ABSTRACT
Plant growth in general and crop production in particular normally occur in environments characterized by fluctuations in soil water supply and evaporative demand, which produce water stress of varying duration. Maintenance of metabolic processes usually depends upon minimization of water loss from cells, and this is often accomplished by intracellular accumulation of solutes such as potassium, amino acids (or derivatives), and sugars.[1-3] This type of adaptational response occurs widely in plants, fungi, microorganisms as well as some animal cells[1,2] and is usually known as osmoregulation, osmotic regulation, or osmotic adjustment. These terms are etymologically identical (‘‘to adjust’’ means ‘‘to regulate’’ or ‘‘conform to a standard’’), and are correctly used interchangeably though some have argued for a distinction.[4,5] The expressions can be taken to mean controlled (e.g., regulated by specific genes) change or maintenance of osmotica; generally the former in plants and the latter in animals, though in each, cell hydration is maintained to varying degrees. This may be expressed as volume maintenance (particularly in wall-less cells such as plant gametes and marine algae) or turgor maintenance (walled cells). Osmotic adjustment is also used to describe solute accumulations in plants where the control system (either genetical or physiological) is not clearly understood.
