestication of cultivated rice, which has conferred morphological, genetic, and growth behavior traits similar to those of cultivated rice (QIU et al., 2017). These similarities hinder its management, making it more difficult to control than other weed species (DELOUCHE et al., 2007), especially when using chemical control based on selectivity (OLOFSDOTTER et al., 2000). The release of Clearfield™ rice technology enabled selective post-emergence application of acetolactate synthase (ALS)-inhibiting herbicides from the imidazolinone chemical group, which aims to control weedy rice populations (SUDIANTO et al., 2013). However, the continuous use of this technology, driven by intensive monocropping, has resulted in the evolution of resistance in weedy rice biotypes, mainly due to gene flow between herbicide-resistant and weedy rice (BZOUR et al., 2018). Other factors that have contributed to this scenario included the repeated use of the same herbicides and modes of action (OFOSU et al., 2023) and the occurrence of spontaneous mutations in weedy rice populations (SALES et al., 2008). These factors pose a risk for the evolution of resistance in other technologies.
Technologies conferring resistance to acetyl coenzyme-A carboxylase inhibitor (ACCase) herbicides represent an alternative for managing weedy rice biotypes that are resistant to imidazolinone herbicides. In Brazil, these technologies were commercially introduced during the 2022/23 growing season and confer resistance to herbicides from the aryloxyphenoxy-propionates chemical group, specifically propaquizafop and quizalofop-p-ethyl (QPE) in the Max-Ace® (RICETEC, 2024) and Provisia™ (PV) (BASF CORPORATION, 2024) platforms, respectively. Based on recommendations, PV technology enables up to two post-emergence applications of the herbicide Provisia® 50 EC (50 g QPE L-¹) at rates ranging from 70 to 120 g a.i. ha-¹ to exclusively control weed species from the Poaceae family (GODARA et al., 2022; HINGA et al., 2016). This technology was developed from a mutation in the gene encoding the ACCase enzyme, conferring QPE resistance due to enzyme insensitivity. A single dominant gene governs the target-site resistance mechanism in QPE rice, but the consistency of resistance mechanism expression is line specific and could vary depending on environmental conditions (CAMACHO et al., 2020).
Among these environmental factors, the influence of the El Niño-Southern Oscillation during spring and early summer stands out in southern Brazil. When it occurs in the El Niño phase, this phenomenon, characterized by ocean-atmosphere interactions, increases rainfall and cloudiness, reducing the incidence of global solar radiation (FONTANA, 2024). Lower radiation levels during plant growth cause stress and limit production, especially in C3 species, such as rice, which have a lower carbon assimilation efficiency and may be more affected by abiotic stress. In these plants, the electron transport rate can be significantly reduced under low light availability (TAIZ et al., 2017).
In addition to direct effects on plant physiology, light intensity, as well as air temperature and soil moisture, also influence the efficacy of ACCase-inhibiting herbicides by affecting the physicochemical processes of target plants, such as absorption, translocation, and metabolism (VARANASI et al., 2016). In this context, lower temperatures and prolonged cloudy conditions have been shown to increase QPE phytotoxicity in resistant cultivars after post-emergence applications (GODARA et al., 2022). However, the influence of low light intensity on the response of QPE-resistant rice genotypes to herbicide application is still poorly understood under southern Brazil growing conditions. Therefore, the present study aimed to evaluate the effects of shading on QPE herbicide selectivity in PV rice.
MATERIALS AND METHODS
The experiment was conducted under field conditions during the 2024/25 growing season at the lowland experimental area of the Departamento de Fitotecnia at the Universidade Federal de Santa Maria, Santa Maria, RS, Brazil (29.7207° S and 53.7224° W). According to the Köppen classification, the region has a humid subtropical climate (Cfa) with no defined dry season (ALVARES, 2013). The soil is classified as an Planossolo Háplico Eutrófico arênico, belonging to the Vacacaí mapping unit (SANTOS et al., 2018).
