Abstract: Perennial ryegrass was grown in soil column systems in which soil water content was monitored by regular weighing the column and was maintained within a predetermined range by controlling watering frequency and water amount. Three types of water stress were imposed after the simulated swards were well established, cyclic, sustained and progressive, each of which was further divided into mild, moderate and severe levels, and an adequately-watered column was used as control. There were two periods of watering treatment in the growing season. Newly-expanded leaf blades were sampled at regular intervals for the determination of water content and free proline concentration.There was a cumulative rather than an instant effect of water stress on proline accumulation in plant leaves, and different levels of stress caused different rates of accumulation. This effect became evident when the whole period of watering treatment was taken into account. Proline accumulation, on the time course, did not follow the pattern of soil water content variation but seemed to be negatively associated with the leaf water content. The physiological ecology significance of the results is discussed with particular reference to soil column use , age of swards and types and levels of wate stress.