Cortical senescence is an important feature of the roots of a number of graminaceous species because it may contribute to rhizodeposition of carbon and influence the efficiency of nutrient uptake. A major limitation to understanding the physiological control of senescence and its impact on rhizosphere processes has been the lack of reliable techniques for characterizing the progress of senescence along the root. The use of a single-cell pressure probe was evaluated for quantifying cell integrity in different regions of wheat roots. The percentage of locations with turgid cells declined with increasing distance behind the root apex. The decline preceded visible collapse of the cortex but after the loss of nuclear staining in the outer cortex. The percentage of locations with turgid cells was closely associated with root diameter, which suggests that measurements of changes in diameter, made using minirhizotrons under well-watered conditions in the field, could provide estimates of in situ rates of cortical senescence.