Cobalt Combined with Potassium Humate as Beneficial Applications in Alleviating Water Stress Impacts on Groundnut During Sensitive Growth Stages

Enhancing plant nutrition during critical periods of water stress is a paramount objective. As such, this study aims to examine the effects that arise from implementing water stress regimes during the sensitive growth phases of groundnut. Furthermore, we will explore the impact of applying cobalt and potassium humate individually or in combination, either through foliar application (KH-F) or soil amendments (KH-S), to mitigate the adverse effects on nutrient accumulation, yield, and irrigation water use efficiency (Iwue). A field experiment was conducted between 2021 and 2022 using a split-split plot design. In the main plots, an additional control of full irrigation requirement scheme (IR100) was implemented, along with three other schemes: the first involved applying 75% of the irrigation requirement during the development stage (IRD75), the second applied 75% during the flowering till pod stage (IRF75), and the third involved applying 85% from the development stage till the pod stage (IR85). In the subplots, cobalt was applied at rates of 0 and 7.5 mg l−1 to the plants. The sub-sub plots were further divided to investigate the effects of KH-F and KH-S. Plants that experienced water stress during the flowering stage were found to be more vulnerable. However, when treated with KH-S, there were significant improvements in yield compared to KH-F treatment. This effect was particularly pronounced under the IRF75 irrigation scheme. Interestingly, there was an antagonistic relationship observed between cobalt and iron or manganese, in contrast to other nutrients. When KH-S was applied in combination with cobalt, it showed the potential to enhance various characteristics and increase the uptake of iron or manganese. The highest yield, nutrient accumulation, and Iwue were achieved through the combined application of cobalt and KH-S, along with adopting the IR85% irrigation scheme.


