Experimental and modelling data contradict the idea of respiratory down-regulation in plant tissues at an internal [O2] substantially above the critical oxygen pressure for cytochrome oxidase.

• Some recent data on O(2) scavenging by root segments showed a two-phase reduction in respiration rate starting at/above 21 kPa O(2) in the respirometer medium. The initial decline was attributed to a down-regulation of respiration, involving enzymes other than cytochrome oxidase, and interpreted as a means of conserving O(2). As this appeared to contradict earlier findings, we sought to clarify the position by mathematical modelling of the respirometer system. • The Fortran-based model accommodated the multicylindrical diffusive and respiratory characteristics of roots and the kinetics of the scavenging process. Output included moving images and data files of respiratory activity and [O(2)] from root centre to respirometer medium. • With respiration at any locus following a mitochondrial cytochrome oxidase O(2) dependence curve (the Michaelis-Menten constant K(m) = 0.0108 kPa; critical O(2) pressure, 1-2 kPa), the declining rate of O(2) consumption proved to be biphasic: an initial, long semi-linear part, reflecting the spread of severe hypoxia within the stele, followed by a short curvilinear fall, reflecting its extension through the pericycle and cortex. • We conclude that the initial respiratory decline in root respiration recently noted in respirometry studies is attributable to the spread of severe hypoxia from the root centre, rather than a conservation of O(2) by controlled down-regulation of respiration based on O(2) sensors.