NDVI, nutrient accumulation and yield of onion fertilized with amorphous silica in a semiarid setting in Brazil

doi:10.21203/rs.3.rs-2104708/v1

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NDVI, nutrient accumulation and yield of onion fertilized with amorphous silica in a semiarid setting in Brazil

https://doi.org/10.21203/rs.3.rs-2104708/v1

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Purpose

Silicon (Si) can reduce abiotic stresses in plants and increase crop yield in semiarid climates. We aimed to assess the effect of an amorphous silica-based fertilizer (ASF) applied to soil on the yield, mineral nutrition, and health of an onion crop grown in northeast Brazil.

Methods

The ASF was applied at the rates of 0, 75, 125, 175, and 225 kg ha^− 1. Yield and quality of bulbs as well as leaf concentrations of Si and nutrients were evaluated. Stress amelioration was assessed through Normalized Difference Vegetation Index (NDVI).

Results

ASF increased the yield of onions and enhanced the concentration of Si, N, P, K, Ca, Mg, B, Mn, Zn, and Cu in leaves. There was no ASF effect on bulb quality. NDVI indicated that applying 225 kg ha^− 1 ASF to soil promoted healthier onion plants and the highest yield.

Conclusions

Silicon increased the yield of onions due to improved plant nutrition and stress amelioration.

Abiotic Stress

Plant nutrition

Allium cepa

Vegetation indices

SAVI.

Over 140 countries grow onion (Allium cepa L.) owing to its nutritional and medicinal properties. Onions can be consumed as raw, fresh, or processed and contain antioxidants and anti-inflammatory compounds that reduce cholesterol levels, diabetes, high blood pressure, and the risk of cardiovascular diseases [1–3]. The global production of 98 Mt of onion makes it the second most important vegetable crop after tomato [4–6]. The crop is grown in different regions of Brazil and is the country's third most relevant vegetable crop, behind potatoes and tomatoes. The production of onions in Brazil covered 58,000 ha and amounted to approximately 460 million USD in 2020 [6, 7].

Onion production in northeast Brazil lies mainly in the semiarid [5, 8], in which several plant stresses such as low precipitation, high temperature, and high evapotranspiration pose drawbacks to the onion's yield and quality. Abiotic stress factors are responsible for 51–82% of global crop productivity reduction [9, 10]. Therefore, implementing management strategies to mitigate these abiotic stresses is crucial to increasing the yield and quality of onions grown in semiarid regions.

Silicon (Si) stands out among mineral elements for its ability to reduce the severity of several biotic and abiotic stresses [9–12]. Moreover, given the rapid pace of global warming, Si application to Si-deficient soils can ameliorate the stressful conditions for plants under drought and heat stress [13, 14]. Although the most notorious effect of Si on plants has been reported for gramineous species such as rice, sugarcane, rice, wheat, and maize, horticultural crops can also benefit from Si fertilization [15–17]. For instance, Cao et al. [18] found that Si restrains drought-induced ROS accumulation by promoting energy dissipation in the leaves of tomatoes, while Gou et al. [19] reported that Si improved seed germination and ameliorated oxidative damage of bud seedlings in cucumber under salt stress.

The average Si total concentration in soils is high (28%); however, most Si-containing minerals (silicates, aluminosilicates, and silicon oxides) have low solubility and release only small amounts of Si into the soil solution to plant uptake. The form of Si accessible to plants (orthosilicic acid; Si(OH)₄) is soluble in water up to 2 mmol L^− 1 at 25°C, above which polymerization into silica (SiO₂) gel begins to take place [20, 21]. Highly weathered soils such as Oxisols and Ultisols and sandy soils composed mainly of silica are depleted in soluble Si and responsive to amendments containing the element [17, 22]. There is an awareness that Si deficiencies in soils are a limiting factor in crop yield and quality but using Si as a fertilizer has not become a routine in the field [23].

