Effect of Water Stress on Brazilian Soybean Production

Abstract

Among the strategies used for water management in agriculture, deficit irrigation stands out, which introduces the term water use efficiency (WUE). There is evidence that suggests that deficient irrigation can increase WUE without causing losses in the productivity of several crops; however, information is limited on the level of water replacement that does not cause negative effects on soybeans. Thus, the present work aimed to determine the level of water replacement under deficit that does not cause damage to productivity per area of soybean cultivation, identify water stress through the thermal response patterns of the soybean plant canopy, and verify the relationships between the Crop Water Stress Index (CWSI) and income and between CWSI and WUE.

1. Introduction

Soybean (Glycine max L.), a member of the Fabaceae family, is extensively cultivated worldwide as an annual legume species originating from China and introduced into large-scale production in the United States during the nineteenth century [1]. Its significant oil content (approximately 18%) and protein content (approximately 40%) render it valuable for both human and animal nutrition, thereby contributing to a reported 1200% increase in production over the past six decades. The global production landscape is primarily dominated by the United States, Brazil, and Argentina, collectively accounting for approximately 80% of the world’s soybean supply. The substantial demand for soybeans, particularly from China, has spurred a progressive upsurge in soybean production across Latin American countries affiliated with Mercosur, notably Brazil, Argentina, and Paraguay [2, 3].

The soybean complex is the main axis of Brazilian agribusiness, occupying more than 50% of the planted area [4]. In Brazil, cultivation is carried out in all regions (South, Southeast, Central-West, Northeast, and North), mainly in the states of Mato Grosso, Paraná, Rio Grande do Sul, and Goiás. Estimated soybean production for the 2020 harvest/2021 should reach 135 million tons, making the country the world’s largest oilseed producer with a planted area of more than 38.5 million hectares and average productivity of 3517 kg ha⁻¹ [5].

Brazil surpassed the United States, which produced more than 112.5 million tons of soybeans in a planted area of more than 33 million hectares. In 2021, 37% of the soy produced worldwide (almost 363 million tons) originated in Brazil. The State of Mato Grosso is the largest Brazilian producer of soybeans, with almost 36 million tons in a planted area of more than 10 million hectares. Next, the States of Paraná, Rio Grande do Sul, and Goiás stand out, which in 2021 produced 19.9, 20.2, and 13.7 million tons of soybeans, respectively. These four states that stand out in soybean production in Brazil together have a planted area of 25.7 million hectares, that is, 66.7% of the planted area in Brazil (Table 1).

Production (millions of tons)	Planted area (million hectares)	Productivity (kg ha^−1)
Soybean in the world
362,947	127,842	2839
Soybean in Brazil (the largest producer of the grain in the world)
135,409	38,502	3517
Soy in the United States (second world producer of the grain)
112,549	33,313	3379
Mato Grosso State (largest Brazilian soybean producer)
35,947	10,294	3492
Paraná state – Brazil
19,872	5618	3537
Rio Grande do Sul state – Brazil
20,164	6055	3330
Goiás state – Brazil
13,720	3694	3714

Table 1.

Soybean in numbers (2020/2021 harvest).

Source: [6].

Soybean has trifoliate or compound leaves, which are responsible for photosynthesis and have a scale structure that protects immature flowers before anthesis [7]. The plant has a branchy, hispid stem, which has a raceme-type inflorescence at its apex in determinate-growing varieties [8, 9]. Soy is an autogamous plant; that is, it has complete flowers appearing in the reproductive stage from node number 5 or higher [10]. After the inflorescence, the plant forms pods, seeds, and matures [7]. The pods are slightly arched, hairy, composed of two simple valves, varying in length from 2 to 7 cm, which house one to five seeds which, in turn, are smooth, oval, globose, and elliptical, and maybe yellow in color, black or green, with a hilum, predominantly brown, black, or gray.

Soy is a plant with a C3-type carbon fixation mechanism. However, it is one of the C3 that undergoes photosynthetic carbon reduction and does not have a CO₂concentration mechanism [7], differing from other grains such as rice, corn, and sorghum. Its production cycle is quite varied and depends on the genotype (cultivar) interaction with the environment [11, 12]. Soybean is extremely influenced by the photoperiod, so in short days (<hours of light), the plant tends to accelerate flowering, negatively affecting productivity. For this reason, latitude is an important element when adapting certain genotypes. Thus, varieties in America are divided into 13 maturation groups [10]. In Brazil, cultivated varieties vary between maturity groups VI and VIII [13].

