Evaluation of droplet deposition and efficiency of 28-homobrassinolide sprayed with unmanned aerial spraying system and electric air-pressure knapsack sprayer over wheat field

Highlights

• 28-Homobrassinolide (HBR) various dosages were sprayed by UAV and EAP on wheat anthesis stage.

• HBR applied by UAV had higher penetration rate but lower spray uniformity as compared to EAP.

• HBR delayed leaf senescence, increased photosynthesis rate and prolonged grain filling duration.

• HBR medium and high dosage applied by UAV enhanced source and sink balance and significantly increased yield production.

Abstract

Brassinolides are plant-specific Polyhydroxylated steroid hormones that play a vital role in plant growth regulation and actively participate in inducing various stress tolerance. Our objective was to investigate the influence of 28-Homobrassinolide (HBR) dosages and compare the droplet deposition of unmanned aerial spraying system (UASS) with electric air-pressure knapsack (EAP) sprayer over the wheat field. Three HBR dosages (19, 24, 30 mg a. i. ha⁻¹) with spray volume (SV) of 15, 30 L ha⁻¹ were applied by UASS, and HBR dosage of 24 mg a. i. ha⁻¹ with a spray volume of 450 L ha⁻¹ was applied by EAP. The droplet deposition of UASS 15 L ha⁻¹ was 15.51 %, 33.57 % higher than UASS 30 L ha⁻¹ and EAP. While spray distribution uniformity in both UASS spray volumes was less than EAP. HBR (24, 30 a. i. mg ha⁻¹) significantly increased the photosynthetic traits, which led to a hastened grain filling rate and increased biomass of reproductive plant organs, yield, and yield components. HBR dosage of 24 mg a. i. ha⁻¹ with a spray volume of 15 L ha⁻¹ significantly increased grain yield by 17.94 %, 16.31 % than CK and 12.13 %, 15.31 % than EAP in two years, respectively. It is concluded that the application of HBR 24 ∼ 30 mg a. i. ha⁻¹ with UASS 15 L ha⁻¹ at the wheat anthesis stage could be an effective approach to improve the wheat yield.

Introduction

Wheat (Triticum aestivum L.) is a major cereal crop and contributes around 40 % of the world's food sources (Giraldo et al., 2019). Wheat grains are rich in calories and the primary source of dietary carbohydrates, proteins, sugar, fiber, and fat (Kumar et al., 2011). Wheat production is critical for global food security (Zou et al., 2018). According to (Erenstein et al., 2021), with the increasing demand for wheat, a 4 % decrease is estimated in global wheat farms (from 135 to 130 million) in 2030 due to limited resources such as diminishing fertile lands and agricultural water (Hawkesford et al., 2013). Furthermore, environmental stresses (biotic and abiotic) and extreme climatic conditions especially rising global temperature and changes in precipitation frequencies, also have a direct and indirect impact on the overall yield of crops (Daloz et al., 2021, Rosenzweig et al., 2014, Shen and Cao, 2012, Zhao et al., 2017). Abiotic factors such as heavy metals, salinity, drought, and extreme temperature are major yield-limiting factors (Zörb et al., 2019), which cause around 70 % of yield loss in major crops (Mantri et al., 2012). Estimation Based on the crop yield models has predicted the loss in agricultural productivity of major crops, including wheat, rice, and maize, which is a serious threat to food security (Tigchelaar et al., 2018). It is reported that wheat production is estimated to decrease by 6 % per °C in further global warming (Asseng et al., 2015). A decrease in wheat yield is due to the decrease in the duration of the wheat-growing season (Yang et al., 2014). In this situation, providing high grains production is a complex challenge that needs to be solved by multidisciplinary approaches in crop improvements (Zheng et al., 2011). Improving stomatal activity and photosynthesis increases the grains filling duration, which increases yield (Derkx et al., 2012, Dwivedi et al., 2018).

Plant growth regulators (PGRs) are extensively used to combat environmental factors, strengthen photosynthetic machinery and regulate senescence (Maghsoudi et al., 2020). Brassinolides (BRs) are the naturally occurring steroidal hormones involved in physiological functions, improve the plant tolerance against abiotic stresses (Naservafaei et al., 2021) and elevate reactive oxygen species (ROS) in high temperature and drought conditions to improve plant growth, yield and quality parameters in different crops (Fahad et al., 2016, Sharma et al., 2017, Sridhara et al., 2021). There is evidence that exogenously applied BRs have an ameliorative role in resistance against heavy metal stress, salinity stress and drought stress (Naveen et al., 2021, Nazir et al., 2021, Nazir et al., 2019). Furthermore, these phytohormones accelerate the photosynthesis rate, improve the antioxidant system, and intensify physio-biochemical attributes (Sanjari et al., 2019). Previous studies revealed the influence of BRs on morpho-physiological processes of different plants such as, stomatal movement, root morphology, protein synthesis, stem elongation, proline accumulation, photosynthesis, leaf exchange attributes, antioxidant system, cell viability, and osmoprotectant induction (Ahmad et al., 2018, Anjum et al., 2016, Krishna, 2003, Shahzad et al., 2018). Foliar application of BRs on flowering (anthesis) stage of wheat, increased period of physiological maturity, prolonged grains filling duration by controlling the sink functions and resulted in increased grain yield by 7.3 %, grain number per spike by 9.17 %, thousand-grain weight (TGW) by 6.62 % compared with control (CK) in wheat and rice (Saka et al., 2003). Moreover, foliar application of phytohormones also increased the number of spikes and spike length per plant, grains per spike, leaf area and grain yield under the salt stress and non-stress conditions (Lal et al., 2019).

