In the present study, severe water stress (25% FC) had a significant negative effect on the biometric, physiological, anatomical, and water status characteristics of the soybean cultivar during the vegetative stages. Understanding the impact of drought stress on soybeans is the key to improving agricultural production in an environment characterized by constant changes. The data from this study enhance our understanding of how water deficit affects soybeans during the vegetative stages, thereby contributing to improving soybean tolerance and yield potential in environments with low water availability. Soil moisture stress is a crucial limiting factor during the vegetative phase of soybean growth and establishment, affecting cell elongation, leaf expansion, and development. Among different growth and developmental traits, node number, plant height, internode length, and leaf area expansion have been suggested as indicators of tolerance to soil moisture deficit in soybeans (Khan et al. 2014; Wijewardana et al. 2019). According to Staniak et al. (2023), understanding how soybean plants respond to stress factors and how they react to stress is essential for innovation in agronomic practices that benefit soybean production in the face of climate change.
The growth rate, physiological responses, and morphology of above-ground and root components were affected by water deficit. Among the water regimes studied, the 25% FC treatment compromised the largest number of variables (height, diameter, gas exchanges, biomass, leaf area, and number of nodules). Crop growth and yields are severely impacted by inadequate water supply, reducing carbon assimilate contents. According to Hasanuzzaman et al. (2013), drought stress in plants can alter gene expression and cellular metabolism, reduce mitotic cell division in mesophyll tissues and other organs, and decrease stomatal conductance.
Drought stress compromises dry matter accumulation, leaf area index, and photosynthetic efficiency (Li et al. 2019). The degree of soybean sensitivity to water is closely related to the growth period, with short-term and moderate water deficit during vegetative growth, generally not reducing soybean yield significantly (Xiong et al. 2020). Under drought stress, plants may partially or fully close stomata to reduce water loss; the resulting decline in the photosynthetic electron transport chain forces excitation energy to dissipate through nonphotochemical quenching, producing reactive oxygen species that damage the photosynthetic apparatus (Xiong et al. 2020). As observed in Fig. 4B, there was a reduction in stomatal conductance over the exposure period to stress in the treatment with 25% FC.
The decline in A is mainly attributed to stomatal closure, as observed in Fig. 4, in line with the findings of Wijewardana et al. (2019). The stomatal limitation is likely the primary cause of reduced net photosynthesis in water-stressed soybeans. Typically, the response of leaf stomata is regulated to minimize water loss or maximize carbon gain, depending on the plant's water status and photosynthetic CO2 assimilation. Some studies have reported reduced A and gs in response to soil moisture stress (Makbul et al. 2011; Ku et al. 2013).
Soil moisture stress decreases leaf water potential (LWP), which reduces turgor pressure and, subsequently, stomatal closure. Midday LWP is a reliable measure to assess plant water status, given its relationship with leaf gas exchange and other growth and development parameters (Wijewardana et al. 2019).
Water use efficiency is an important trait in plants and a key factor linking water and carbon cycles. Understanding the physiological mechanisms involved in this trait and predicting their response in a constantly changing environment is a considerable challenge. Instantaneous water use efficiency (A/E) and intrinsic water use efficiency (A/gs) are obtained from gas exchange and express how efficiently plants use water while simultaneously assimilating carbon. High WUE values are characteristic of plants tolerant to low water availability and work as a parameter that indicate physiological plasticity to abiotic factors (Costa et al. 2022).
Factors limiting photosynthesis are generally attributed to restricted CO2 conductance from the atmosphere to the carboxylation site and limited CO2 assimilation in the stroma of chloroplasts under saturated light conditions (Hu et al. 2019). Rubisco is a fundamental catalyst of carbon fixation, a process regulated primarily by chloroplast CO2 concentration and the content and activity of the Rubisco enzyme. Photosynthesis depends on CO2 diffusion and assimilation capacity and coordination, which are critical in achieving maximum photosynthetic capacity (Galmés et al. 2017).
Under prolonged water deficit, plants undergo morphological changes to cope with the adverse effects of stress (Wang et al. 2022). A complex network of anatomical adaptations, such as reduced vessel size with greater wall thickness, decreased formation of cortical and mesophyll parenchyma, and increased stomatal density, is needed to maintain water potential and energy storage under drought stress (Boughalleb et al. 2014). Confirming the authors mentioned earlier, in the present study, we found a reduction in the thickness and area of the cortex in the leaf midrib (Table 1). Additionally, an increase in root xylem vessels with a diameter of < 20 µm and a reduction in vessels > 50 µm was observed (Table 2).
Root vascular systems showed smaller xylem vessel diameters under water deficit (Table 2). A decrease in root xylem diameter in response to drought is a mechanism to prevent xylem embolism (Comas et al. 2013). Makbul et al. (2011) also reported a decline in stele and xylem diameter in soybean roots as a drought tolerance mechanism. In the present study, severe stress (25% FC) reduced root volume and xylem vessel diameter, leading to a decline in root hydraulic conductivity (RHC) (Fig. 6; Table 2). According to Suku et al. (2013), within a stable water potential gradient, RHC is altered via two pathways: by increasing root surface area or enhancing intrinsic properties (transport volume). Furthermore, water flow resistance in the xylem is partly determined by the diameter and length of conducting vessels responsible for water transport (Figueiredo et al. 2014). Kisman et al. (2022) reported that drought stress increased epidermal and phloem thickness and decreased cortex, stele parenchyma, and xylem thickness in root anatomy. Similarly, in stem anatomy, epidermal, xylem, and phloem thickness decreased, but there were no differences in leaf anatomy. Similar responses were observed in our study (Table 1).
Efficient water and nutrient uptake by the roots significantly affect shoot development, which is governed by the balance between carbon assimilation through photosynthesis and carbon loss via respiration (Oliveira et al. 2019). Consequently, alterations that decrease hydraulic conductivity, particularly in the early stages of the pathway such as the root, increase stress and may reduce leaf gas exchange, with a potentially adverse effect on growth and yield (Gilbert et al. 2011).
The 25% reduction in field capacity (75% FC treatment) did not compromise shoot and root responses, as shown in Fig. 8. This demonstrates the potential for water resource conservation and more effective water management.