Critical Oxygen Pressure for Growth and Respiration of Excised and Intact Roots

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Abstract
A method based on the measurement of ATP/ADP ratios is described. It permits the determination of critical respiratory O2 pressure of any organ, or part of any organ, of an intact plant. The data obtained by this method with intact maize(Zea mays L. INRA 508) root tips are compared with polarographic determinations on similar excised tissues. When internal O2 transport from the aerial part was prevented, the critical O2 pressure found for the respiration of intact tips was similar to that found with excised tips. It was close to 10 kilopascals in a humid atmosphere and about 30 kilopascals in a liquid medium. Flooding of the gas spaces by vacuum infiltration did not modify these results. When internal O2 transport from the aerial parts of the plant occurred, significantly lower values were obtained in liquid medium for the critical O2 pressure, which shifted from more than 21 to 6 kilopascals. The higher values observed with excised root tips, compared to those obtained with intact tissues, can be explained by the lack of internal O2 transport, rather than by the flooding of gas spaces. Data are presented which show that root growth started to be limited at a significantly higher pressure than the respiration. These results are attributed to nonrespiratory oxidative processes with a low affinity for O2 involved in root elongation.