Osmotic Adjustment Under Drought Conditions

Abstract

In broad terms, plants adapt to drought either by decreasing water loss (reduced stomatal conductance) or by maintaining water uptake. The latter process is facilitated within plant cells by osmotic adjustment (OA), a biochemical mechanism that helps plants to acclimatize to dry and saline conditions. OA results in a net increase of the number of osmotically active substances in the cell. This increase in solutes leads to a more negative osmotic potential, which in turn can improve the degree of cell hydration, maintaining turgor in leaf tissue and in other metabolically active cells. In other words, plants can survive longer and maintain metabolic processes in drying soil if OA occurs. In particular crop cultivars, OA has positively affected growth and yield under drought stress. A wide range of substances can contribute to OA, including inorganic cations and anions, organic acids, carbohydrates, and amino acids. OA is often associated with an accumulation of specific solutes with protective functions. These compatible solutes—rich in hydroxyl (−OH) groups—such as sugars, cyclitols, proline and glycine betaine, can accumulate in the cytoplasm and help to protect cellular proteins, enzymes, and cellular membranes against dehydration. Still, it is important to recognize that individual solutes do not contribute greatly to OA in many species and that OA is mainly achieved by the accumulation of a multitude of solutes. As OA requires the metabolism or uptake of solutes it is generally a slow process, and is sensitive to the timing and intensity of stress. Adding to this inherent variation in expression of OA, there is some evidence that studies have underestimated leaf relative water content (RWC) when quantifying OA. The correct measurement of plant water status, such as RWC, is vital to ensuring an accurate assessment of the relative capacity for OA in different plants.