**3.5 Osmotic adjustment**

Osmotic adjustment is an adaptation technique for the acclimation of increased cell turgor and water retention after stress. Osmotic leaf adjustment is significantly associated with drought tolerance in other crop species. The osmotic adjustment of compatible solutes in the cytosol is carried out in plant cells in response to

water stress. This lowers the cell's osmotic potential for cell turgor and cell growth. Compatible solutes such as proline, sorbitol, and glycine betaine are more soluble and do not interfere with metabolism in cells, even at high levels. Proline is a widely appropriate drought stress response in plants [24]. Proline accumulation in drought plants is, however, different and relies on cultivar and growing phase (e.g., proline accumulation in cotton ovaries was higher than in the leaves). Ref. [115] have suggested that during reproductive phases, the osmotic adjustment may be greater than in vegetative stages and may depend on tissue.

The water shortage affects the turgidity and osmotic balance of the cells at the cellular level. Osmotic adaptation is key to reducing the impacts of crop damage caused by drought. Mechanisms of plant protection also include osmoprotectants or osmolytes that control homeostasis after drought and cellular salinity stress. The effect of drought stress on osmotic balance is adverse and hence plants collect various organic and inorganic components to lower the osmotic potential of the dry weight [75]. Osmotic adjustment is involved in numerous organic compounds including amino acids (proline and glycine), sugar (trehalose and fructan), sugar alcohols (mannitol, sorbitol, D-monitor-monitor and polyamine (polyamine and betaine), polyols, ectoin, alkaloids, and inorganic ions known as osmoprotectants/osmolytes [75, 116]. Such solutes help protect proteins and membranes from harm owing to high concentrations of inorganic ions and oxidative damage caused by drought stress [31]. The exogenous use of osmoprotective agents (proline and glycine betaine) has proven to be beneficial in decreasing the deleterious effects of cotton drought stress [117]. Transgenic cotton plants were more drought-tolerant than controlling plants and were more photosynthesized, had greater relative water content, improved osmotic adjustment, reduced lipid membrane peroxidation, and less ion leakage [76]. AnnBj1 ectopic annexin gene expression improved the content of the proline and sucrose, which increased the tolerance of drought in cotton [118]. In addition, the overexpression of the GhAnn1 cotton annexin gene improved dryness and salt tolerance by boosting the activity of superoxide dismutase (SOD) [119].

Compatible solutes protect the proteins and membranes from damage due to high levels of inorganic ions and water-deficit oxidant damage [31], and salinity [6]. Foliar application of glycine betaine and proline could be a useful approach for increasing tolerance in cotton cultivations [117]. More drought tolerance showed in cotton plants more glycine betaine in accumulation. The promotion of physiological processes, such as leaf photosynthesis, relative water, improved osmotic adjustment, and low lipid stability, might thus increase crop performance under drought by transgenic/ nontransgenic method [42]. For example, a rise in proline and sucrose content of the AnnBj mustard annexin gene in cotton led to a higher tolerance of drought [118]. In addition, GhAnn1 overexpression, annexin cotton gene, drought, and salt tolerance have been enhanced by the enhancement of superoxide dismutase [119]. Further osmotic adjustment research in reproductive organs is needed to completely understand this process in drought cotton plants.