Normalized Difference Vegetation Index (NDVI) amplifies the contrast between the visible red and near-infrared (NIR) spectral bands to estimate crop variables such as yield, photosynthesis, nitrogen content, and plant vigor. Crop monitoring through remote sensing has been common for commodities such as wheat, soybean, and maize, while other crops, including onions, have been underrepresented [24, 25]. Considering that NDVI can assess crop health [26], the technique is especially relevant to studying the effects of amendments that can ameliorate plant stresses. For instance, applying Si to cotton increased the green color index, reflecting photosynthesis and plant height gain, positively increasing NDVI [27].

Sources of Si tested as fertilizers in Brazil are mainly industrial by-products and naturally occurring minerals rich in silica, such as Wollastonite [28–30]. Research on amorphous silica as fertilizers are still scarce [5, 17]. Here we assessed the effects of applying an amorphous silica-based fertilizer to onion grown in a semiarid region of northeast Brazil. The yield and quality of onions and the accumulation of Si and nutrients (N, P, K, Ca, Mg, B, Cu, Fe, Mn, and Zn) in leaves were assessed. Also, we used NDVI and Soil-Adjusted Vegetation Index (SAVI) to evaluate the effect of Si on plant vigor. Our results indicate the potential of Si fertilizers to mitigate abiotic stress and improve crop productivity not only in the study area but also in other semiarid regions worldwide, in which the beneficial effects of Si are particularly evident due to stress conditions.

2.1 Experimental area

The experiment was carried out at AM Frutas farm, located in the municipality of Juazeiro, Bahia state, northeastern Brazil. The area was selected based on the soil's sandy texture, low CEC, and low Si contents, characteristics that increase crop responses to silicon. The region's climate is a hot desert (BWh) according to the Köppen climate classification. The average annual temperature is 26.6 0C, and the average yearly rainfall is 560 mm. The soil of the experimental area was collected and analyzed for chemical and physical characteristics (Table 1) following standard methods. Plant-available Si in the soil was determined by colorimetry in extracts of acetic acid 0.5 mol L^− 1 and CaCl2 0.01 mol L^− 1 following the modified molybdenum blue colorimetry procedure [31]. Silicon-available contents estimated for both extractants were low, and Si deficiency is likely.

Table 1
Chemical and physical characteristics of the soil in the experimental area
pH	water (1:2.5)	5.98
Ca²⁺	mmol_c kg^− 1	89.4
Mg²⁺	mmol_c kg^− 1	14.7
K⁺	mmol_c kg^− 1	6.2
Na⁺	mmol_c kg^− 1	1.0
Al³⁺	mmol_c kg^− 1	0.0
H⁺ + Al³⁺	mmol_c kg^− 1	10.1
SB	mmol_c kg^− 1	95.5
CEC	mmol_c kg^− 1	99.5
P	mg kg^− 1	43.7
SOC	g kg^− 1	5.16
SOM	g kg^− 1	8.90
Si_a	mg kg^− 1	4.3
Si_c	mg kg^− 1	2.2
Sand	g kg^− 1	570.0
Silt	g kg^− 1	80.0
Clay	g kg^− 1	350.0
SB sum of bases; CEC cation exchange capacity; SOC soil organic carbon; SOM soil organic matter; Si_a silicon available in the soil extracted with acetic acid 0,5 mol L^− 1; Si_c silicon available in the soil extracted with CaCl₂ 0,01 mol L^− 1.

2.2 Experimental setup

Onion (hybrid Num 1205 - Bayer CropScience) planting was performed by direct seeding with a population of 1 million plants per hectare. The fertilization followed the program adopted at the farm. Basal and top-dressing fertilization amended 302 kg ha^− 1 N, 157 kg ha^− 1 P₂O₅, 364 kg ha^− 1 K₂O, 46 kg ha^− 1 Ca, and 30 kg ha^− 1 Mg. Water was supplied to plants by drip irrigation.

The Si fertilizer used (Agrisilica®, Agripower Australia Limited) is natural diatomaceous earth of the Melosira granulata species that contains total SiO₂ (52%), calcium (2%), magnesium (1%), and iron (1%) in the form of granules (2–5 mm size). The fertilizer was broadcast applied when seedlings were 8 days old at the rates of 0 (control), 75, 125, 175, and 225 kg ha^− 1, with four replicates, in 20 experimental plots of 11 m² (10 m x 1 m) each. The distance between adjacent plots was 0.5 m. The trial was laid out as a randomized complete block design and treatments were assigned to each plot within each block randomly.

