Crop & Pasture Sciences, School of Agriculture, University of Western Australia, Nedlands, Western Australia 6009 Australia.
Plant Physiol. 1991 Aug;96(4):1294-301. doi: 10.1104/pp.96.4.1294.
This investigation presents metabolic evidence to show that in 4- to 5-day-old roots of maize (Zea mays hybrid GH 5010) exposed to low external O(2) concentrations, the stele receives inadequate O(2) for oxidative phosphorylation, while the cortex continues to respire even when the external solution is at zero O(2) and the roots rely solely on aerenchyma for O(2) transport. Oxygen uptake rates (micromoles per cubic centimeter per hour) declined at higher external O(2) concentrations in excised segments from whole roots than from the isolated cortex; critical O(2) pressures for respiration were greater than 0.26 moles per cubic meter O(2) (aerated solution) for the whole root and only 0.075 moles per cubic meter O(2) for the cortex. For plants with their shoots excised and the cut stem in air, ethanol concentrations (moles per cubic meter) in roots exposed to 0.06 moles per cubic meter O(2) were 3.3 times higher in the stele than in the cortex, whereas this ethanol gradient across the root was not evident in roots exposed to 0 moles per cubic meter O(2). Alanine concentrations (moles per cubic meter) in the stele of roots exposed to 0.13 and 0.09 moles per cubic meter O(2) increased by 26 and 44%, respectively, above the levels found for aerated roots, whereas alanine in the cortex was unchanged; the increase in stelar alanine concentration was not accompanied by changes in the concentration of free amino acids other than alanine. For plants with their shoots intact, alcohol dehydrogenase and pyruvate decarboxylase activities (micromoles per gram protein per minute) in roots exposed to 0.13 moles per cubic meter O(2) increased in the stele by 40 to 50% over the activity in aerated roots, whereas there was no appreciable increase in alcohol dehydrogenase and pyruvate decarboxylase activity in the cortex of these roots. More convincingly, for roots receiving O(2) solely from the shoots via the aerenchyma, pyruvate decarboxylase in the cortex was in an "inactive" state, whereas pyruvate decarboxylase in the stele was in an "active" state. These results suggest that for roots in O(2)-free solutions, the aerenchyma provides adequate O(2) for respiration in the cortex but not in the stele, and this was supported by a change in pyruvate decarboxylase in the cortex to an active state when the O(2) supply to the roots via the aerenchyma was blocked.
本研究提供代谢证据表明，在暴露于低外部 O(2)浓度的 4-5 天大的玉米（Zea mays 杂种 GH 5010）根中，中柱接收的 O(2)不足以进行氧化磷酸化，而皮层在外部溶液为零 O(2)且根仅依赖通气组织进行 O(2)运输时仍继续呼吸。与从分离的皮层相比，从整个根切取的切段在较高的外部 O(2)浓度下的氧气摄取率（每立方厘米每小时微摩尔）下降；整个根的呼吸临界 O(2)压力大于 0.26 摩尔/立方米 O(2)（充气溶液），而皮层仅为 0.075 摩尔/立方米 O(2)。对于将地上部分切除且茎切口暴露在空气中的植物，在 0.06 摩尔/立方米 O(2)下暴露的根中，茎中的乙醇浓度（每立方米摩尔）比皮层高 3.3 倍，而在暴露于 0 摩尔/立方米 O(2)的根中，这种根中的乙醇梯度并不明显。在暴露于 0.13 和 0.09 摩尔/立方米 O(2)的根中，中柱的丙氨酸浓度（每立方米摩尔）分别比充气根高 26%和 44%，而皮层中的丙氨酸不变；中柱丙氨酸浓度的增加并未伴随着除丙氨酸以外的游离氨基酸浓度的变化。对于地上部分完整的植物，在暴露于 0.13 摩尔/立方米 O(2)的根中，茎中的酒精脱氢酶和丙酮酸脱羧酶活性（每分钟每克蛋白质微摩尔）比充气根高 40%至 50%，而这些根的皮层中酒精脱氢酶和丙酮酸脱羧酶活性没有明显增加。更有说服力的是，对于仅通过通气组织从地上部分接收 O(2)的根，皮层中的丙酮酸脱羧酶处于“非活性”状态，而中柱中的丙酮酸脱羧酶处于“活性”状态。这些结果表明，对于处于无氧溶液中的根，通气组织为皮层中的呼吸提供了足够的 O(2)，但中柱中没有，当通过通气组织向根供应 O(2)受阻时，皮层中的丙酮酸脱羧酶向活性状态的变化支持了这一观点。