Introduction
Currently, one of the most critical global challenges is the availability of an adequate freshwater supply. Although water covers 71% of Earth's surface, freshwater only accounts for 3%, leading to potential limitations in crop production, especially in sandy soils with arid climates (Chavarria and Dos Santos 2012;Khatri and Tyagi 2015). These conditions impose abiotic stresses that hinder plant growth and development, resulting in significant crop yield reductions (He et al. 2018).
Consequently, there is a growing emphasis on the ability to retain water within plants for extended periods and in sufficient quantities, as up to 90-95% of absorbed water is lost through evaporation (Rao et al. 2018). The plant's natural defense mechanisms are insufficient to endure prolonged water stress, making it crucial to identify effective materials that can enhance plant defense efficiency and water conservation.
Potassium humate (KH) is one such material with unique properties that can improve both plant defense systems and nutrition using various techniques. It can function as an antitranspirant by forming a double layer that reduces transpiration rates from plant leaves, thereby enhancing water status in plant tissues (Mahdi et al. 2021;Elshamly 2023). Additionally, KH acts as a plant biostimulant and a highly soluble source of potassium (Ameen et al. 2019). Therefore, KH is an effective natural resource that plays a significant role in improving organic matter content, enhancing soil nutrition, promoting plant growth, and increasing Iwue (Ibrahim and Ali 2018;Saad 2020). Furthermore, potassium plays a vital role in regulating osmotic potential, stomatal movement, activating enzymes involved in respiration and photosynthesis, promoting cell division, improving yield, carbohydrate transformation, and the synthesis of sugars and starch (Banerjee et al. 2016;Hasanuzzaman et al. 2018). While several studies have highlighted the benefits of applying KH to plants under stressful conditions as mentioned above, few have determined the optimal method of application, whether foliar or soil, when groundnut plants are exposed to water stress. Therefore, one of the key objectives of this research is to identify the most effective method of applying KH under water stress conditions and assess its effects on mitigating these conditions during the most sensitive growth stages.
On the other hand, the importance of cobalt (Cb) in plants remains somewhat unclear, although several studies have demonstrated its multiple benefits for plant growth, particularly in the Leguminosae family. Cb is essential for numerous enzymes involved in nitrogen fixation and supports a group of N 2 -fixing bacteria known as diazotrophs, which are associated with the roots of various crops such as barley, corn, rice, sugarcane, and wheat (Hu et al. 2022). Cb influences plant growth and metabolism at different stages, depending on its concentration and the rhizosphere conditions (Elshamly and Nassar 2023). Despite the existence of favorable effects of low Cb on plant metabolism (El-Seoud et al. 1994), its full potential is yet to be realized. Nonetheless, various researchers have reported positive effects of Cb on plant growth, yield, and the content of both macronutrients and micronutrients (Fang et al. 2017;Carvalho and Foulkes 2018;Gad et al. 2019;Hong et al. 2019;Elshamly and Nassar 2023).
Based on the aforementioned information, it can be hypothesized that the application of Cb or KH has the potential to reduce water loss, depending on the intensity of the irrigation scheme and the effectiveness of these applications in mitigating the impacts of water deficiency. However, it is essential to consider whether it would be more effective, under stress conditions, to focus on reducing water loss through foliar application or on improving soil nutrition status through soil application. Additionally, it is important to investigate whether the simultaneous use of Cb and KH can enhance the benefits for groundnut growth, which is one of the objectives of the current research.
To test this hypothesis, it is crucial to understand the main factors that affect plant performance under water stress conditions. One of these factors is pH, which significantly influences the availability of macronutrients and micronutrients, particularly in arid regions. Meng et al. (2013) pointed out that soil pH reflects the dynamic equilibrium between proton production (such as the dissociation of carbonate, organic acids, and sulfur oxidation) and proton consumption (including soil mineral weathering and soil organic matter mineralization) in a soil-plant system. Therefore, comprehending the pH dynamics during different irrigation schemes and its interactions with Cb and KH is crucial in mitigating water stress. This is particularly important for plants like groundnut, which have mechanisms to facilitate the activation and absorption of insoluble nutrients in the soil (Wang and Gao 2019). These mechanisms may include controlling pH by adjusting the balance between cation and anion nutrients, modifying nitrate uptake, or increasing root exudates (Sugiyama and Yazaki 2012; Zhang et al. 2016;Dayoub et al. 2017;Duchene et al. 2017;Guinet et al. 2020).
In arid regions, various factors contribute to the limitation of sustainable agricultural development, including water scarcity and the ability of plants to withstand the associated impacts, which directly affect crop production. Furlan et al. (2012) observed that water stress negatively affected groundnut growth, resulting in reduced shoot dry weight, nodule count, harvest index, specific leaf area, and nitrogen content, while root dry weight and chlorophyll content increased. Another study mentioned that although groundnut is relatively tolerant to water stress, it still experiences undesirable effects on yield and seed quality (Zhang et al. 2021). However, plants have developed mechanisms to tolerate water stress, such as increased root density, activation of antioxidant systems, accumulation of osmolytes, and synthesis of aquaporins (Ma et al. 2020;Kapoor et al. 2020). Therefore, ensuring adequate irrigation for groundnut plants is crucial for pod formation and yield (Sezen et al. 2022). Understanding the most sensitive growth stage to water stress and the resulting impacts on plants in these stressful conditions is essential for developing technologies to enhance crop performance in water-limited environments. Numerous studies have differed in identifying the most sensitive growth stage to water stress and the optimal intensity of water regimes for achieving the best groundnut yield. Generally, they have concluded that the highest yield under water stress conditions is obtained during the vegetative stage, followed by the flowering stage, and finally the pod-filling stage (Kandoliya et al. 2015;Boydak et al. 2021;Rachaputi et al. 2021). Regarding the preferred water regime, Xia et al. (2020) demonstrated that a moderate irrigation scheme has better impacts on yield compared to severe or full schemes, especially when combined with the application of 150 kg ha −1 of nitrogen. Rathore et al. (2021) showed that implementing deficit irrigation by applying 90% or 80% of crop evapotranspiration resulted in the highest yield, Iwue, and quality of groundnut. Emara et al. (2022) reported that applying 100% of the crop water requirement to the Giza 6 variety under a drip irrigation system achieved the highest yield. Meanwhile,  indicated that soil moisture should not fall below 70% of field moisture capacity from flowering to ripening stages. However, it is worth noting that only a few studies have investigated the optimum irrigation schemes during the sensitive growth stage of groundnut from vegetative to pod-filling stages, as most of them focus on determining the best irrigation scheme for the entire growth season, which is one of the objectives of the current study.
Globally, groundnut (Arachis hypogaea), or peanut, is an important summer oil crop and food grain legume. In Africa, groundnuts are cultivated on 40% of the arable land, while worldwide, they cover an area of 27.7 million hectares and yield an average of approximately 1.59 tons per hectare (Khudaykulov et al. 2021). In Egypt, the cultivated area for groundnut was around 66,000 hectares, with a total yield of 231,223 tons in 2019 (Abdelghany et al. 2022).
While previous studies have examined the effects of applying Cb and KH individually to plants, only a few articles have investigated their combined application in mitigating the adverse impacts of water stress. Therefore, the main objective of the current research is to assess the individual and combined effects of Cb and KH applications on alleviating water stress during the sensitive growth stage. The study aims to determine whether these applications can enhance the accumulation of macronutrients and micronutrients, improve yield, and increase Iwue in groundnut crops under arid conditions. Additionally, the research aims to identify the preferred method of applying KH, either through foliar spraying or ground drenching, in combination with Cb when groundnut plants are exposed to water stress during the sensitive stage.

Study Site
The experiment site was conducted in the summer of 2021 and 2022 growing seasons at the experimental farm of water studies and researches station, National Water Research Center, Toshka, Egypt, which is located at the latitude of 22°, 24′.11′ N, longitude of 31°, 35′.43′ E, and altitude of 188 m.

