Silicon increases chlorophyll and photosynthesis and improves height and NDVI of cotton ( Gossypium hirsutum L . r . latifolium Hutch )

Silicon (Si) it is a beneficial element that relieves biotic and abiotic stresses in plants. However, cotton plants are not considered Si accumulators, with low potential for uptake the element by roots. The objective of this study was to evaluate the effect of combinations of Si rates applied by leaf spray and soil on the physiology, growth and yield of cotton (Gossypium hirsutum L. r. latifolium Hutch). The experimental design was a randomized complete block in a 3 x 4 factorial scheme with four replications. Leaf spraying consisted of three Si concentrations (0, 500, and 1000 mL ha) corresponding to 0, 100, and 200 ml ha of monosilicic acid, with spraying split into three applications at stages V4, V6 and V8. Soilbased fertilization consisted of four Si rates in (0, 2.5, 5.0, and 10.0 kg ha) corresponding to 0, 0.5, 1.0, and 2.0 kg ha of SiO2. At flowering, photosynthesis, green color index (GCI), plant height, and NDVI were evaluated. The application of Si in the planting furrow near the rhizosphere increased the green color index, reflecting a gain in photosynthesis and plant height, which positively increased NDVI. The use of high solubility Si in the planting furrow can increase the concentration of monosilicic acid in the area with the highest root distribution, enhancing the effect of this element in a non-accumulator crop such as cotton, by improving the green color index, photosynthesis and hence reflecting on gains in plant height and plant leaf area demonstrated by NDVI.


Introduction
Silicon (Si) is a beneficial element that relieves multi-stresses, both biotic and abiotic (Vasanthi et al., 2014). Si is absorbed by plants as monosilicic acid and deposited on the leaf epidermis as amorphous silica, increasing its rigidity through interaction with pectin and polyphenols (Pilon-Smits et al., 2009).
Si accumulation below the cuticle also improves leaf architecture by increasing plant height of cotton, providing a higher incidence of light on the leaf surface (Ferraz et al., 2014), increasing the light stimulus to photosynthetic pigments and hence increasing the green color index (GCI) and the normalized difference vegetation index (NDVI). NDVI is a variable that strongly correlates with several phenological variables in cotton crops, including leaf area Development, v. 9, n. 7, e548973826, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.3826 5 index, photosynthetic rate, and plant leaf mass (Silva et al., 2017).
However, cultivated species differ in their ability to accumulate Si by the presence or absence of proteins specialized in element absorption (Mitani et al., 2008). Cotton plants are considered non-accumulators (Katz, 2014), with low potential for uptake the element by roots. Thus, application in the planting furrow associated with leaf spraying could better enrich the plant, being able to reflect in improvements in the crop physiology and yield.
In this sense, the hypothesis of this study is that the use of high solubility Si in the planting furrow near the rhizosphere and its complementation by foliar spraying during vegetative development may improve the effect of this element in a non-accumulator crop such as cotton. Therefore, the objective was to evaluate the effects of Si rates, applied in the planting furrow with complementation via leaf spraying, on the photosynthesis, NDVI and cotton growth.

Plant material and growing conditions
The study was carried out in the period from February to September 2015, using the cotton cultivar FM 975 WS Bayer ®, long cycle and technology Wide Strike (resistance to Lepidoptera) with an approximate final population of 100,000 plants ha -1 . The experiment was installed an area of the Chapadão Agricultural Research Support Foundation, in Chapadão do Sul, State of Mato Grosso do Sul, located at 18º41'33" S and 52º40'45" O, with 840 m of altitude.
The region climate according to the Köppen classification is Aw, defined as humid tropical (Kottek et al., 2006). Rainfall (mm) and maximum and minimum temperature (°C) data over the experiment period are shown in Figure 1. Temperature and rainfall ranges in the experimental area were favorable to cotton crop growth, with average temperatures of 30°C during the day and 22°C at night, close to those considered suitable to crop (Lamas and Yamaoka, 2012), and rainfall between 500 mm and 1,500 mm year -1 . The soil of the experimental area is classified as Latossolo Vermelho distroférrico (Santos et al., 2018). A sample was previously collected for chemical analysis, according to the method described by Raij et al. (2001) and for particle size analysis (Embrapa, 1997) ( Table 1). The fertilization was performed by applying 250 kg ha -1 of MAP (11-52-00) before sowing. Topdressing was performed by applying 150 kg ha -1 of KCl (00-00-60) at pre-sowing and 260 kg ha -1 of urea (46-00-00) divided into stages B1 (first visible flower bud) and F1 (first floral bud at the first branch transformed into a flower), with expected seed yield higher than 6 t ha -1 (Sousa & Lobato, 2004).

