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.

P>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.