Meteorological, Soil, and Irrigation Water Data
The daily meteorological data for the period of May to September at the two growing seasons were obtained from the Toshka agrometeorological station allocated at the experiment site. The mean monthly relative humidity, mean maximum, and minimum temperature data for the groundnut crop growing season in the region are presented in Table 1. The physical and chemical properties of the experimental soil and irrigation water are given in Table 2. The main source of irrigation water is groundwater through a well that was dug in the studied area.

Experimental Design and Treatments
The experiment was carried out in a split-split plot design with three replications under a drip irrigation system. The main plot was allocated for water irrigation schemes as follows: first scheme by applying 100% of irrigation requirements at all growth stages (IR100), secondly by applying 75% of irrigation requirements during the development stage while applying the full irrigation requirements in the remaining growth stages (IRD75), third scheme by applying 75% of the irrigation requirements during flowering and pod stage while applying 100% in the remaining growth stages (IRF75), finally by applying 85% of irrigation requirements starting from development till pod stage and applying the full irrigation requirements in the remaining growth stages (IR85). The selection of the irrigation schemes in this study was depended on the recommendations of previous studies such as (Xia et al. 2020;. Additionally, before the experiment was started, soil water parameters were measured; then, the reductions in the soil moisture till it reach 45% of available water were recorded, which previous studies demonstrated was the critical limit on yield. Accordingly, based on this knowledge, the irrigation was every 2 days. Furthermore, regarding the irrigation amounts that applied to groundnut in the different irrigation schemes, they were proportionally obtained from IR100 as will be handled later. Moreover, there was a buffer zone (3 m width) between irrigation units to avoid interactions, and each irrigation plot was equipped with a manometer valve to control the operating pressure at 1 bar, and also with a flow emitter its discharge was 4.0 l h -1 to control the mounts of the targeted irrigation water requirements at each irrigation scheme. While in the subplots, two concentrations of Cb application were assigned (Cb at a rate of 0 and 7.5 mg l −1 ). The Cb applications in a form (CoSO 4 . 7H 2 O) were purchased from Sigma-Aldrich Co. On the other hand, the subplots were equipped with a valve and Cb was injected through the drip irrigation systems once time only after 30 days from the planting date. The selection of the Cb application at 7.5 mg l −1 and after 30 days from sowing in the current study was depended on the recommendation of a previous study (Gad 2012). Whereas the sub-sub plots were assigned for KH application as follows {control spray tap water, 3 g l −1 of KH was applied as foliar spraying after 30, 45, and 60 days from planting, denoted KH-F, KH applied as soil drench: 6 (kg ha −1 ) was injected by drip irrigation systems twice after 30 and 60 days from emergence in two equal doses, denoted KH-S}. The KH was purchased from Central China Trading Co. Ltd (10% K 2 O). It is worth noting that KH application rates as KH-S or KH-F and the intervals of these applications were implemented according to the manufacturer's recommendations. The experimental site was irrigated using a drip irrigation system. All plots were irrigated equally after sowing, and in regard to growth stages, the different plant growth stages of groundnut were divided into four stages: • Initial stage that ranges between 0 and 35 days after sowing. • Development stage that ranges between 35 and 55 days after sowing. • Flowering and pod formation stage that ranges between 55 and 95 days after sowing. • Late-season stage, ranges between 95 and 115 days after sowing.
These growth stages were determined according to the physiological and morphological features of the groundnut based on (Kandoliya et al. 2015).

Crop Husbandry
The area of the experiment was well prepared, and the fertilization and other field practices recommendations of the Ministry of Agriculture and Land Reclamation in Egypt were implemented for newly reclaimed soil. Groundnut crop (Arachis hypogaea L.) variety Giza 6 at rates of 180 kg pods ha −1 (120 kg seeds ha −1 ) were sown in hills on the 1st week of May 2021 and the 8th of May 2022 in the first and second season, respectively. After inoculation with root nodules bacteria (Rhizobium leguminosarum), two seeds were drilled per hill and the seeds were sown on one side of the dripper's jet. The plant spacing was planting with space of 30 cm within rows and 60 cm between rows, and the sowing depth was 5 cm.

The Calculations of Irrigation Water Requirements
The IR100 was calculated by entering specific data that were obtained from the Toshka Agrometeorological Station, in CROPWAT package, version 8.0 (FAO 2009), which is a software program for planning and managing irrigation, using the Penman-Monteith equation to calculate ETo on a daily basis from the measured climatic data as the following equation: where ETo = reference evapotranspiration (mm day −1 ). Rn = net radiation (MJm −2 day −1 ). G = soil heat flux (MJm −2 day −1 ). Δ = slope vapor pressure and temperature curve (kPa °C −1 ). γ = psychrometric constant (kPa °C −1 ). U 2 = wind speed at 2 m height (ms −1 ). es-ea = vapor pressure deficit (kPa). T = mean daily air temperature at 2 m height (°C).
Finally, IR100 amounts were calculated according to the following equation: where IR = irrigation requirements (mm). ETc = crop evapotranspiration (mm). Lr = leaching requirement %, equaled 10% since EC of soil solution is low, Lr was neglected. Ei = the efficiency for the irrigation system %, the efficiency for drip irrigation = 85%.

