Molecular Soil Biology 2024, Vol.15, No.5, 205-215 http://bioscipublisher.com/index.php/msb 207 Paraheliotropism, the movement of leaves to minimize direct sunlight exposure, is another strategy employed by some plants to reduce water loss. This mechanism can decrease leaf temperature and reduce transpiration rates, thereby conserving water (Lobato et al., 2020). Additionally, the accumulation of osmolytes, such as proline, helps in maintaining cell turgor and protecting cellular structures during dehydration (Giorio et al., 2018). In summary, plants employ a range of physiological adaptation mechanisms, including stomatal regulation, root structure adjustments, and leaf morphology changes to cope with water deficit conditions (Wahab et al., 2022) (Figure 1). These strategies are crucial for maintaining water balance, optimizing WUE, and ensuring plant survival under drought stress. 3 Biochemical Regulation Mechanisms 3.1 Accumulation of osmoprotectants (e.g., proline, sugars) Plants under water deficit conditions often accumulate osmoprotectants such as amino acid compounds (proline), amine compounds (glycine betaine and polyamines), soluble sugars, and trehalose, mannitol, and other compounds, play a major role in maintain cellular osmotic balance and protect cellular structures. For instance, in Moringa oleifera, proline content increased significantly with the severity of moisture stress, particularly in the leaves under severe stress conditions (Chitiyo et al., 2021). Similarly, in Scrophularia striata, soluble sugars like glucose, mannose, rhamnose, and xylose were found to accumulate under osmotic stress, serving as compatible solutes and aiding in the production of phenolic compounds (Falahi et al., 2018). Additionally, Quercus robur and Q cerris seedlings showed species-specific accumulation of osmoprotectants, with Q. robur primarily accumulating glycine betaine and Q. cerris accumulating dimethylsulphoniopropionate (DMSP) under water deficit conditions (Kebert et al., 2022). 3.2 Activation of antioxidant systems and free radical scavenging Water deficit conditions lead to the overproduction of reactive oxygen species (ROS), causing oxidative stress in plants. To mitigate this, plants activate antioxidant systems, including both enzymatic and non-enzymatic antioxidants. For example, in Isatis indigotica, activities of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) increased significantly under moderate and severe water deficits (Zhou et al., 2023). Similarly, the tea plants (Camellia sinensis), tolerant genotypes exhibited higher activities and expression levels of antioxidative enzymes, including superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase (POX) and catalase (CAT), which helped in reducing oxidative damage compared to susceptible cultivars (Nalina et al., 2021). Furthermore, in Moringa oleifera, antioxidant activity increased with drought progression, indicating a robust defense mechanism against oxidative stress (Chitiyo et al., 2021). Figure 1 Drought stress impacts plants’ morphological, physiological, and biochemical processes (Adopted from Wahab et al., 2022)
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