Armstrong William, Webb Trevor, Darwent Marcus, Beckett Peter M
Department of Biological Sciences, University of Hull, Kingston upon Hull HU6 7RX, UK.
Ann Bot. 2009 Jan;103(2):281-93. doi: 10.1093/aob/mcn177. Epub 2008 Sep 26.
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.
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.
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.
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.
根据完整或切除根周围介质中氧气消耗率确定的呼吸临界氧压（COPR），对于无氧介质和湿地中呼吸对氧气的依赖性而言，是不可靠的指标。本文使用数学模型对此进行说明，并讨论了用于测定完整植物根中COPR的更合适的极谱法。
使用套管铂电极通过径向氧损失（ROL）间接监测根尖附近的皮层[O₂]（豌豆幼苗），或使用微电极直接监测（玉米）；通过控制地上部周围的[O₂]来控制根中的[O₂]。利用米氏酶动力学对根的径向扩散和呼吸特性进行数学建模。
仅当根尖皮层中的氧分压（OPP）降至0.5 - 4.5 kPa以下时，呼吸才会下降，这一数值与微电极测量和数学建模所证实的中柱中心附近呼吸抑制一致。建模预测表明，根中心显著核心区域的OPP可能低于氧微电极通常的检测限，但仍能支持一些有氧呼吸。
在无氧介质中，如在湿地中一样，根的COPR可能相当低，这取决于中柱/中柱分生组织的呼吸需求、尺寸和扩散特性以及细胞色素氧化酶的酶动力学。非湿地植物根的COPR与湿地植物根的COPR可能没有太大差异。在低皮层OPP诱导发酵的中柱“厌氧”核心区域，可能仍存在微量的O₂。