Treatments and experimental design
The crop was sown in February, 2015. The experimental design was factorial (3 x 4) based on a randomized block design with four replications. The treatments consisted of combinations of Si rates applied to soil and by foliar spray, being: three Si rates applied by foliar spray (0, 500 and 1000 mL ha -1 ) corresponding to 0; 100 and 200 ml ha -1 of monosilicic acid using as source the ZumSil ™ (79.3 g L -1 of Si and density of 1.25); and four Si rates applied to soil (0; 2.5; 5.0 and 10.0 kg ha -1 ) corresponding to 0; 0.5; 1.0 and 2.0 kg ha -1 of SiO2 using as source the Silcoat ® (200 g kg -1 of Si) (Table 2), where Si coating was performed on the base fertilizer formulation. Research, Society and Development, v. 9, n. 7, e548973826, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.3826 8 Leaf spraying was performed at the phenological stages V4 (from the end of V3 until when the central vein of the fifth leaf reached 2.5 cm), V6 (from the end of V5 until when the central vein of the fifth leaf reached 2.5 cm) and V8 (from the end of V7 until when the central vein of the fifth leaf reached 2.5 cm), by using a backpack sprayer pressurized by CO2 Herbicat.
The sprayer used has a six-nozzle spray boom, spaced 0.50 m and calibrated to the 150 L ha -1 application rate, using 2.5 MPa pressure and tips Teejet XR 110015 (green), which produced fine drops (148 µm). The application speed was 1 m s -1 .

Evaluated variables
When the plants emitted the first flower bud in the first flowering reproductive branch (F1), photosynthesis (mmol CO2 s -1 m -2 ) assessments were performed by means of an infrared gas analyzer (LICOR, Inc., LI-6400), under ambient CO2 concentrations (372 ± 10 mol s -1 m -2 ), with photosynthetically active photon flux of 1000 mol s -1 m -2 . Measurements were performed on the fifth leaf from the main stem apex in four plants per treatment, between 9:00 and 12:00 am. The green color index (GCI) was performed on the fifth leaf from the apex to the base of the plants, carrying four measurements per plot using Falker® Clorofilog chlorophyll Research, Society and Development, v. 9, n. 7, e548973826, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.3826 9 meter. Plant height was measured using a continuous plant sample from the plot useful area using measuring tape, measuring from the base of the stem to the beginning of the last fully developed leaf.

Statistical analysis
Data were subjected to analysis of variance and, subsequently, regression analysis.
When there was significance between the contrast check versus factors, the t-test was applied using the Rbio statistical software (Bhering, 2017). In all analyses, a significance at 5% probability level was adopted. Subsequently, the correlations between the variables and the result were estimated, expressed graphically by the correlation network. Finally, principal component analysis was performed to identify the association between the treatments and the evaluated variables.

Results
The application of Si via soil in the planting furrow provided effects on GCI, photosynthesis (Photo), plant height (PH) and plant leaf mass (indicated by NDVI). Si supply by leaf spraying on cotton plants did not influence (p>0.05) any of the evaluated variables.
The combination of Si rates with soil supply and foliar spray complementation did not promote significant interaction in any of the evaluated variables (Table 3). The GCI showed a linear response to Si application by soil in the planting furrow, increasing 0.37 units per 1 kg ha -1 of Si applied to the soil (Figure 2a). Although the F test Research, Society and Development, v. 9, n. 7, e548973826, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.3826 detected differences between Si rates in soil for photosynthesis, there was no adjustment for the tested models (linear and quadratic) regarding this variable (Figure 2b).  There was a positive and significant correlation between plant height (PH) and NDVI, and between conductance (Cond) and transpiration (Trmmol) (data not shown) (Figure 4).

Moderate magnitude correlations were observed between NDVI and photosynthesis (Photo)
and yield (YIE) (data not shown). The other variables presented low correlation estimates ( Figure 4). Principal component analysis demonstrating the relationship between treatments and evaluated variables is shown in Figure 5. Treatment 4 (0 mL ha -1 by leaf spraying + 10 kg ha -1 by soil) is the one closest to the yield vector, being the most effective for increasing its average yield (298.31 @ ha -1 ) ( Figure 5). The proximity of NDVI and PH vectors to treatment 8 (10 kg ha -1 by soil and 500 mL ha -1 by leaf spraying), demonstrates that it was effective in increasing the mean of this variable ( Figure 5).

