Studies of onion root respiration I. Velocity of oxygen consumption in different segments of root at different temperatures as a function of partial pressure of oxygen

Abstract

The rate of oxygen consumption and carbon dioxide production in different partial pressures of oxygen at 15°, 20°, 30° and 35°C were measured manometrically in segments of onion root tip 5, 10 and 15 min in length. The rates for the regions 5–10 mm and 10–15 mm above the tip were determined by difference. Curves for the three zones of root tip relating oxygen uptake to pressure of oxygen were hyperbolic at all temperatures but the pressure just supporting the maximum rate (critical pressure) increased with temperature. R.Q.'s were essentially unity at pressures equal to or greater than the critical pressure and exceeded unity when the pressures were less than the critical pressure. Rate of carbon dioxide production paralleled that of oxygen consumption but showed a smaller proportionate reduction at low oxygen tensions. Log k vs 1/T curves each root segment at different pressures of oxygen permitted calculations of activation energy. For the two basal zones, μ values of approximately 12000 calories were obtained at all pressures. For the apical 5 mm, a μ value of 4300 calories was found at 5% oxygen and one of about 13000 calories at 100% oxygen. The curves at pressures of 10%, 15% and 21% oxygen for the same segment hadbreaks with the slopes of the two straight-line portions approaching these extremes. From these results, it is concluded that diffusion is the limiting factor in rate of respiration at the lower pressures of oxygen.