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
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- •Some recent data on O2 scavenging by root segments showed a two-phase reduction in respiration rate starting at/above 21 kPa O2 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 O2. 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 [O2] from root centre to respirometer medium.
- •With respiration at any locus following a mitochondrial cytochrome oxidase O2 dependence curve (the Michaelis-Menten constant Km = 0.0108 kPa; critical O2 pressure, 1–2 kPa), the declining rate of O2 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 O2 by controlled down-regulation of respiration based on O2 sensors.