Wheat is a major cereal, accounting for approximately 29% (806.50 million tons) of total cereal production from 2022–2023, as reported by the FAO [1]. Additionally, after rice, it is Bangladesh’s 2nd most important cereal crop, with an area of 0.32 million ha under production and a yearly yield of 1.17 million metric tons, but the average yield (3.69 t ha⁻¹) is lower than that of other countries [2]. According to the IPCC (Intergovernmental Panel on Climate Change) awareness report, the potential yield of wheat in Bangladesh is projected to decline by 32% by the year 2050 [3]. On the other hand, the worldwide demand for wheat will be increased by as much as 60% by 2050 [4] because of the faster growth of the economy, expanding urbanization, and consequent changes in lifestyle [5]. National wheat production can only fulfil approximately 20% of its overall consumption, creating a demand–supply gap that imports can only fill [6].

The rapid expansion of the world population has led to a problematic situation regarding food security. This is caused primarily by global climate change and increased unpredictability in weather precedents, resulting in drought-prone regions worldwide, including the crop-growing areas in northwestern Bangladesh, which severely hinder crop productivity [7]. Bangladesh is confronted with a significant risk of moisture stress, as almost 3.50 million hectares of land are experiencing drought, accounting for approximately 47% of the country’s total area [8].

Drought has been identified as a polygenic stress that primarily occurs due to a reduction in rainfall and the subsequent period of drought [9], which limits crop productivity and quality as well as the overall potential of land globally [10]. The detrimental impacts of drought on crops are well documented. Drought inhibits nutrient uptake, shortens phenology during regular growth and development periods, and modifies physio-biochemical processes. As a result, it decreases the production of dry matter, grain output, and quality of wheat [11]. The main constraint of water shortages during wheat grain filling can shorten its duration, lowering grain size and quantity and ultimately leading to a decrease in grain output [12]. The effects of drought stress on wheat may be minimized in two ways: first, by managing optimal agronomic practices efficiently, and second, by developing drought-resistant wheat cultivars [13]. Researchers have developed strategies to mitigate drought stress through the use of film mulch [14], drought-tolerant varieties [15], nanoparticles [16], superabsorbent hydrogels and biochar [17], humic acid [18], and plant growth-promoting rhizobacteria [19]. In fact, in China, humic acid has been widely added to common urea and phosphate fertilizers to increase their effectiveness.

Humic acid is a vital element of soil organic materials that strengthens the physicochemical and biological aspects of soil while enhancing grain production and crop quality [20]. It is an active ingredient of organic fertilizers and may provide a replacement for conventional soil fertilization and quick reservoirs of nitrogen, especially in semiarid areas [21]. Humic acid applied to leaves improves photosynthesis activity, gas exchange capacity, chlorophyll pigments, protein and starch ratios, and physiological processes and increases tolerance to abiotic stress [22]. Several researchers have reported that the application of humic acid under drought stress conditions can reduce drought-induced adverse impacts on wheat [23], maize, sorghum, mungbean [24] and rapeseed [25]. Therefore, the present study aimed to reduce the adverse effects of drought stress on wheat through the administration of humic acid and to observe the effectiveness of various application methods.

The grain yield of agricultural crops depends on photosynthesis, and leaves are vital determinants for improved photosynthetic capacity [30]. According to the results of the present study, the use of humic acid led to an increase in the number of leaves compared with that of stressed plants. Research conducted by de Freitas Moura et al. [31] also corroborates our findings. Additionally, Arslan et al. [32] reported that humic acid suppressed the activity of the GRF gene, which is known to be sensitive to drought in wheat, promoting wheat development in environments prone to drought. The flag leaf is the primary supplier of photosynthates and acts as a nearby food source for the grain during the reproduction stage [33]. We observed that water deficit stress substantially hampers the morphological traits of the flag leaves of wheat plants. These results align with the findings reported by Ahmad et al. [34]. However, humic acid accelerates nutrient uptake, photosynthesis, translocation, and photosynthate utilization, resulting in rapid cell division, which might be the reason behind the increase in leaf morphology [35].

The data in Table 3 also confirmed that, compared with soil application, exogenous application of humic acid in adverse situations improved flag leaf shape. Sindabad et al. [36] reported that water deficiency stress considerably reduces the relative leaf water content in wheat. In our investigation, the use of humic acid considerably improved the RLWC of the flag leaves of wheat compared with that under water scarcity stress. Research conducted by Abou Tahoun et al. [37] indicated that humic acid molecules increase the osmotic adjustability and water retention of plants and maintain their photosynthetic and antioxidant metabolism when they are subjected to drought stress. The findings of Wang et al. [38] align with our findings, indicating that drought reduces photosynthetic activity because of both stomatal and nonstomatal restrictions. However, humic acid enhances photosynthesis, respiration, hormone synthesis, and protein synthesis in plants [39]. Humic acid promotes chlorophyll production by stimulating ascorbate peroxidase activity, which removes ROS in the thylakoids and minimizes chlorophyll breakdown in drought-stressed plants [39]. Tohidi [40] reported that humic acid increased the wheat chlorophyll content under water deficit stress, which justifies our present findings. Proline acts as an enzyme stabilizing agent that helps control and decrease water loss from cells during periods of water scarcity. It also helps to maintain the osmotic equilibrium and contributes to the structure of subcellular components [41]. A study by Nowsherwan et al. [42] demonstrated that higher crop plants significantly increase proline levels in response to abiotic stresses. Similarly, Liang et al. [41] reported countable increases in proline concentrations in wheat leaves under conditions of water scarcity, which is in accordance with our present results. The decline in greenness (SPAD value) under water deficit stress may be explained by photooxidation and degradation of chlorophyll in the leaves [43]. Water deficit stress impairs the greenness and photosynthetic machinery of green plants.

