Genomics and Applied Biology 2024, Vol.15, No.6, 296-306 http://bioscipublisher.com/index.php/gab 297 implications of these findings for soybean breeding programs aimed at developing drought-resilient cultivars. By integrating insights from various studies, this study seeks to provide a comprehensive understanding of the strategies employed by soybean plants to cope with drought stress and to inform future research and breeding efforts aimed at enhancing drought tolerance in this vital crop. 2 Physiological Responses to Drought During Flowering and Podding 2.1 Water relations and osmotic adjustment Drought stress significantly impacts the water potential in soybean plants, leading to a decrease in leaf water potential. This reduction in water potential is a critical response mechanism that helps the plant to maintain cellular turgor and continue metabolic activities under water-limited conditions. For instance, in a study involving soybean genotypes, it was observed that water deficit conditions led to a marked decrease in leaf water potential, which was more pronounced in drought-sensitive varieties compared to drought-tolerant ones (Wang et al., 2022a; Silva et al., 2022). Osmolytes such as proline and soluble sugars play a crucial role in osmotic adjustment under drought conditions. These compounds help in stabilizing proteins and membranes, scavenging free radicals, and maintaining cell turgor. Research has shown that the content of proline and soluble sugars increases significantly in soybean leaves under drought stress, contributing to enhanced drought tolerance. For example, drought-tolerant varieties exhibited higher levels of proline and soluble sugars compared to drought-sensitive varieties, which helped them to better withstand water deficit conditions (Wang et al., 2022a; Silva et al., 2022). 2.2 Stomatal conductance and transpiration Stomatal conductance is a critical factor in regulating water loss and gas exchange in plants. Under drought conditions, soybean plants typically exhibit reduced stomatal conductance to minimize water loss through transpiration. This response is essential for conserving water and maintaining cellular hydration. Studies have indicated that drought-tolerant soybean varieties tend to have better control over stomatal closure, which helps them to reduce water loss more effectively than drought-sensitive varieties (Buezo et al., 2018; Silva et al., 2022). Drought stress leads to a reduction in transpiration rates as a result of decreased stomatal conductance. This reduction is a protective mechanism to conserve water. However, it also impacts the plant's ability to assimilate carbon dioxide, affecting photosynthesis. High-yielding soybean varieties have been observed to enhance their water use efficiency under drought conditions by optimizing their transpiration rates and maintaining higher productivity levels despite reduced water availability (Figure 1) (Buezo et al., 2018; Molinari et al., 2021). 2.3 Photosynthetic activity and chlorophyll fluorescence Drought stress adversely affects photosynthetic activity in soybean plants by reducing the rate of carbon assimilation. This reduction is primarily due to stomatal closure, which limits the availability of carbon dioxide for photosynthesis. Additionally, drought stress can lead to damage in the photosynthetic apparatus, further reducing photosynthetic efficiency. Studies have shown that drought-tolerant soybean varieties are able to maintain higher photosynthetic rates under water deficit conditions compared to sensitive varieties, which is crucial for sustaining growth and productivity (Buezo etal., 2018; Iqbal et al., 2019; Silva et al., 2022). Chlorophyll fluorescence is a valuable tool for assessing the impact of drought stress on the photosynthetic efficiency of plants. It provides insights into the health of the photosynthetic apparatus and the extent of stress experienced by the plant (Pereira et al., 2019). Research has demonstrated that drought-tolerant soybean varieties exhibit less reduction in chlorophyll fluorescence parameters under drought conditions, indicating better maintenance of photosynthetic activity and lower stress levels compared to sensitive varieties (Buezo etal., 2018; Du et al., 2020; Delavar et al., 2023). 2.4 Reproductive development and yield components Drought stress during the flowering stage can significantly impact the reproductive development of soybean plants. It can lead to delayed flowering, reduced pod formation, and ultimately lower yield. High-yielding soybean
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