Plant Growth and Antioxidative Enzymes in Sunflower Supplemented With Selenium

. Abstract The use of soil additives such as selenium can positively influence the antioxidative system of plants, making them more tolerant to abiotic stresses. The aim of this work was to evaluate the concentration of sodium selenite or sodium selenate applied to the substrate that causes improve in plant growth and antioxidative enzyme activities in sunflower plants. The treatments were divided in: control (absence of selenium); 0.2 mg L -1 of sodium selenate; 0.4 mg L -1 of sodium selenate; 0.8 mg L -1 of sodium selenate; 0.2 mg L -1 of sodium selenite; 0.4 mg L -1 of sodium selenite and 0.8 mg L -1 of sodium selenite. The analysis of Shoot Dry Mass (SDM) production and activities of the antioxidantive enzymes: Ascorbate Peroxidase (APX), Guaiacol Peroxidase (GPX) and Catalase (CAT) was performed. For SDM and APX the concentration of 0.8 mg L -1 of sodium selenite caused higher values. CAT showed greater activity in treatments that received 0.4 and 0.8 mg L -1 of sodium selenate and 0.4 and 0.8 mg L -1 of sodium selenite than the control treatment. GPX showed superior activity in the treatments 0.8 mg L -1 of sodium selenate, 0.2 mg L -1 of sodium selenite and 0.8 mg L -1 of sodium selenite than the control treatment. It was concluded that selenium promoted improvements in the antioxidant activity and in the production of shoot dry mass of sunflower plants.


Introduction
The cultivation of oilseeds has been encouraged for the production of biofuels, such as biodiesel. The main crops used are soybeans and corn, which are also in great demand for the food industry (Diniz, Sargeant, & Millar, 2018). Therefore, other crops such as palm, peanut, forage radish, castor bean, sesame, canola and sunflower have become an object of study for the development of techniques that improve agricultural management (Cavalcante Filho, Buainain, & Benatti, 2019).
Sunflower (Helianthus annus L.) is an annual crop with good acclimatization to different climate and soil conditions. It can be used as ornament, food and for the production of biofuels (Araújo, D. Silva, V. Silva, Magalhães, & Barros, 2018). However, there are regions, such as those with a semi-arid climate, in which water and climate limitations impact the crops negatively. Due to low rainfall regimes, evapotranspiration is higher, which can cause drier soils that accumulate salts, leading plants to a stress condition (Gul, Dinler, & Sarısoy, 2017).
Fertilization and irrigation are widely used techniques to remediate abiotic stresses (Araújo et al., 2018). The use of additives, such as selenium, has shown that there is a possibility of a positive effect on crops subjected to stress, when used in adequate concentrations (Al-Kazzaz, 2018).
Selenium is a trace element that may be naturally present in soils. However, about 70% of the world has selenium deficient soils. In soils, selenium comes from rocky weathering and volcanic magma. Its availability depends on the parameters that guide the chemical and biochemical reactions of the soil. Regions whose climate is temperate and humid are mostly poor in selenium (Hossain et al., 2021). For plants, the presence of selenium can present benefits, such as increased productivity, a positive influence on the quality of products (such as fruits and seeds) and on plant senescence. The bioavailable forms for plants are those soluble in water, which are organic selenium, selenite and selenate (Garduño-Zepeda & Márquez-Quiroz, 2018). Gül et al. (2018), evaluated sunflower plants under different salt stress conditions in the Black Sea region. The authors found that salt stress conditions reduced the percentage of seedling germination. Habibi (2017) carried out a study in the region of Poland, where the climate is temperate, subjecting sunflower plants added with sodium selenate to salt stress and concluding that selenium helped to reduce damage to the plant. Hachmann et al. (2019), when testing selenium concentrations in cauliflower plants subjected to water stress, observed higher productivity in those treatments that contained selenium.
Considering the scarcity of studies that quantify the ideal selenium concentration applied in soil/substrates for the cultivation of sunflower plants in tropical and semi-arid climates, such as in Northestern Brazil, the aim of this work was to evaluate the concentration of sodium selenite or sodium selenate applied to the substrate that causes improve in plant growth and antioxidative enzyme activities in sunflower plants.

