Measuring and interpreting respiratory critical oxygen pressures in roots.
OBJECTIVE
Respiratory critical oxygen pressures (COPR) determined from O(2)-depletion rates in media bathing intact or excised roots are unreliable indicators of respiratory O(2)-dependency in O(2)-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 [O(2)] near the root apex was monitored indirectly (pea seedlings) from radial oxygen losses (ROL) using sleeving Pt electrodes, or directly (maize) using microelectrodes; [O(2)] in the root was controlled by manipulating [O(2)] around the shoots. Mathematical modelling of radial diffusive and respiratory properties of roots used Michaelis-Menten enzyme kinetics.


RESULTS
Respiration declined only when the O(2) 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 O(2)-microelectrodes but still support some aerobic respiration.


CONCLUSIONS
In O(2)-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 O(2) may still be present in stelar 'anaerobic' cores where fermentation is induced at low cortical OPPs.
Armstrong
William
W
Department of Biological Sciences, University of Hull, Kingston upon Hull HU6 7RX, UK. w.armstrong@hull.ac.uk
Webb
Trevor
T
Darwent
Marcus
M
Beckett
Peter M
PM
D019069:Cell Respiration; D004058:Diffusion; D008365:Manometry / Q000379:methods*; D008954:Models, Biological; D010100:Oxygen / Q000494:pharmacology*; D010101:Oxygen Consumption; D010313:Partial Pressure; D018532:Pisum sativum / Q000502:physiology; D018517:Plant Roots / Q000502:physiology*; D018520:Plant Shoots / Q000502:physiology; D011312:Pressure; D003313:Zea mays / Q000502:physiology
D010100:Oxygen