Armstrong, W. and Webb, T. 1985. A critical oxygen pressure for root extension in rice.—J. exp. Bot. 36: 1573–1582. The relationship between oxygen partial pressure and root extension was investigated using a method designed to circumvent boundary layer resistance effects. Roots were made wholly dependent upon oxygen transported longitudinally from the atmosphere through the aerenchymatous and intercellular spaces and the internal oxygen pressure in the extension zone was altered when required by adjusting the oxygen content of gas mixtures applied to the shoot (whole plants) or basal cut-end of the root (excised roots). Platinum cathodes sleeving the root in the extension zone were used to monitor root surface oxygen partial pressure continuously, root extension was measured half-hourly by means of travelling microscopes. The results confirmed that internal oxygen transport is sufficient to support root extension in rice and that, at least in the short term, extension is not greatly influenced by oxygen concentration until very low values are reached. A certain degree of variability was noted when using whole plants but this was no longer evident with excised roots. The results from the latter suggest a figure of ≤ 0.8 kPa for the critical oxygen pressure of root extension, which is much lower than any previously published values. It is supposed that such a low value is probably not a unique characteristic of rice roots but rather is the result of the measurements having circumvented both the boundary layer effects external to the root and the normal influence of the epidermal and hypodermal layers.