Measurements of Nutrient Content in Seeds and Soil pH
After harvest, the dried groundnut seeds were weighed and ground into a fine powder. The macronutrients {nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca)} and micronutrients {iron (Fe), zinc (Zn), manganese (Mn), and copper (Cu)} were estimated according to (Nankya et al. 2021;Kumssa et al. 2022). It is worth noting that by the visual observation of the soil profile, there was a difference in soil color, which was confirmed in the mechanical analysis reflected in water status as can be seen in Table 2. Therefore, soil samples were taken from two depths (0-20 and 20-40 cm) horizons. It was air-dried and the soil was passed through a (1 mm) mesh sieve. The soil samples were used to determine soil pH by potentiometric method (digital ionalyzer/501, Orion research multifunctional pH meter), and the soil: water ratio was 2.5:1.

Yield Measurements
The groundnut crop was harvested at full maturity, and record the average of the following traits: the average root length, the average shoot fresh weight, the average seed weight plant −1 , the average pod weight plant −1 , pods yield, and seed yield.

Statistical Analysis
Means of variance (ANOVA) were analyzed in all data to determine any statistically significant differences using SAS software. The means were separated through a revised least significant difference (LSD) test at the 0.05 level as per Casella (2008).

The Effects of the Examined Irrigation Schemes, Cb, and KH as (KH-F or KH-F) on Macronutrients (N, P, K, and Ca)
The individual and interaction effects of the examined irrigation schemes, Cb, and KH on the investigated parameters are given in Table 3.
The impacts of irrigation water schemes, Cb, and KH as KH-F or Khgd on the uptake of N are presented in Fig. 1a. In general, by comparing IR100 and IRD75 irrigation schemes under Cb 0 treatment, applying the applications of KH-F or KH-S, there had no significant difference in N content. While, under IRF75 and IR85 irrigation schemes, there was a significant difference in N content with the adoption of KH-S. Likewise, under Cb treatment, where the results indicated that there had no significant difference in N content under IR100 and IRD75 irrigation schemes, while there was a significant difference in N content by the adoption of KH-F or KH-S. The obtained results indicated that under Cb 0 treatment, IRF75 led to the lower N content or either by the adoption of KH-F and the implementation of IRF75. In the same context, a lower N content under Cb treatment was obtained by the adoption of IRD75 or even by the adoption of KH-F and the implementation of IRD75. On the other hand, adopting IR85 and applying Cb 7.5 + KH-F or KH-S attained the highest increase in N content. On the other side, the results demonstrated that by comparing control in Fig. 1b, it was found that IRF75 irrigation scheme significantly attained the lowest values of the P content, although the adoption of KH-F or KH-S in combination with Cb application achieved significant improvements, while without supplying these auxiliary applications of Cb, there had no significant difference. On the other hand, the gained results illustrated that the maximum increase of P content was observed with the adoption of Cb 7.5 + KH-F and the implementation IR85 irrigation  Fig. 1c, the content of K was decreased by adopting IRF75. While by adopting IR85 water irrigation scheme × KH-S, the highest K content in the groundnut seeds was attained.
As can be seen in Fig. 1d, it was found that IRF75 irrigation scheme significantly achieved the lowest values of the Ca content, although the adoption of KH-F or Khgd in combination with or without Cb application achieved significant enhancements. Likewise, adopting IR100 water irrigation scheme × (Cb 7.5 + KH-S) applications were significantly equaled in applying combined applications of (Cb 7.5 + KH-S) × IR85 irrigation scheme, for attaining the highest Ca content.