Soybean varieties can have three types of growth: determinate, indeterminate, and semi-determinate. Predominantly, soybeans planted in Brazil have a specific type, which is characterized by the following attributes: zero vegetative growth after flowering, uniform flowering throughout the plant, development of pods at the top and base of the plant, similar leaf size, and concentration of pods at the terminal node [14].

2. Water relations in soybean crops

In Brazil, agricultural producers generally aim for high productivity by ignoring the rational use of water used in agriculture. This inadequate management of water resources may be a consequence of the fact that in most irrigated areas, the volume of water applied is not charged but rather the costs of driving and pumping [15]. However, water resources are becoming more limited due to climate change, which suggests increasing the efficiency of water use in agriculture to ensure food security [16]. The negative effects of water waste can lead to economic and environmental losses, such as leaching of agrochemicals, water eutrophication, and electricity costs, among others [15].

Two perspectives for defining water use efficiency: hydrological and physiological [17]. Thus, hydrologically, the United States is defined as the ratio between productivity and the volume of water used, where the volume of water corresponds to rain and/or irrigation [18]. From a physiological point of view, the term United States refers to the radius between the biomass of the crop and the loss of water from the plant to the atmosphere [17]. Using these definitions, United States can be measured on different scales: instantaneous at leaf level or crop level; however, there are difficulties in relating them [19]. In this sense, the decision on the level at which will be measured will depend on the ease or type of study.

Water scarcity is a significant constraint on biodiversity and crop productivity on a global scale, particularly in arid and semiarid regions characterized by limited precipitation and high rates of evapotranspiration. This scarcity is worsened by the unsustainable use of water resources. Drought conditions in water-stressed environments are expected to increase in response to the phenomenon of global warming [20].

Scientific research has devoted considerable attention to understanding the mechanisms of plant responses to water stress over the last few decades. In the context of plants, water stress triggers imbalances in the reactive oxygen species (ROS) system and antioxidant mechanisms. The excessive increase in ROS, caused by water stress, limits plant growth and compromises the physiological integrity of cell membranes. Furthermore, drought stress results in a decline in photosynthetic activity, causing disruptions in enzymes responsible for nitrogen metabolism, such as nitrate reductase (NR), glutamine synthetase, glutamine dehydrogenase, and glutamate synthase [19].

Water and energy are important resources for economic and social development, as well as environmental integrity, while both are essential for irrigated agriculture in Brazil and the world [21]. One approach to improving water use efficiency is to replace surface irrigation systems with more efficient pressurized systems such as spot irrigation [22, 23]. This interdependence, which is often referred to as the “water-energy nexus,” has increasingly been highlighted as an important issue for future strategic public policy considerations [24].

Soybean is a crop that has its peak water requirement at the time of flowering and grain filling (7–8 mm day⁻¹). Depending on the cultivar and the climatic conditions of the region, the soybean-growing period varies from 90 to 135 days, requiring a total irrigation depth to complete its cycle without water restrictions of 450–700 mm [20, 25].

Deficit irrigation (DI) strategies to increase water productivity have been used in many crops of agricultural interest [26, 27]. It is a water conservation technique that exposes crops to a specific level of water stress during a certain stage of development or throughout the growing season without a significant reduction in yield [28]. To date, no detailed comparative study has been presented in the scientific literature to determine what would be the level of water replacement that does not cause physiological damage and consequently productivity for the current soybean varieties planted in Brazil.

3. Water stress in soybean crops

Given the economic significance of soybeans in Brazil, understanding the crop’s responses to inadequate irrigation is imperative. Moreover, with anticipated climate change effects, drought scenarios are projected in soybean-growing regions [11, 12]. Water stress during soybean growth emerges as the primary driver of yield variability, resulting in reductions ranging from 46–74% [11, 12].

Soybean’s sensitivity to water deficit manifests diverse responses depending on the phenological stage, severity, and duration of stress [14]. Generally, soybean plants exhibit heightened vulnerability to severe water stress during reproductive stages. Plant response to drought is manifested through biophysical or chemical mechanisms [29], wherein, under water deficit conditions, plants may augment stomatal resistance [30], enhance water use efficiency, and adapt root systems to access moisture from deeper soil layers [31].

At the morphological level, soybean plants subjected to water deficit may undergo alterations in both aerial and root structures. These changes are more pronounced during prolonged drought periods and may involve reductions in stem and trifoliate leaf size [7]. Studies on 48 Brazilian soybean genotypes under deficient irrigation noted decreases in plant stature and root volume [32].

