Genomics and Applied Biology 2024, Vol.15, No.6, 296-306 http://bioscipublisher.com/index.php/gab 302 4.3 Physiological and biochemical markers for efficient screening Physiological traits are critical markers for screening drought tolerance in soybeans. Traits such as leaf water potential, chlorophyll content, and stomatal conductance provide insights into the plant's ability to maintain water status and photosynthetic efficiency under drought conditions. For instance, drought-tolerant soybean varieties were found to maintain higher leaf water potential and chlorophyll content, which are indicative of better water retention and photosynthetic capacity (Fatema et al., 2023). Additionally, stomatal conductance measurements revealed that drought-tolerant genotypes could regulate water loss more effectively, contributing to their overall drought resilience (Buezo et al., 2018). Biochemical markers such as antioxidant enzyme activities and osmolyte accumulation are also valuable for screening drought tolerance. Increased activities of enzymes like peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX) have been associated with enhanced drought tolerance, as they help mitigate oxidative stress caused by drought. Moreover, the accumulation of osmolytes such as proline and soluble sugars plays a crucial role in osmotic adjustment and cellular protection under water deficit conditions. Studies have shown that drought-tolerant soybean varieties exhibit higher levels of these osmolytes, which aid in maintaining cell turgor and stabilizing proteins and membranes during drought stress (Moloi and Merwe, 2021; Wang et al., 2022b). In summary, controlled environment screening using growth chambers and greenhouses, along with the application of drought simulating techniques, provides a robust framework for evaluating drought tolerance in soybeans. The selection of physiological and biochemical markers further enhances the efficiency of screening, enabling the identification of drought-tolerant varieties that can be utilized in breeding programs to develop resilient soybean cultivars. 5 Case Studies and Successful Breeding Programs 5.1 Case studies of drought tolerant varieties Several soybean varieties have been identified for their notable drought tolerance characteristics. For instance, the variety N-3001 (HY1) has demonstrated a high intrinsic water use efficiency and a strategy of faster transition into the reproductive stage to avoid drought periods, enhancing its photoprotective defenses and investing in growth and productivity under water-limited conditions (Buezo et al., 2018). Another variety, AGS383, has shown minimal adverse effects under drought conditions, maintaining healthier root and shoot growth, greater leaf area, and higher photosynthesis, which contribute to its heavier grains and higher yield under both drought and normal conditions (Fatema et al., 2023). Additionally, the variety Heinong 44 (HN44) has been identified for its significant increase in antioxidant enzyme activities and osmotic regulatory substances under drought stress, which helps in maintaining cell membrane stability and overall plant health (Wang et al., 2022a; Wang et al., 2022b). Field performance and yield stability of drought-tolerant soybean varieties have been extensively studied. For example, the variety AGS383 has been noted for its yield stability, producing heavier grains and maintaining higher seed yield under drought conditions compared to other genotypes (Fatema et al., 2023). Similarly, the variety Heinong 44 (HN44) has shown better performance under drought conditions by maintaining higher levels of antioxidant activities and osmotic regulatory substances, which contribute to its drought tolerance and yield stability (Wang et al., 2020; Wang et al., 2022b). Furthermore, a study on various soybean genotypes identified LD24, JD36, and TF31 as highly drought-tolerant during the vegetative growth phase, and TD37 and LD26 during the reproductive growth phase, indicating their potential for stable yield under drought conditions (Yan et al., 2020). 5.2 Breeding strategies for enhancing drought tolerance Traditional breeding approaches have focused on selecting traits that enhance reproductive resilience under drought conditions. For example, the selection of traits such as leaf area index (LAI) and root-to-shoot ratio (RSR) has been effective in identifying drought-resistant cultivars during the vegetative growth phase. Additionally, traits like the number of seeds per pod (SPP) and yield per plant (YPP) have been used to screen for drought
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