2.3 Leaf sampling and analyses

Leaves were sampled for foliar analysis of Si and nutrients on the 60th day of cultivation. The content of Si in the dry leaf matter was determined according to Elliot and Snyder [32]. Nitrogen was obtained by the digestion of 0.2 g of the plant material in concentrated sulfuric acid at 350°C, according to the method of Kjeldahl, while K, P, Ca, Mg, Fe, Mn, Zn, and B were determined in the digestion extract of 0.5 g of samples in nitro-perchloric solution at 200°C. The determinations of P were performed by colorimetry and that of K by flame spectrometry. Calcium, Mg, Fe, Mn, Zn, and B were determined by ICP-OES [33].

2.4. NDVI and SAVI

The aerial survey was carried out 90 days after planting the onion using a Phantom® 4 quadcopter (SZ DJI Technology Co., Ltd, Shenzhen, China) equipped with a MAPIR® Survey 3W multispectral camera (MAPIR, San Diego, USA). This camera features a passive sensor with an 87° field of view (FOV) and captures images composed of three bands of the electromagnetic spectrum, referring to red (R) at 660 nm, green (G) at 550 nm, and near-infrared (NIR) at 850 nm. The flight was automated following a plan generated by the DroneDeploy® software (DroneDeploy, San Francisco, USA), whose parameters were: 5 m s^− 1 of aircraft speed, 1.5 s of image capture interval, 70 m of altitude, overlap front (overlap) of 80% and side (sidelap) of 70%, and ground sampling distance (GSD) of approximately 3 cm pixel^− 1. The cited adjustments for the flight parameters were based on the recommendations of the sensor manufacturer and the PhenoFly Planning Tool [34]. The flight was carried out between 2 pm and 3 pm, with a high incidence of sunlight and low presence of clouds.

Images captured in raw format (minimally processed) were radiometrically corrected using reflectance values of the MAPIR calibration target in the MAPIR Camera Control software (MAPIR, San Diego, USA). Subsequently, the images were processed in Agisoft Metashape v1.5.2 software (Agisoft LLC, St. Petersburg, Russia) to obtain georeferenced orthomosaics, which, in Quantum GIS v3.14.16 software (QGIS Development Team, www.qgis.org), were cut based on the vector files (shapefile) of the plots in the field, selecting the areas of interest and avoiding the edge effect on the pixels. In the same software, in the useful area of each plot, the values of the weighted average of reflectance were taken in a total of 237,968 pixels, and the information was used to calculate the vegetation indices.

We obtained the Normalized Difference Vegetation Index (NDVI) and the Soil-Adjusted Vegetation Index (SAVI) to evaluate plant vigor.

The NDVI is an index ranging from − 1 to 1 and is sensitive to changes in vegetation vigor and density. Additionally, it was calculated using the equation described by Rouse et al. (1973): $$NDVI= \frac{NIR-R}{NIR+R}$$

NIR and R refer, respectively, to the spectral signatures of the plant canopy in the near-infrared and red bands of the electromagnetic spectrum.

The SAVI is an index that considers the effects of the exposed soil in the analyzed images. Therefore, it is used for evaluations in which the vegetation does not entirely cover the environment [35]. This index was described by Huete [36] and is calculated using the equation: $$SAVI= \frac{NIR-R}{NIR+R+L}*(1+L)$$

L is the adjustment factor of the SAVI index, which assumes a value of 0.25, 0.50, and 1.00, respectively, for dense, intermediate, and low vegetation conditions. The present work used 0.50 for the L factor.