A randomized block design with ten treatments and four replications was used. Treatments consisted of 24-h shading periods relative to the timing of the two QPE applications: before the first application; after the first application; before and after the first application; before the second application; after the second application; before and after the second application; before both the first and second applications; after both the first and second applications; normal conditions; and an untreated control without QPE application. To simulate shading conditions in rice, black high-density polyethylene (HDPE) shade cloths that reduced solar radiation by 50% were used. The shade cloths were installed 1.5 m above the plant canopy and extended 2.0 m and 5.0 m laterally beyond the experimental units. Two post-emergence QPE (Provisia® 50 EC, 50 g a.i. L-1; BASF SE, Ludwigshafen, Germany) applications were performed at a rate of 80 g a.i. ha⁻1 along with the adjuvant Dash® vegetable oil (0.5% v v-1; BASF SE, Ludwigshafen, Germany). Applications were carried out using a tractor-mounted sprayer equipped with hollow cone ceramic nozzles (Mag 1 MJ052 80°, blue), operating at 620 kPa with a spray volume of 100 L ha-1, a travel speed of 5.5 km h-1, and a boom height of 0.60 m above the plant canopy. Applications were conducted under suitable environmental conditions with a wind speed of 5-10 km h-1, relative humidity above 60%, temperature below 30 °C, and no dew present at the time of spraying. The first application was performed at 14 days after emergence (V4) on November 13 and the second at 26 days after emergence (V6) on November 25. Control plots, which did not receive QPE, were protected during application with black impermeable tarps, fully covering each experimental unit to prevent spray drift and contamination.
Sowing was carried out on October 21, 2024, at a seeding rate of 40 kg ha-1 with hybrid rice cultivar 132 PV, which is resistant to QPE (Provisia® 50 EC, 50 g a.i. L-1). Basal fertilization consisted of 17.5, 70, and 70 kg ha-¹ of N, P₂O₅, and K₂O, respectively. At the V7 growth stage, 110 and 52 kg ha-1 of N and K₂O were applied; respectively, followed by an additional 50 kg N ha-1 at the R0 stage, following the scale described by COUNCE et al. (2000).
For weed management, the following herbicides were applied at the S3 stage: glyphosate (Braddock SL, 1920 g a.e. ha-1), pendimethalin (Prowl® H2O, 1137.5 g a.i. ha-1), and clomazone (Gamit® 360 CS, 144 g a.i. ha-1). Due to crop rotation in the area and low weed pressure, no additional post-emergence applications were required. Other crop management practices were performed following the Arroz Irrigado: Recomendações Técnicas da Pesquisa para o Sul do Brasil (SOSBAI, 2022). One day after the second post-emergence QPE application, the first topdressing fertilizer was applied, and flood irrigation was initiated at a 10-cm water depth when the crop was at the V7 stage, which was maintained until the R8 stage.
Photosynthetically active radiation (PAR) was measured for 24 h before and after the applications in both shaded and non-shaded areas. Measurements of the photosynthetic photon flux density (PPFD; µmol m-² s-1) were recorded using an LI-191R Line Quantum Sensor coupled to an LI-1400 Data Logger (LI-COR Environmental, Lincoln, NE, USA), which records (Figure 1A and 1B). Data on the global solar radiation and mean, maximum, and minimum air temperatures were obtained from automatic weather station A803, located in Santa Maria, RS, Brazil, and provided by the Meteorological Database for Teaching and Research (BDMEP) of Brazil’s National Institute of Meteorology (INMET, 2025) (Figure 1).
Photosynthetically active radiation (PAR) in shaded and non-shaded areas, global solar radiation (MJ m-²), and temperature (°C), measured 24 h before and after the first (A) and second (B) quizalofop-p-ethyl application. Daily global radiation and mean, maximum, and minimum temperatures on days close to quizalofop-p-ethyl applications (C). Source: Author’s data and INMET (Automatic Weather Station A803 - Santa Maria, RS, 2025).
The following variables were analyzed: chlorophyll index, phytotoxicity, plant canopy cover fraction, yield components, and grain yield. The chlorophyll index was evaluated using a portable Minolta SPAD-502 chlorophyll meter (Konica Minolta, Osaka, Japan) with the sensor placed at the midpoint of the last fully expanded leaf with a visible collar on 10 randomly selected plants at 3, 5, 7, and 11 days after the first application (DAFA) and 3, 5, 7, and 14 days after the second application (DASA). Phytotoxicity was visually evaluated at 7, 14, 21, and 28 days after application (DAA), in which 21 and 28 DAA corresponded to 7 and 14 DASA, respectively, using a visual scale ranging from 0 to 100% with 0 indicating no phytotoxicity symptoms and 100% indicating plant death. The plant canopy cover fraction was determined using the Canopeo mobile application based on images captured at a height of 1.5 m above the canopy at 11 and 27 DAA, corresponding to 1 day before the second application and 15 DASA, respectively. The following yield components were evaluated: number of panicles per area (NPA), determined by counting the number of panicles in 2 m linearly; number of filled spikelets per panicle (NFS), evaluated on 20 randomly selected panicles; and 1000-grain weight (TGW), obtained by weighing 8 subsamples of 100 grains. The grain yield (GY) was determined by manually harvesting 2 m2 and adjusting the moisture content to 13%. Data was tested for model assumptions and subjected to analysis of variance (ANOVA) using the F-test. When significant, means were compared using the Scott-Knott test at the 5% probability level in R software (R CORE TEAM, 2019) version 4.5.0.
