**5.2 Proline as an osmolite**

The osmoregulatory role of proline in plant cells exposed to hyperosmotic stress has been the subject of numerous studies and under environmental conditions that result in cellular dehydration such as drought, freezing or extreme salinity, it is widely accepted that proline accumulates and acts as a compatible solute helping to protect cells from damage (Heur, 1994). Accumulation of cytoplasmic osmolytes, such as proline, is thought to aid in reducing the cellular water potential to a level below the external water potential, this enables water to move into the cell and be maintained there, while at the same time minimising potentially deleteriously increases in ionic strength. However, there is some debate in the published literature as to whether increased cytosolic levels of free proline has any direct adaptive value (Heur, 1994). While there are many reports of positive correlations between the capacity for proline accumulation and dehydration and cold tolerance (see section 3.1), some researchers still challenge the value of the ability of plant cells to accumulate proline as a positive index for osmotic stress resistance (Heur, 1994 & references therein).