Method
The experiment was carried out between August and September 2021, under greenhouse conditions at Instituto Federal de Educação, Ciência e Tecnologia do Ceará (IFCE) Maracanaú campus-CE, Brazil. The geographical location is 24 M 543133.84 m E/9571989.84 m S. The climate is tropical, with dry winters according to the Köppen classification, the average temperature was 26 °C and the average humidity was 65% (Climate-Data, 2022).
The experimental design was completely randomized, with seven independent treatments containing five replications each one. Two sources of selenium were used, sodium selenate (Na 2 SeO 4 ) and sodium selenite (Na 2 SeO 3 ), applied directly to the soil on the day of sowing. The concentrations chosen were based on the study of Nasser (2015). The treatments were divided into control (absence of selenium); 0.2 mg L -1 of sodium selenate; 0.4 mg L -1 of sodium selenate; 0.8 mg L -1 of sodium selenate; 0.2 mg L -1 of sodium selenite; 0.4 mg L -1 of sodium selenite and 0.8 mg L -1 of sodium selenite.
Each treatment contained five replications, with three plants in plastic vases of 5 L. The substrate that was used contained four volumes of sand and a volume of commercial earthworm humus, plus the selenium concentration corresponding to the treatment. The sunflower seeds used were cultivar BRS 323, provided by the Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA). The irrigation was daily at 80% of field capacity.
At 19 days after sowing (DAS), a destructive harvest was performed. The material was separated into two parts. The first one for the quantification of Shoot Dry Mass (SDM) and the other one for the enzymatic analyses. In order to obtain the SDM, the plants were placed in paper bags in a forced circulation oven, with a temperature of 60°C, until the material had a constant mass. Subsequently, the dry material was weighed on an analytical balance to obtain the SDM.
In the enzymatic analysis, the material was used in the form of extracts, produced according to the methodology of Nunes et al. (2017). All analyzes used spectrophotometric methods. Catalase (CAT) activity was determined by the methodology of Havir and McHale (1987). The determination of Ascorbate Peroxidase (APX) concentration was performed by the method of Nakano and Asada (1981). Finally, the activity of the enzyme Guaiacol Peroxidase (GPX) was determined by the methodology of Kar and Mishra (1976).
Data were tested by analysis of variance (ANOVA) and means compared by Tukey's test (P ≤ 0.05). The Sigma Plot 11.0 program was used to perform statistical analysis and graph construction.

Results and Discussion
The plants that received selenium in the form of sodium selenate (Na 2 SeO 4 ) and sodium selenite (Na 2 SeO 3 ), in most concentrations, had higher values than those obtained in the control treatment. For the variable Shoot Dry Mass (SDM), the treatment with the highest value was the one that received a concentration of 0.8 mg L -1 of sodium selenite, with an average value of 0.27 g plant -1 , as shown in Figure 1. In comparison to the average of the other treatments that received selenium, the percentage of growth was 30% higher. And in relation to the average of the control treatment, the values were 50% higher. cultivating Brassica napus, were able to perceive beneficial effects on the antioxidant system in some treatments submitted with dosages of sodium selenite. Selenium, in adequate concentrations, is a regulator of enzymatic reactions involving the elimination of ROS, according to studies carried out by Hernández-Hernández et al. (2019), who showed incremental effects on CAT, APX and GPX activity in tomato crops added to selenium.

Conclusion
In the experimental conditions used, selenium promoted improvements in the antioxidative enzyme activities and in the Shoot Dry Mass of sunflower plants. In general, the treatment with a concentration of 0.8 mg L -1 of sodium selenite outperformed the control treatment in all variables analyzed. When compared to the other treatments containing selenium, it was superior in the variables MSPA and APX.
Studies involving abiotic stresses, such as salt and water stress are indicated to corroborate the positive effects of selenium on the antioxidant system of sunflower plants. In addition, analyzes that can quantify the selenium concentration absorbed by the plants are necessary to confirm its effectiveness.