RESULTS AND DISCUSSION
No significant phytotoxicity symptoms were observed at 7 and 14 DAA (data not shown), even though mild temperatures were recorded following application (Figure 1A). Similar results have been reported, showing no differences in phytotoxicity symptoms in QPE-resistant rice lines after the first application, regardless of the light intensity. However, in the same study, higher phytotoxicity levels were observed under low-temperature conditions (20 °C) (GODARA et al., 2022). In the present study, the absence of significant phytotoxicity symptoms even under mild temperatures may be related to the amplitude and/or duration of thermal fluctuations. Although, reduced temperatures during early rice growth stages can decrease the selectivity of ACCase-inhibiting herbicides, resulting in greater injury and reducing yield (MARTINI et al., 2015; MARTINI et al., 2021), the fact that the experiment was conducted in a single rice-growing region in southern Brazil limits the generalizability of these findings.
At 21 DAA, phytotoxicity symptoms ranging from 6 to 10% were observed under all treatments receiving the herbicide, regardless of the shading conditions. Plants subjected to shading before and after the second application showed a higher phytotoxicity, averaging 19%. At 28 DAA, a reduction in symptoms to levels below 10% was observed in all herbicide-treated plots (Table 1), confirming findings from other studies that have reported plant recovery from herbicide-induced phytotoxicity (GALVIN et al., 2022; MARTINI et al., 2023). The occurrence of phytotoxicity symptoms at 21 DAA may be associated with stress from sequential QPE applications and the onset of flood irrigation. Although QPE is selective for resistant rice, its application may trigger biochemical and physiological changes as a secondary effect, disturbing plant metabolism (DAYAN et al., 2019) and reducing selectivity (XIMENEZ et al., 2019). Additionally, the initiation of flood irrigation may intensify physiological stress in the plant, enhancing symptom expression. In studies evaluating irrigation systems and herbicide application, phytotoxicity averaging 7% was observed in rice grown under flood and sprinkler irrigation systems; however, in both cases, injuries were transient and of low intensity at 21 DAA (HELGUEIRA et al., 2018). Similarly, a greater phytotoxicity severity in resistant rice lines has been reported when QPE applications occurred close to periods of soil saturation compared to dry soil conditions (GODARA et al., 2022).
Phytotoxicity (%) at 21 and 28 days after application (DAA), plant canopy cover fraction (%) at 11 and 27 DAA, number of panicles per area (NPA), number of filled spikelets per panicle (NFS), 1000-grain weight (TGW), and grain yield (GY) of hybrid rice 132 PV affected by shading at periods close to quizalofop-p-ethyl applications.
Regarding solar radiation, after the second QPE application, plants maintained under a low light intensity (600 µmol m-2 s-1) exhibited phytotoxicity ranging from 18 to 31% at 7-28 DASA (GODARA et al., 2022). In the present study, which was conducted under simulated shading conditions in southern Brazil, the most pronounced phytotoxicity symptoms were observed after the second application, reaching approximately 19% injury under shading before and after the second application (Table 1). These results are likely more related to the duration of exposure to shading than to its timing relative to QPE application. On dates close to the second application due to the reduced global solar radiation (Figure 1C), PAR remained low during the 24 h before and after application, because of shading, consistently remaining below 200 µmol m-2 s-1 (Figure 1B). Under these conditions, the electron transport rate was significantly reduced due to low light availability (below 300 µmol m-2 s-1) (TAIZ et al., 2017). A lower global radiation and PAR may have limited plant metabolic activity and increased the phytotoxicity observed at 21 DAA, which corresponds to 7 days after the sequential application (Table 1). Additionally, recorded temperatures between 20 and 25 °C indicate that temperature was likely not a limiting factor for herbicide selectivity under the study conditions (Figure 1B). However, considering that the study was conducted under a single experimental environment, further studies under different environments are needed for a more comprehensive understanding of the environmental effects on injury occurrence in PV rice.