In our study (data in Table 4), humic acid remarkably increased the SPAD value of wheat leaves under stress conditions. As stated by Nikbakht et al. [44], humic acid improves the nutrient uptake rate (N and K) in plant shoots, thereby increasing the number of chloroplasts per cell and increasing the synthesis of chlorophyll as well as the greenness of green plants such as wheat [45]. Leaf temperature serves as a valuable measure of drought tolerance because of the significant role of stomata in regulating water loss. Plants respond to drought by experiencing a reduction in water content, which decreases leaf water potential. Thus, there is a growing inclination towards higher leaf temperatures and a decline in leaf area, leading to a decrease in both crop productivity and resilience. Humic acid, when applied to plants, particularly through foliar application, enhances stomatal conductance, leading to stomatal transpiration. This process results in the cooling of plant leaves and a decrease in leaf temperature. Water scarcity can lead to stunted plant growth through dehydration of the protoplasm, decreased turgidity, and impaired cell growth and division [46]. Our results agree with the studies of Jannat et al. [47], who demonstrated that water deficit stress lowered the plant height and tillering capacity of wheat. The decrease in the number of tillers in plant⁻¹ might be due to the roots receiving less water and nutrient availability under drought conditions, which disrupts plant tissue and reduces meristematic activity and cell proliferation. Humic acid enhances the transportation and accessibility of nutrients [48], which encourages the growth and development of plants during both vegetative and reproductive stages. Haghighi et al. reported that organic compounds such as humic acid decrease watering intervals, improve WUE, and minimize the effects of drought stress on crop plants, ultimately improving vegetative development. El-Sayed et al. [49] reported that humic acid administration during water stress increased the tiller density of bread wheat cultivars. According to Forgane [50], spikes plant⁻¹ are a significant factor that has a favourable relationship with grain yield. In this study, water deficit stress significantly reduced this component. These findings are supported by those of Jannat et al. [47], who confirmed that water shortage diminished the number of spikes and spike length compared with nonstress conditions. In our investigation, humic acid produced a greater increase in the spikes of wheat than that of under stressful conditions. Nasiroleslami et al. [51] reported a positive relationship between humic acid application and the number of spikes. Humic acid may improve wheat spike length by adjusting the cellular osmotic pressure and increasing enzymatic and antioxidant activities [52], which increase plant reproduction. The spike weight is a crucial component that is strongly correlated with the overall grain yield. Reduced photosynthesis and a lower supply of photosynthates due to water shortage are the primary drivers that hinder the growth of vegetative and reproductive development [15]. Nowsherwan et al. [42] reported that water scarcity stress decreased spike length, spikelets, and grains per spike, lowering grain yield. Water shortage stress reduces the number of grains, affecting dry matter production, pollen viability and ovule function. Our findings align with those of Mehrinfar et al. [53], who reported that foliar humic acid spray altered bread wheat cultivar grain yield and quality under water stress. Rana et al. [54] reported that water deficiency stress reduces grain weight, which is consistent with our findings. Humic acid increases plant photosynthetic pigments and biochemical activity. Arjumend et al. [55] reported that varying humic acid rates and NPK mineral fertilizers affected wheat thousand grain weight and yield, that is supporting our findings on 100-grain weight. In this study, both varieties presented decreased grain yield (plant⁻¹) under water deficit stress. This might be because stress may reduce photosynthetic activity and assimilate supply, reducing grain number and size [15]. Similarly, Helmy Hegab et al. [56] demonstrated that humic acid increased wheat grain yield and components. Our study revealed that above-ground biological yield decreased due to water stress at the vegetative stage, resulting in the production of fewer tillers and less biomass than under well-watered conditions. Other authors, such as Zhang et al. [57], reported that drought lowered the biomass and yield of wheat by 25.00% and 27.50%, respectively that is congruent to our present findings on above ground biological yield.

The current investigation revealed that both wheat varieties, BWMRI Gom 1 and BWMRI Gom 3, presented significant reductions in key growth and yield parameters under water deficit stress compared with well-watered conditions. All the parameters (except proline) were declined considerably under WDS, indicating the detrimental impact of water stress on wheat productivity. However, the application of humic acid, particularly via foliar spraying, has shown promising effectiveness in mitigating these negative effects. Foliar application of humic acid consistently improved plant resilience under WDS, with increases in spike number, spike weight, grain number, grain yield, and biological yield in both varieties. Finally, BWMRI Gom 3 appears to be more resistant to drought conditions than BWMRI Gom 1, and foliar application of humic acid was found to be comparatively more effective than soil application in mitigating the adverse effects of water deficit stress on wheat.