The Effects of the Examined Irrigation Schemes, Cb, and KH as (KH-F or KH-S) on Micronutrients (Fe, Mn, Zn, and Cu) and Soil pH
The impacts of irrigation water schemes, Cb, and KH as KH-F or KH-S on the accumulation of micronutrients are presented in Fig. 2. Generally, the obtained results indicated that increasing the water stress pattern intensity IRD75 and IRF75 attained the lowest absorption contents for the micronutrients. Where, as can be seen in Fig. 2a, Fe content in groundnut seeds increased with the adoption of high irrigation quantities than IRF75 irrigation scheme. Furthermore, the obtained results indicated that  Likewise, observing Fe, the results showed that the adoption of supplying combined applications of Cb attained the lowest values of Mn content in groundnut seeds under various irrigation schemes. Whereas in the obtained results in Fig. 2b, the lowest Mn contents in groundnut seeds were obtained by the adoption of IRF75 irrigation scheme. Likewise, the highest Mn concentrations were recorded by the adoption of KH-S × IR85 irrigation scheme. While according to the obtained results in Fig. 2c, the contents of Zn in groundnut seeds were enhanced significantly by the adoption of supplying a combined application of Cb compared to the non-Cb applications. The adoption of Khgd × IR85 irrigation scheme was pronounced for attaining the highest Zn concentrations. Likewise, the lowest Zn contents in groundnut seeds were obtained under IRF75 irrigation scheme.
The Cu contents in groundnut seeds increased by the adoption of Khgd than KH-F as can be seen in Fig. 2d. The highest Cu contents were significantly obtained by applying KH-F or KH-S + Cb 7.5 applications × adopting IR85 Fig. 2 The impacts of implementing different water stress schemes and applied cobalt applications combined with foliar and soil applications of potassium humate on the average a iron (Fe), b manganese (Mn), c zinc (Zn), and d copper (Cu) in the groundnut seeds at two seasons of 2021/2022. Error bars indicate standard errors from the mean. Bars with different letters are statistically significant at p ≤ 0.05. Abbreviations: control, spray with pure water; KH-F (applying potassium humate as foliar application); KH-S (applying potassium humate as soil application); Cbo (without applied cobalt); Cb7.5 (applied 7.5 mg l −1 cobalt injection in the irrigation water); IR100 (applying the full irrigation requirement at all stages); IRD 75 (applying 75% of irrigation requirements during development stage; IRF75 (applying 75% of irrigation requirements from flowering till pod stage; and IR85 (applying 85% of irrigation requirements from development till pod stage) According to the obtained results in Fig. 3, generally under control treatment and without Cb applications, the soil pH attained the highest values by the reduction of water irrigation requirements, particularly at the topsoil (20 cm) while at 40 cm from the surface, the highest values were obtained by the adoption of IRD75 irrigation scheme. The highest pH values were significantly obtained under control treatment and the adoption of IRF75 irrigation scheme, while the lowest pH values were significantly obtained by applying KH-S + Cb 7.5 applications × the adoption of IR85 irrigation scheme.

The Effects of the Examined Irrigation Schemes, Cb, and Kh as (KH-F or KH-S) on Agronomic Traits, Yield, and Iwue
As can be seen in Fig. 4a, the obtained results indicated that root length was affected negatively by increasing the reduction of the irrigation water requirements, and positively by the adoption of KH-S with or without Cb applications. Whereas the adoption of KH-S + Cb 0 or Cb 7.5 applications × IR85 irrigation scheme attained the highest increase of the groundnut root length, while the lowest groundnut root length was recorded under KH-F × IRF75 irrigation scheme, which significantly equaled adopting control IRF75 irrigation scheme. On the other hand, the highest shoot fresh weight values were obtained with the adoption of KH-S + Cb 7.5 applications × the implementation of IR85 irrigation scheme ( Fig. 4b), while the lowest shoot fresh weight values were obtained by the adoption of control IRD75.
Also, the implementation of IR100 and IR85 irrigation schemes was statistically similar in attaining the highest seed weight per plant under the adoption of various examined application schemes (Fig. 4c). Moreover, the obtained results indicated that the highest seed weight per plant was obtained by adopting IR85 irrigation scheme and applying KH-S + Cb 7.5 applications. While the lowest seed weight per plant values was obtained by the adoption of control IRD75.
Likewise, the obtained results in Fig. 4d indicated that the highest pod weight per plant was obtained by adopting IR85 irrigation scheme and applying KH-S + Cb 7.5 applications. While the lowest pod weight per plant values was obtained by the adoption of control IRD 75, which significantly equaled the adoption of KH-F × IRF75 irrigation scheme.
As presented in Fig. 5a, the obtained results indicated that combined additions of KH-S + Cb 7.5 applications under IR85 irrigation scheme have achieved the best pod yield, which significantly equaled the adoption of KH-F × IR85 irrigation scheme. In addition, the adoption of IRF75 scheme under differently examined applications attained the lowest significant values for pod yield except with KH-S + Cb 7.5 applications; the results demonstrated that there was an improvement.
The obtained findings in Fig. 5b showed that with or without Cb application, the KH-S applications worked in enhancing groundnut seed yield under the adoption of the different examined irrigation schemes except under IRD 75   Fig. 3 The impacts of implementing different water stress schemes and applied cobalt applications combined with foliar and soil applications of potassium humate on the average soil pH values at two seasons of 2021/2022. Error bars indicate standard errors from the mean. Bars with different letters are statistically significant at p ≤ 0.05. Abbreviations: control, spray with pure water; KH-F (applying potassium humate as foliar application); KH-S (applying potas-sium humate as soil application); Cbo (without applied cobalt); Cb7.5 (applied 7.5 mg l −1 cobalt injection in the irrigation water); IR100 (applying the full irrigation requirement at all stages); IRD 75 (applying 75% of irrigation requirements during development stage; IRF75 (applying 75% of irrigation requirements from flowering till pod stage; and IR85 (applying 85% of irrigation requirements from development till pod stage) there had no significant difference. In the same context, the highest groundnut seed yield was obtained by the adoption of KH-S + Cb 7.5 applications under IR85. On the other hand, the obtained results indicated that the lowest groundnut seed yield was significantly obtained under the adoption of IRF75 irrigation scheme. As can be seen in Fig. 5c, the obtained results indicated that the highest shelling values were obtained with the implementation of IR100 or IRD 75 irrigation schemes and the adoption of (control or KH-F or KH-S) under Cb 0 applications. While the lowest shelling values were obtained by the adoption of control IRF75 irrigation scheme.
On the other hand, Iwue was enhanced by adopting KH-S + Cb 7.5 applications under IR85, although that significantly equaled the adoption of KH-F + Cb 7.5 applications under IR85 irrigation scheme (Fig. 5d). Furthermore, the results indicated that whether KH-F or KH-S applications were applied in combination with Cb 7.5 applications under IRD 75, they had caused Iwue values equal to or greater than the Iwue values obtained when adopting IR100. While the finding demonstrated that the adoption of IRF75 irrigation scheme under Cb 0 or Cb 7.5 applications significantly decreased Iwue compared to the control IR100 irrigation scheme and attained the lowest values. Fig. 4 The impacts of implementing different water stress schemes and applied cobalt applications combined with foliar and soil applications of potassium humate on the average groundnut a root length, b shoot fresh weight, c seeds weight per plant, and d pods weight per plant at two seasons of 2021/2022. Error bars indicate standard errors from the mean. Bars with different letters are statistically significant at p ≤ 0.05. Abbreviations: control, spray with pure water; KH-F (applying potassium humate as foliar application); KH-S (applying potassium humate as soil application); Cbo (without applied cobalt); Cb7.5 (applied 7.5 mg l −1 cobalt injection in the irrigation water); IR100 (applying the full irrigation requirement at all stages); IRD75 (applying 75% of irrigation requirements during development stage; IRF75 (applying 75% of irrigation requirements from flowering till pod stage; and IR85 (applying 85% of irrigation requirements from development till pod stage)