2.5 Yield and quality of onion bulbs

Harvest took place when leaves were drying and falling over (senescence). First, bulbs were pulled and sun-cured for three days. Then, they were graded by diameter according to commercial standards, in the size ranges: 2.5–4.5 cm; 4.6–5.5 cm; 5.6–8 cm; and 8.1–9 cm. Around 80% of the bulbs fall into the range of 5.6–8 cm, the preferred onion size by Brazilian consumers, but there was no difference among treatments regarding onion diameters. The onion yield was obtained by weighing each experimental plot's bulbs and then extrapolating them to tons per hectare. Data were collected from plants in the plot's central rows, leaving aside plants in the outermost rows and at the end of each row to avoid edge effects.

2.5 Statistical analyses

The experiment was designed as randomized blocks, with five treatments and four replications. The results were expressed as means of four replicates ± standard deviation. Data were subjected to analysis of variance using the SAS Statistical Software package. Yield means that differed significantly were separated using the Least Significant Difference (LSD) test procedure at a 5% significance level. At the same time, Si and nutrient concentrations in plants were submitted to regression analysis using the same software. Regression models were selected, considering their significance and highest R2 values.

3.1. NDVI and SAVI

The NDVI and the Si rates (p < 0.01) were adjusted to the linear regression (Fig. 1). On the other hand, there was no relationship between Si rates and SAVI. Santos et al. [27], evaluating the growth of cotton plants, found an increase in the NDVI of 0.0043 units for each 1 kg ha^− 1 of Si applied. Because chlorophyll molecules reflect the infrared band and absorb the red band, changes in the photosynthetic apparatus can be detected by the spectral signature and, consequently, by vegetation indices such as NDVI [37, 38]. Additionally, it has been reported that the application of Si affects the concentration of chlorophyll molecules in various crops such as cucumber, pepper, corn, as well as onion [19, 39–41].

The vegetation indices for the experimental plots indicated that the plants were under some stress, characterized by values farther from 1 since the NDVI and SAVI had maximum values of 0.47 and 0.64, respectively (Fig. 2). Therefore, remote sensing could be a valuable tool to detect early stress in onion crops and the effects of stress-amelioration amendments. Beisel et al. [26] demonstrated that single-image NDVI enables early detection of ammonium nitrate and salinity stress in arugula plants and Arabidopsis seedlings. Additionally, our data showed that NDVI detected increased plant vigor, as seen in the maps by the darker shades, and consequently better conditions in the photosynthetic apparatus in plots with higher Si rates.

3.1 Accumulation of silicon and nutrients by plants

The regression analysis showed a significant effect of the amorphous silica fertilizer (ASF) rates on the Si accumulation in the onion leaves (Fig. 3a). There was a sharp increment in the Si leaf concentration with the lowest fertilizer rate (75 kg ha^− 1), as a reflection of the naturally low Si available content in the soil (Table 1), followed by a more slightly yet steady increment in the highest fertilizer rates applied. Silicon concentration in onion leaves had an exponential trend and seemed to reach a plateau close to 0.26%, suggesting a limitation of the crop in responding to rates of ASF applied to the soil higher than 225 kg ha^− 1. Plants containing less than 0.5% Si in shoots are regarded as non-Si-accumulators [42]. On the other hand, sugarcane, known for accumulating high Si contents, showed a linear increase in leaf Si concentration to rates as high as 750 kg ha^− 1 of the same fertilizer [43].

Plants species are classified as active, passive, and rejective based on the basic mechanistic understanding of Si uptake [44]. Active accumulators (gramineous species, e.g., rice and wheat) have shoot Si concentrations between 1.5 and 10%. Passive accumulators (mainly dryland Gramineae) have shoot Si concentrations ranging from 0.5 to 1.5%. Lastly, most dicots are rejective and have < 0.2% of Si in shoots [14]. It is worth pointing out that Si concentrations in the onion leaves were similar to P concentrations and higher than Mg (Figs. 1C and 1F, respectively) and micronutrient (Fig. 4) concentrations. Therefore, even passive and rejective species such as onions can reach leaf Si concentrations > 0.1%, typical of macronutrients [45], making Si a principal mineral constituent of plants. A better understanding of Si accumulation in vegetable crops gained from our work and others can aid in predicting whether they will benefit from Si application.

