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.
Department of Biological Sciences, University of Hull, Kingston upon Hull, UK. w.armstrong@hull.ac.uk
William
W
Armstrong
Peter M
PM
Beckett
D015687:Cell Hypoxia; D019069:Cell Respiration; D003576:Electron Transport Complex IV / Q000378:metabolism*; D008954:Models, Biological; D009928:Organ Specificity; D010100:Oxygen / Q000378:metabolism*; D010101:Oxygen Consumption; D018532:Pisum sativum / Q000166:cytology / Q000201:enzymology* / Q000378:metabolism*; D018517:Plant Roots / Q000033:anatomy & histology / Q000166:cytology / Q000378:metabolism; D011312:Pressure; D011773:Pyruvates / Q000378:metabolism; D013696:Temperature
D011773:Pyruvates; D003576:Electron Transport Complex IV; D010100:Oxygen