28-Homobrassinolide (HBR) is an analogue of naturally occurring phytohormones that enhance the growth, yield, and quality traits and have a great role in photosynthetic traits, which leads to better grain filling, increase in photo-assimilate production and final yield (Yusuf et al. 2011). In literature, research showed that high-temperature stress and heavy metal down the regulation of PS II activity, HBR helps alleviate the effect of these stresses and leads to up-regulation of PS II activity (Hayat et al., 2010). Foliar application of HBR on tomatoes increased the chlorophyll, which increased the size, length, breadth, and fruit quality (Sridhara et al., 2021). However, the application of HBR on the wheat crop by different sprayers has not been studied.

The selection of high-efficiency spray equipment is a critical factor in achieving a better effect of agrochemicals (Yang et al., 2018). Electric air-pressure knapsack (EAP) and boom sprayers are commonly used in wheat fields. Although boom sprayers are helpful in spraying in large fields, because of driven by ground machinery, it has damaging effects on wheat plants and results in reduced yields, while EAPs are not suitable for large fields because of low working efficiency (Weicai et al., 2016). To improve the crop protection and efficiency of agrochemicals, unmanned aerial Spraying system (UASS) have been developed (Xiongkui et al., 2017). UASS as an emerging plant protection machinery, could not only improve the spraying efficiency of chemicals but also reduce the cost of wheat production. In order to reduce the chemical dosage and residue, the precise variable rate of pesticide spraying can be adjusted according to crop type (Lou et al., 2018, Wang et al., 2019a). The use of adjuvants in the spray composition increases the UASS spray efficiency, and chemical deposition by decreasing fine droplets and droplets winding spectra (Wang et al., 2018). Droplet deposition distribution and penetration depend on the size of the droplets; deposition of chemicals on the upper and lower canopy of the plant was increased with increasing droplet size at an extent of 185.09 μm volume median diameter (VMD) (Chen et al., 2020). The research community has shown interest in the droplet distribution efficiency of UASS and EAP by comparing the deposition of two sprayers. UASS had higher chemical deposition as compared to EAP, but less coverage rate and droplet density (Q. Xiao et al., 2020), and Wang et al. (2019b) concluded that spray deposition characterizations of the UASS were higher working efficiency and less losses to the ground as compared to EAP and boom sprayer. Another study revealed that spraying fungicide on heading stage, UASS had higher droplet distribution on the lower canopy of wheat as compared to manual sprayers, and the addition of adjuvant in UASS spray formulation showed better effect when chemical dosage reduced by 20 % as compared to EAP (Qin et al., 2018). Furthermore, application of weedicides in wheat field by UASS had more suppressive effects as compared to artificial sprayers (Zhang et al., 2020). Application of HBR by UASS on maize flowering stage had high deposition rate on plant leaves and resulted in high photosynthesis, grain filling rate and then higher yield in comparison with EAP (Hussain et al., 2022). The droplet distribution and penetration is characterized by the different spray volumes (SV). UASS with speed of 4.5 m s⁻¹, 3.5 m height have 49.1–57.1 % more deposition than EAP (Xiao et al., 2019) and UASS working efficiency is 20 times higher than EAP. Chemical formulation of 10 L ha⁻¹ with addition of adjuvant applied by UASS has better penetration in plant leaves (Wang et al., 2019b). UASS 15 L ha⁻¹ had better results than EAP 300 L ha⁻¹ spray volume when chemical dosage was same in both sprayers, while the same results were recorded in both sprayers when the chemical dosage was decreased by 20 % with the addition of adjuvants in UASS (Sarri et al., 2019). However, there are no reports of applying multiple dosages of HBR by different sprayers with various spray volumes during the wheat flowering stage.

In this research, we set different dosages of HBR and spray volumes for UASS and compared its effects with the EAP sprayer. We hypothesized that HBR applied by UASS would have better effects on wheat growth and development as compared to EAP sprayer. The objectives of this work were to: (1) compare the possible effects of HBR on wheat crop applied by UASS and EAP, (2) obtain the optimum dosage of HBR and spray volume for wheat crop applied by UASS, (3) evaluate the droplet deposition of EAP and spray volumes of UASS on different wheat canopies, (4) examine the effects UASS application of 20 % decreased HBR dosages than EAP.