Applying ASF to soil caused a significant increase in the concentration of macronutrients in leaves, except Ca (Fig. 3E). Higher accumulation of nutrients in Si-amended plants compared to control has been reported in several works [16, 17, 43, 46, 47]. Concentrations of N, K, and Mg in leaves had a linear increase in response to ASF application, while P showed an exponential rise that stabilized in the highest Si rates. Amorphous silica can contribute to water retention and cations in the soil due to its porous nature and cation exchange capacity [48]. Such an effect can be partially related to the results found here.

Results showed that the application of ASF caused a significant increase in the content of all the micronutrients tested in leaves of onions over control, except Fe, which decreased with the Si rates applied to soil (Fig. 4). Greger et al. [16] also found reduced Fe content in the shoot of maize amended with Si in the nutrient solution. Other works have shown increased micronutrient accumulation in the leaves of Si-treated plants, including melon [17], sugarcane [43], and barley [49]. Generally speaking, the role of Si in transporting micronutrients from roots to shoots may be related to the increase in citrate concentration, which would make the redistribution of micronutrients more efficient [50]. For example, some works demonstrated that Si promoted Mn translocation to the shoot rather than its root uptake [16, 51].

3.2 Yield of onion bulbs

On average, the Si fertilizer rate of 225 kg ha^− 1 increased the onion yield significantly from 110 t ha^− 1 to 118 t ha^− 1 (Fig. 5). On the other hand, rates lower than 225 kg ha^− 1 had no significant effect on onion yields despite the increasing Si concentrations in the leaves with the increasing Si rates (Fig. 6A). Such a yield increment is noteworthy given the high yields (over 100 t ha^− 1) routinely obtained on the farm.

The increased crop yield with Si nutrition likely resulted from several factors, including reduced heat, light, salt, and drought stresses common in semiarid settings and improved nutrient use efficiency. For instance, some works have shown that Si increases the root hydraulic conductance and cell wall stability [52, 53], imparting high tolerance to water deficit, besides increasing water uptake by roots [54]. In addition, silicon fertilization promoted onion plants' salt stress acclimation, resulting in improved bulbs yield in semiarid conditions [5].

Our results showed that applying 225 kg ha^− 1 of ASF increased the yield of onions and enhanced the concentration of Si, macronutrients (N, P, K, Ca, and Mg), and micronutrients (B, Mn, Zn, and Cu) in plants. Additionally, NDVI images indicated that the 225 kg ha^− 1 ASF application promoted healthier onion plants than the control treatment. Therefore, Si could increase the yield of onions grown in semiarid settings due to improved crop performance driven by enhanced plant nutrition and stress amelioration.

Ethics approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

All authors have provided consent to publication.

Availability of data and materials

All data and materials are available under reasonable request.

Authors' contributions

Clístenes Williams Araújo do Nascimento was responsible for the study conception and design. Material preparation, data collection and analysis were performed by Marcos Kleber Dias de Barros, Fernando Bruno Vieira da Silva, Luiz Henrique Vieira Lima, Franklone Lima da Silva, Venâncio de Lima Veloso, Josévaldo Ribeiro Silva, and Simone Aparecida da Silva Lins. The first draft of the manuscript was written by Clístenes Williams Araújo do Nascimento and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Acknowledgements

The authors are grateful to AM Frutas farm for providing the experimental area and the Study Group on Ecophysiology and Plant Stress (GEESP, Dr. Ana Rita Leandro) of the Federal Institute of the Sertão Pernambucano for helping with the fieldwork.

Funding and/or Competing interests

All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

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NDVI, nutrient accumulation and yield of onion fertilized with amorphous silica in a semiarid setting in Brazil

Status:

Version 1

Abstract

Purpose

Methods

Results

Conclusions

Figures

1 Introduction

2 Material And Methods

2.1 Experimental area

2.2 Experimental setup

2.3 Leaf sampling and analyses

2.4. NDVI and SAVI

2.5 Yield and quality of onion bulbs

2.5 Statistical analyses

3 Results And Discussion

3.1. NDVI and SAVI

3.1 Accumulation of silicon and nutrients by plants

3.2 Yield of onion bulbs

CONCLUSIONS

Declarations

References

Additional Declarations

Status:

Version 1