Discussion
Given the unpredictable and extreme climatic fluctuations experienced by crops during their growth, it is crucial to understand the periods most adversely affected by these stresses. Proper selection of nutrition and irrigation schemes under stress conditions is considered an effective strategy for maintaining productivity.
In this study, it was observed that groundnut plants are sensitive to water stress, particularly when subjected to the IRD75 or IRF75 irrigation schemes. These findings align with the research by (Maheswari et al. 2018). Notably, adopting the IRF75 scheme during the flowering and pod stages resulted in a significant reduction in groundnut yield, indicating the severity of this irrigation regime. It is hypothesized that the decrease in water requirements led to a higher rate of evaporation, resulting in the accumulation of undissolved alkaline salts in the topsoil layers. This aligns with the findings of Ren et al. (2019) and Liu et al. (2021). Consequently, soil pH increased, negatively impacting plant and microbiome activities. According to the proton budgets theory, the change in soil pH under these conditions is attributed to increased proton consumption processes rather than production processes. This is mainly Fig. 5 The impacts of implementing different water stress schemes and applied cobalt applications combined with foliar and soil applications of potassium humate on the average a groundnut seeds yield, b groundnut pods yield, c shelling, and d water use efficiency (Iwue) at two seasons of 2021/2022. Error bars indicate standard errors from the mean. Bars with different letters are statistically significant at p ≤ 0.05. Abbreviations: control, spray with pure water; KH-F (applying potassium humate as foliar application); KH-S (applying potassium humate as soil application); Cbo (without applied cobalt); Cb7.5 (applied 7.5 mg l −1 cobalt injection in the irrigation water); IR100 (applying the full irrigation requirement at all stages); IRD75 (applying 75% of irrigation requirements during development stage; IRF75 (applying 75% of irrigation requirements from flowering till pod stage; and IR85 (applying 85% of irrigation requirements from development till pod stage) Applied treatments d due to a decrease in root and microbiome exudates in the rhizosphere. The increase in soil pH leads to a decline in root length and a decrease in the availability of macronutrients (N, P, K, and Ca) and micronutrients (Fe, Mn, Zn, and Cu). Interestingly, even though groundnut plants were exposed to prolonged water stress under the IR85 irrigation scheme, which represents a moderate irrigation regime, positive impacts on yield and Iwue were observed. This highlights the importance of determining the appropriate stress pattern for each crop, especially during the sensitive period of water shortage. It is concluded that the differences in results are attributed to the intensity of water stress experienced by the plants. Under the IR85 regime, although the plants and microbiome had to cope with prolonged water stress, the reduction in applied water stimulated their activities and secretions. This led to the generation of an auxiliary source of proton production, resulting in some decrease in soil pH. Additionally, the reduction in applied water promoted deeper root penetration into the soil profile, enabling increased root length and expansion of the root's influence range. Consequently, water, macronutrient, and micronutrient accumulations improved, resulting in enhanced growth traits, yield, and Iwue under these circumstances (Cai and Ahmed 2022). In a similar context, Xia et al. (2020) discussed the reasons behind the superiority of moderate irrigation schemes in terms of yield. Moderate soil moisture content was found to improve nitrification intensity, leading to increased soil nitrogen content, stable photosynthetic products, and accelerated nutrient transport to sink organs, thereby contributing significantly to the final groundnut yield. Furthermore, they noted that under full irrigation, groundnut leaves did not always exhibit high chlorophyll content. Excessive irrigation water and nitrogen could easily cause diseases in groundnut leaves, negatively impacting yield.
In contrast, previous studies have demonstrated that legume crops have the ability to acidify the soil solution (Bouray et al. 2021;Lai et al. 2022) have shown that legume plants possess a stronger proton-releasing ability and the potential to enhance nutrient acquisition through various biophysical and chemical mechanisms, such as altering the production rate of extracellular enzymes and secreting organic anions, thereby acidifying the rhizosphere. However, the current study indicates that the continuity of this ability is influenced by the irrigation scheme pattern and the ability to improve growth conditions. When the IRF75 irrigation scheme was implemented without Cb applications, critical reductions were observed in several biophysical and chemical processes in the plants, surpassing their tolerance limits. To enhance this ability, supplementary applications, particularly those applied to the soil, were found to increase proton production and improve the bioavailability or phytoavailability of absorbed nutrients by soil organisms or plants (Antoniadis et al. 2017;Oburger et al. 2018), as opposed to foliar applications. Consequently, the results showed that KH-S applications resulted in greater improvements in yield compared to KH-F, especially under the IRF75 irrigation scheme. It can be concluded that the application of KH-S to stressed plants improved their water status due to the beneficial effects of K and humic substances (Mahdi et al. 2021). Additionally, KH-S applications improved soil structure and water-holding capacity, leading to enhancements in shoot fresh weight and nutrient absorption in the current study (Savarese et al. 2022). Furthermore, KH-S applications acted as a source of protons, causing a decrease in soil pH and positively affecting plant and microbiome activities, including exudate production, which is consistent with the findings of Msimbira and Smith (2020). Moreover, KH-S applications improved nutrient availability by facilitating chelation processes, particularly with micronutrients, through the ability of humic substances to form stable colloidal aggregates and provide potential binding sites for chelation (Tahoun et al. 2022). On the other hand, KH-F applications enhanced water status due to the benefits of K and humic substances already present in the applications. Although some reduction in soil pH was observed with KH-F applications, potentially attributed to increased plant secretions, the reductions in