The plant canopy cover fraction was not affected by the shading period or QPE application, as it did not differ from the untreated control. The observed increase from approximately 11% at 11 DAA to 80% at 27 DAA (15 DASA) reflected plant growth (Table 1) associated with leaf expansion and tillering throughout the vegetative cycle. These results indicated that there was no significant impairment of the plant’s structural growth even in the presence of phytotoxicity symptoms, including at higher levels, as evidenced by the maintenance of the canopy cover. In contrast, a reduction in the canopy cover fraction was observed in QPE-resistant rice lines PVL01 and RTv7231 MA when subjected to a low light intensity (600 µmol m-2 s-1) compared to a high light intensity (1150 µmol m-2 s-1) with values decreasing from 99 to 77% and from 82 to 38%, respectively (GODARA et al., 2022). This difference may be associated with the fact that shading was temporary in the present study, occurring only 24 h before, after, or before and after QPE applications, whereas plants were continuously under low or high light intensity throughout the experimental period. This intensified phytotoxicity under low light and reduced the canopy cover (GODARA et al., 2022). Prolonged exposure to low irradiance may reduce the photosynthetic and metabolic capacity of plants, limiting their stress recovery (TAIZ et al., 2017) and impairing normal metabolism, slowing herbicide detoxification by the crop and causing phytotoxicity (PETTER et al., 2011). However, the exploratory design of the study-imposed limitations on the robustness of these inferences.
Regarding the chlorophyll index, after the first QPE application, the only difference observed occurred at 5 DAFA with no clear relationship to either the QPE application or crop shading (Table 2). At 3 DASA, a higher chlorophyll index was recorded under shading conditions applied after as well as before and after the second application, reaching approximately 42.5 SPAD (Table 2). This difference may be associated with a temporary slowdown in vegetative growth due to the combined effects of shading and herbicide application, considering that leaf growth causes a nitrogen dilution effect, which reduces the chlorophyll index (POCOJESKI et al., 2015). Chlorophyll meter readings are influenced by changes in the leaf blade, such as diseases and phytotoxicity (HAEFELE et al., 2010), suggesting that the reduction from 44 SPAD at 5 DASA to 30 SPAD at 7 DASA was exclusively the result of QPE application since only the untreated control had a higher chlorophyll index. However, at 14 DASA, the chlorophyll index returned to approximately 44 SPAD, with no further differences observed among treatments (Table 2). This recovery may be associated with the retranslocation of nitrogen accumulated in physiologically older leaves to younger plant organs, promoting the restoration of the photosynthetic apparatus (CANCELLIER et al., 2013).
Yield components were not affected by phytotoxicity, even under conditions in which greater symptoms were observed at 21 DAA. The NPA was approximately 263 panicles m-2, with about 136 filled spikelets per panicle and a TGW of 27 g (Table 1). These components are important as they determine rice crop productivity (MARCHEZAN et al., 2005), which was also not affected by QPE or shading, showing average yields between 11 and 12 Mg ha-1, including in the untreated control (Table 1). The maintenance of yield components could be associated with the phenotypic plasticity of rice and its ability to redistribute assimilates, enabling compensation for moderate stress before the critical phases of yield determination (YOSHIDA, 1981). In addition, hybrid vigor may confer attributes such as greater robustness, improved root development, increased fungal disease resistance, and high tillering potential (LUZZARDI et al., 2008). This last attribute may have maintained the number of panicles since delayed irrigation and phytotoxicity tend to stimulate tillering. Furthermore, heterosis increases yield and enhances plant adaptability and resistance to environmental stressors, such as drought, salinity, flooding, and wind (VIRMANI, 1994).
CONCLUSION
Prolonged shading after the second quizalofop-p-ethyl (QPE) application may increase phytotoxicity symptoms in ProvisiaTM (PV) rice; however, it does not interfere with yield components or grain yield.
ACKNOWLEDGMENTS
The authors would like to thank the Fundação de Apoio à Tecnologia e Ciência (FATEC) for financial support, the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the research fellowship granted to Dr. André R. Ulguim (Proc. 302130/2025-0), and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil. This study was financed in part by CAPES - Finance Code 001.
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CR-2025-0534.R1
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DATA AVAILABILITY STATEMENT
The data of this manuscript is available directly with the author or from a permanent repository.
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DECLARATION OF USE OF ARTIFICIAL INTELLIGENCE
Artificial intelligence tools were not used to write the manuscript or to replace the authors’ activities and skills, such aspreparing the abstract, keywords, hypotheses, and conclusion ofthe study, among other aspects of the manuscript.
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ASSOCIATE EDITOR:
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SCIENTIFIC EDITOR:
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The data of this manuscript is available directly with the author or from a permanent repository.