Physiologically, water deficit induces alterations in gas exchange, leading to fluctuations in leaf temperature and changes in photosynthetic structures and pigments like chlorophyll and carotenoids [7]. Diminished pigment levels constrain the conversion of solar energy into chemical energy required for metabolic functions. Despite moderate water stress, gas exchange limitations do not compromise photosystem II (PSII), thus maintaining quantum efficiency and electron transport rates unaffected in soybeans [7].

Morphophysiological changes produce large reductions in flowering percentage, pod formation, and other yield components [11]. The biggest drops in productivity occur when water stress occurs during the reproductive stage because high rates of flower abortions have been observed [7]. On the other hand, when stress occurs during the grain-filling period, the size of the seeds is smaller due to the loss of moisture [7].

4. Thermal response to water deficit

Water stress stands as a primary determinant of diminished soybean yields in arid and semiarid territories globally [33]. Within such regions, the sporadic nature of precipitation induces fluctuations in water and nutrient uptake, thereby impacting spatial and temporal productivity within cultivating zones [34]. In semiarid locales, monitoring crop water status becomes pivotal for regulating irrigation protocols to ensure crop stability [35].

Efficient water management in irrigation hinges directly upon factors associated with the crop, atmospheric conditions, and soil composition, which collectively dictate irrigation depth and optimal timing of application. Diverse methodologies have been explored to optimize water utilization in agriculture, given the escalating demand for this resource across multifarious sectors of the economy. Consequently, several approaches assessing plant water stress in situ have been employed, including the assessment of canopy temperature, which serves as a proxy for estimating crop water stress indices [36].

Infrared thermometry emerges as a dependable and easily deployable technique for quantifying leaf temperature [35]. Through canopy temperature measurement, identification of crop water stress [37, 38], irrigation management [39], and even phenotyping drought-tolerant cultivars [40] have been facilitated. Leaf temperature is contingent upon crop water status and prevailing environmental parameters [37], with their interplay encapsulated in the formulation of the Crop Water Stress Index (CWSI) [37].

The genesis of plant temperature investigation vis-à-vis environmental factors dates back to the study by the authors in Ref. [41], who utilized mercury thermometers to ascertain leaf temperature. Subsequently, the adoption of thermocouples became customary in research endeavors aimed at gauging leaf temperature, with these sensors pivotal in one of the inaugural reports highlighting leaf temperature registering lower than ambient air temperature [42].

The authors in Refs. [43, 44] followed the same line, but the authors in Ref. [45] argued that cooling, caused by perspiration, did not satisfactorily explain the results obtained, which were justified by other factors, including incorrect temperature measurements. Only the work using thinner wire thermocouples conclusively demonstrated that the leaf temperature could be lower than that of the air and that this was a function of the air vapor pressure deficit [37].

The estimation of crop canopy temperature gained special emphasis from the empirical and theoretical approach to determining the CWSI proposed by the authors in Refs. [37, 38, 39, 40, 41, 42, 43, 44, 45, 46], respectively, where an infrared thermometer was used to determine temperature, contrasting with individual measurements carried out by thermocouples. According to the authors in Refs. [47], infrared thermometry is an ideal method for monitoring water stress, as it is non-destructive, enabling continuous and less expensive measurement than many alternative methods.

Technological development controls the evolution of parameters such as resolution and precision, which determine the quality and cost of this equipment. Thermal imaging allows information about the temperatures of all areas to be obtained simultaneously in one image. Therefore, it provides an ideal approach for collecting a large number of individual leaf temperatures when used for CWSI determination [48].

Infrared thermometers and thermal cameras are important instruments used to estimate crop surface temperature today. When operated manually, they can be portable or mounted (fixed), supported by equipment such as tripods, platforms, or cranes [49].

To calculate the CWSI for a crop, it is essential to know the upper (canopy temperature of a plant under water stress) and lower (canopy temperature of a plant under full irrigation) baselines [46]. Baselines for calculating CWSI can be defined in several ways [50]. Thus, currently, statistical, empirical, and theoretical methods are used [50, 51]. One of the most used is the theoretical method proposed by the authors in Ref. [37], in which the baselines are estimated from the energy balance equation.

In this way, the upper experimental limit represents a condition of maximum crop stress when the stomata would be completely closed, and the lower limit is the representative measurement of the plant without any water restriction [40]. The position of the maximum stress line can be determined experimentally when soil moisture is close to the permanent wilting point. Therefore, according to experimental limits, a water stress index for crops is between 0 (zero water stress) and 1 (maximum water stress) [52].