Discussion
Leaf spraying of Si did not affect the development of cotton plants (Table 3). Si is an element with low phloem mobility and tends to remain where it was absorbed by leaf limb Development, v. 9, n. 7, e548973826, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.3826 13 (Mitani et al., 2008). Thus, it is believed that the leaf spraying rate was insufficient to promote relevant increase in the analyzed variables.
Despite the low potential of Si uptake by cotton plant roots (Katz, 2014), the supply of the element by soil in the planting furrow provided greater contact with the roots, resulting in positive effects on plant physiology and growth. This demonstrates that the application efficiency is a fundamental aspect to manage the use of Si in cotton.
The green color index is an indirect measure of chlorophyll content. In this study, the application of Si by soil in the planting furrow resulted in an increase of this parameter Upright plants may have a larger light reception area and maintain wavelength uptake by photosynthetic pigments, besides a most efficient energy transference to the photosystem II and hence increasing photosynthesis (Streit et al., 2005). Increased photosynthesis with Si leaf spraying was also demonstrated by Barros et al. (2019), however, the authors attribute this effect to the association of Si with salicylic acid.
The maintenance of the green color intensity in the plants may also have been contributed by the preservation of the photosynthetic pigment integrity caused by the attenuating effect of Si on the oxidative damage reduction, since the excessive accumulation of reactive oxygen species may cause cell membrane disintegration (Bokhtiar et al., 2012), as was also demonstrated in cotton crop under copper toxicity with Si addition in the nutrient solution (Ali et al., 2016).
With the increased morpho-physiological components, NDVI also showed increase with Si supply by soil in the planting furrow (Figure 3b) and showed a strong correlation with plant height (Figure 4). NDVI is the difference between the emission and reflection of the electromagnetic spectrum waves in two lengths, estimating the closure of the rows promoted Development, v. 9, n. 7, e548973826, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.3826 14 by the plant canopy and the green color index (Babolim et al., 2014), a characteristic that can indirectly define yield.
The improved plant mass demonstrated by NDVI is resulting from the Si effects on increasing green color index, photosynthesis and plant height, since NDVI variability is related to several phenological variables in cotton crop, among them the leaf area index, photosynthetic rate and plant leaf mass (Silva et al., 2017).
There are no studies demonstrating the effect provided by Si on plant mass represented by NDVI, which emphasizes the relevance of this study. NDVI has a positive correlation with plant height (Motomiyia et al., 2009) and chlorophyll content (Souza et al., 2017), demonstrating that Si stimulates crop development by improving morpho-physiological parameters.
The high correlation between stomatal conductance and transpiration is significant because the plant increases the loss of water to the environment when the stomata are open.
However, as these factors increase, photosynthesis is also favored, since stomata opening causes CO2 diffusion into the leaf, increasing the internal concentration, which will be used for photosynthesis (Ferraz et al., 2014).
Thus, we can accept the hypothesis that the use of high solubility Si in the planting furrow near the rhizosphere during vegetative development may increase the concentration of monosilicic acid in the area of greater root distribution, enhancing the effect of this element in a non-accumulator crop such as cotton. PCA analysis revealed the existence of an association between the Si rate of 10 kg ha -1 by soil and yield, which suggests that higher yields can be obtained by using Si in soil. By improving the green color index, photosynthesis and reflecting on gains in plant height and leaf area, as demonstrated by NDVI, the use of Si in the soil can be fundamental to obtain greater gains in the cotton yield, which is the main characteristic desired by the growers.
The results provided by our study contribute to the decision making about alternatives to the fertilization management of the cotton, such as Si, which is a technique still little explored in the crop. The use of Si is an easy and low-cost technique that can be associated with the management of soil fertilization, which usually uses only macronutrients. Our findings reveal that the application of Si in the planting furrow benefits the plant by improving its photosynthetic traits and by being associated with an increase in yield However, further studies should be carried out with Si fertilization in the crop evaluating other application techniques and/or other physiological traits in order to provide more information on the effects of this element on cotton. Development, v. 9, n. 7, e548973826, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i7.3826 15

Conclusion
The application of Si in the planting furrow near the rhizosphere has a beneficial effect on cotton plant growth by improving green color index, photosynthesis, plant height and NDVI. The results provided by our study contribute to the decision making about alternatives to the fertilization management of the cotton. Thus, the use of Si can be associated with the management of soil fertilization, contributing to yield gains and consequent increase in the profitability of the cotton. However, further studies addressing different Si application techniques and evaluating other physiological traits should be carried to better elucidate the effects of this element in cotton.