soil pH induced by KH-S applications were more pronounced, especially in the deeper soil layers (40 cm). Consequently, the benefits obtained from KH-S applications outweighed those of KH-F under these conditions. Except for Fe under the IRF75 irrigation scheme and Mn under various irrigation schemes, the current data indicates that the application of Cb leads to some improvements in nutrient absorption when compared to the control treatments. Furthermore, although reductions in Fe and Mn absorption were observed with Cb treatments under the different irrigation schemes, the application of KH-F or KH-S helped alleviate the intensity of these reductions. Specifically, KH-S was more effective in mitigating the reduction compared to KH-F. The reduction in Fe and Mn absorption can be primarily attributed to antagonistic reactions under these conditions. In the control treatment without Cb applications under various irrigation schemes (except IRF75), sufficient moisture was present in the rhizosphere during the major growth periods, leading to increased activities of plants and the microbiome, as well as their secretions, resulting in a decrease in soil pH to appropriate levels (Msimbira and Smith 2020; Zifcakova 2020). With the addition of Cb applications, there was an increase in microbiome activities, as Cb plays an important role in increasing soil microbiome activities (Banerjee and Bhattacharya 2021). Under these circumstances, the increased secretions from roots or the microbiome led to a decrease in soil pH, resulting in improved nutrient availability, particularly for P, as demonstrated in the obtained results. McCauley et al. (2017) have indicated that maintaining a pH range of 5.5-7.5 for P and 6.5-8 for Ca enhances the availability of P and Ca in the soil. However, due to the antagonistic relationships between P or Ca with Fe and Mn, which have been demonstrated in previous studies (Pii et al. 2015;Rietra et al. 2017;Palani and Raju 2019), the unavailable nutrients were formed, leading to reductions in the uptake of micronutrients Fe and Mn. In this context, studies by the International Plant Nutrition Institute (2015) have shown that Cb applications have a positive impact on enhancing the contents of N, P, and K in peanuts. These results are consistent with (Wyszkowski et al. 2009;Kosiorek and Wyszkowski 2019), who demonstrated that low concentrations of Cb increased the uptake of macronutrients, while higher concentrations had the opposite effect. On the other hand, the application of Cb seemed to increase competition with the remaining available nutrients Fe and Mn in the capture sites on the roots, resulting in intensified reductions (Gad 2012). These findings are supported by Gad et al. (2019), who reported that Cb application had a significant favorable impact on the mineral status of N, P, K, Zn, and Cu, with the exception of Fe uptake. They revealed a strong association between Cb and soil Mn and Fe oxides, leading to the presence of unavailable nutrients for plants. Therefore, under such conditions, the results showed that the application of KH provided positive benefits for the uptake of Fe, Mn, Cu, and Zn, with KH-S being more effective than KH-F. According to my hypothesis, the application of Cb increases the secretions from the microbiome, resulting in a decrease in soil pH and increased nutrient availability, as supported by previous studies (Kosiorek and Wyszkowski 2019;Mejia et al. 2023). Additionally, the application of KH enhances the nutritional status, strengthens plant growth, and increases root secretions (Kumar et al. 2013;Puglisi et al. 2013;Canellas et al. 2019), leading to an increase in nutrient availability (Ibrahim and Ali 2018; Mohammed et al. 2021). In the case of KH-S application being more effective than KH-F, this can be attributed to the chelating ability of KH-S, as mentioned previously, which allows for the formation of protective layers to maintain Fe and Mn and prevent fixation induced by P or Ca. Additionally, the presence of Fe and Mn as chelated nutrients improves their ability to compete with Cb for capture sites on the roots, ultimately resulting in the observed enhancements in the current study. These findings are in line with Zanin et al. (2019).
Under the IR85 irrigation scheme, the combined applications of Cb and KH-S demonstrated superior effects on nutrient uptake and yield traits, resulting in the highest yield and Iwue. This can be attributed to the specific conditions imposed by the IR85 irrigation scheme, which subjects the groundnut plants to a certain degree of water stress. As a response to water stress, the plants exhibited physiological changes such as increased root length and efficiency, leading to improved water and nutrient uptake from the soil rhizosphere and ultimately enhancing agronomic traits. The soil applications of KH-S provided significant quantities of required nutrients for the plants and contributed to the integrity of root membranes through osmotic adjustment (Canellas et al. 2008). Additionally, KH-S applications promoted lateral root formation and initiation of root hairs (Olivares et al. 2017), compensating for reductions in root efficiency observed under different irrigation schemes, including IR100 as mentioned by Elshamly (2023). These effects increased the groundnut plants' tolerance to water stress. Furthermore, KH-S applications had positive impacts on the physical properties of the soil, including improved ventilation and aggregation, attributed to the presence of humic substances (Li et al. 2019). Also, KH-S formatted chelate components with cationic metals (Zanin et al. 2019;Tahoun et al. 2022), reflecting for enhancements in nutrient metabolism and photosynthesis (Canellas et al. 2020;Nardi et al. 2021). Moreover, KH-S applications increased soil microorganism populations due to their nutrient content, which included nitrogen, oxygen, hydrogen, sulfur, and approximately 60% organic carbon (Sible et al. 2021;Ampong et al. 2022). Legumes, such as groundnuts, utilize organic carbon to attract beneficial microbes involved in biological processes like rhizobia, leading to increased nutrient uptake (Chagas et al. 2018;Msimbira and Smith 2020).
Similarly, Cb applications under the IR85 irrigation scheme increased groundnut root length, enabling better penetration within the soil profile and resulting in increased exudates. This, in turn, facilitated greater absorption of water and phytoavailable nutrients (Meftah et al. 2016;Minz et al. 2018). The application of Cb also influenced microbiome activities and their exudates, enhancing nutrient mobility and availability. These effects align with the findings of Banerjee and Bhattacharya (2021).