When a crop is under water stress, the stomata reduce their opening, reaching the extreme of total closure, and transpiration decreases so that the leaf temperature increases significantly [51]. A plant without water stress has a leaf temperature of 1–4°C lower than the ambient temperature, and in this case, the CWSI is close to zero. When transpiration decreases, the leaf temperature rises and can be between 4 and 6°C above the ambient temperature. In this case, the water stress is high, the plant has its leaf transpiration strongly reduced, and the CWSI becomes more close to 1 [52].

The CWSI calculation uses three main environmental variables: plant canopy temperature (Tc), air temperature (Ta), and atmospheric vapor pressure deficiency (DPV). All three of these variables have a great influence on the water used by the plant [53, 54].

The method proposed by Jackson has been investigated over the years by several authors on different crops, such as corn [54, 55], soybeans [34], eggplant [56], cherry tomatoes [57], sugarcane [39], and lettuce [58], among others.

The authors in Ref. [59] used a total of 16 infrared thermometers mounted on the lateral line of a central pivot connected to a datalogger to estimate and record canopy temperature in a sorghum crop. The author aimed to compare the direct method based on the neutron probe, with the method composed of infrared thermometers to determine the CWSI, and subsequently develop an irrigation plan. They used two thermal cameras mounted on a truck crane, 15 m above the olive tree canopy under five different irrigation regimes, to investigate the use of thermal images in field estimation of soil and crop water status.

5. Biometric relationships of soybean cultivation and water use efficiency

Reducing irrigation, as noted by the authors in Ref. [60], constrains nutrient absorption, root growth, and the transfer of carbon reserves from plant tissues to grains. However, the response of water use efficiency can vary within the same species depending on the type of water stress imposed on the plant [61]. Moreover, factors such as human intervention in irrigation decision-making [16], as well as diverse irrigation strategies and systems [62], can influence water use efficiency values.

Reference values for water use efficiency differ across various studies, complicating the establishment of clear biometric relationships with water use efficiency in tropical soybean cultivation. Precision drip irrigation holds promise in addressing some of these knowledge gaps, as highlighted by Blum [63], who emphasizes that effective water use entails maximizing soil moisture utilization for transpiration while minimizing water losses through evaporation and drainage.

Studies have linked instantaneous and crop-scale water use efficiency to biomass and leaf area in beans under deficit irrigation, yielding low coefficients ranging from 0.03 to 0.33. Conversely, the authors in Ref. [64] investigated irrigation treatments involving salinity and reported significant coefficients between water use efficiency and wheat crop yield. Additionally, the authors in Ref. [65] employed remote sensing tools to examine soybean cultivation, revealing favorable correlation rates between water use efficiency and soybean productivity during the vegetative stage. However, experiments on soil cover management in soybean plantations indicated minimal correspondence between water use efficiency and productivity [66]. Thus, the factors contributing to the variability in the relationship between biometric variables and water use efficiency in soybeans remain ambiguous.

6. Conclusions

Hence, the current investigation furnishes pertinent insights into water management strategies for soybean cultivation. Profound observations elucidate the significant repercussions of water stress on Brazilian soybean production, given the crop’s acute sensitivity to water availability across its developmental phases.

Diminished Yield: Water deficit during crucial growth stages, notably flowering and pod formation, precipitates yield reduction. Optimal water levels are imperative for pod setting and seed filling, as inadequate moisture fosters flower abortion, diminishes pod formation, and shrinks seed size, ultimately diminishing yield per hectare.

Delayed Maturation: Water scarcity retards soybean plant maturation as survival takes precedence over reproduction, elongating the developmental timeline. This prolongs exposure to adverse weather conditions or diseases, heightening susceptibility.

Increased Vulnerability to Pests and Diseases: Water-deprived plants exhibit weakened defense mechanisms, rendering them susceptible to pest and pathogen attacks, exacerbating yield and quality diminution unless adequately addressed through pest management.

Economic Implications: Reduced yields and compromised quality entail substantial economic losses for Brazilian soybean growers, as diminished production coupled with lower market valuation decreases farm income and profitability.

To alleviate water stress’s impact, Brazilian farmers may implement diverse strategies such as irrigation, drought-resistant soybean varieties, enhanced soil management, and crop rotation. Technological innovations like precision irrigation systems and drought monitoring tools offer avenues to optimize water utilization and mitigate adverse effects on soybean crops.

The CWSI allows for a simpler interpretation of the plant’s water status, as the index varies from 0 (no stress) to 1 (maximum stress). The ease and robustness that thermometry provides would allow farmers to make better irrigation decisions. Furthermore, they could estimate possible drops in soybean yield with some reliability.

Conflict of interest

The authors declare no conflict of interest.