AOBPreview originally published online on September 26, 2008
Annals of Botany 2009 103(2):281-293; doi:10.1093/aob/mcn177
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Measuring and interpreting respiratory critical oxygen pressures in roots
1 Department of Biological Sciences, University of Hull, Kingston upon Hull HU6 7RX, UK
2 The Registry, University of Aberdeen, Aberdeen, UK
* For correspondence. E-mail w.armstrong{at}hull.ac.uk
Received: 10 April 2008 Returned for revision: 21 May 2008 Accepted: 25 July 2008 Published electronically: 26 September 2008
Background and Aims: Respiratory critical oxygen pressures (COPR) determined from O2-depletion rates in media bathing intact or excised roots are unreliable indicators of respiratory O2-dependency in O2-free media and wetlands. A mathematical model was used to help illustrate this, and more relevant polarographic methods for determining COPR in roots of intact plants are discussed.
Methods: Cortical [O2] near the root apex was monitored indirectly (pea seedlings) from radial oxygen losses (ROL) using sleeving Pt electrodes, or directly (maize) using microelectrodes; [O2] in the root was controlled by manipulating [O2] around the shoots. Mathematical modelling of radial diffusive and respiratory properties of roots used Michaelis–Menten enzyme kinetics.
Key Results: Respiration declined only when the O2 partial pressure (OPP) in the cortex of root tips fell below 0·5–4·5 kPa, values consistent with depressed respiration near the centre of the stele as confirmed by microelectrode measurements and mathematical modelling. Modelling predictions suggested that the OPP of a significant core at the centre of roots could be below the usual detection limits of O2-microelectrodes but still support some aerobic respiration.
Conclusions: In O2-free media, as in wetlands, the COPR for roots is likely to be quite low, dependent upon the respiratory demands, dimensions and diffusion characteristics of the stele/stelar meristem and the enzyme kinetics of cytochrome oxidase. Roots of non-wetland plants may not differ greatly in their COPRs from those of wetland species. There is a possibility that trace amounts of O2 may still be present in stelar ‘anaerobic’ cores where fermentation is induced at low cortical OPPs.
Key words: Critical oxygen pressure, cytochrome oxidase, maize, modelling, non-wetland, tissue O2 profiles, pea, radial oxygen loss, root aeration, root respiration, stelar O2 deficiency, wetland
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