Conclusion
The current study demonstrated that groundnut plants are susceptible to the impacts of water stress, even with small changes in applied water requirements during sensitive growth periods. To mitigate these effects, cobalt and potassium humate were used as foliar and soil applications in the study. The findings indicated that under water stress conditions during sensitive stages, soil applications of potassium humate were more effective than foliar applications in improving plant growth, yield, and decreasing soil pH. The results also showed that the applications of cobalt and potassium humate led to enhancements in nutrient accumulation and yield. Although there was a negative impact on the uptake of Fe and Mn nutrients, which was attributed to the irrigation pattern implemented and the antagonistic relationships 1 3 between these nutrients and cobalt, the combination of potassium humate and cobalt reversed the severity of these negative impacts and resulted in improved groundnut yield under different irrigation schemes. Based on the findings, it is recommended to implement irrigation requirements at 85% of the crop water requirements from flowering till the end of the pod stage. Additionally, the combined application of cobalt and soil applications of potassium humate is suggested. This combination resulted in improvements in macronutrient and micronutrient accumulation, yield, and irrigation water use efficiency. The combined applications of cobalt and potassium humate prove beneficial in alleviating the influences of water stress on groundnut plants, particularly during sensitive growth periods. Furthermore, these applications help conserve irrigation water in arid conditions. Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).
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