Gould Nick, Thorpe Michael R, Koroleva Olga, Minchin Peter E H
Department of Biology, University of Waikato, Private Bag 3105, Hamilton, New Zealand.
Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
Funct Plant Biol. 2005 Nov;32(11):1019-1026. doi: 10.1071/FP05036.
According to the Münch hypothesis, a flow of solution through the sieve tubes is driven by a hydrostatic pressure difference between the source (or collection) phloem and the sink (or release) phloem. A high hydrostatic pressure is maintained in the collection phloem by the active uptake of sugar and other solutes, with a concomitant inflow of water. A lower pressure is maintained in the release phloem through solute unloading. In this work we directly test the role of solute uptake in creating the hydrostatic pressure associated with phloem flow. Solute loading into the phloem of mature leaves of barley and sow thistle was reduced by replacing the air supply with nitrogen gas. Hydrostatic pressure in adjacent sieve elements was measured with a sieve-element pressure probe, a cell pressure probe glued to the exuding stylet of aphids that had been feeding from the phloem. Sieve element sap was sampled by aphid stylectomy; sap osmotic pressure was determined by picolitre osmometry and its sugar concentration by enzyme-linked fluorescence assays. Samples were taken with a time resolution of ~2-3 min. In accordance with Münch's proposal a drop in osmotic and hydrostatic pressure in the source phloem following treatment of the source leaf with N was observed. A decrease in sugar concentration was the major contributor to the change in osmotic pressure. By observing these variables at a time resolution of minutes we have direct observation of the predictions of Münch.
根据明希假说，筛管中溶液的流动是由源（或收集）韧皮部与库（或释放）韧皮部之间的静水压力差驱动的。通过糖和其他溶质的主动吸收以及随之而来的水的流入，在收集韧皮部中维持高静水压力。通过溶质卸载，在释放韧皮部中维持较低的压力。在这项工作中，我们直接测试了溶质吸收在产生与韧皮部流动相关的静水压力中的作用。通过用氮气代替空气供应，减少了溶质向大麦和苦苣成熟叶片韧皮部的装载。用筛管元件压力探针测量相邻筛管元件中的静水压力，该探针是一种粘贴在从韧皮部取食的蚜虫分泌的口针上的细胞压力探针。通过蚜虫口针切除术采集筛管元件汁液；通过皮升渗透压测定法测定汁液渗透压，通过酶联荧光测定法测定其糖浓度。以约2-3分钟的时间分辨率采集样本。与明希的提议一致，在用氮气处理源叶后，观察到源韧皮部的渗透压和静水压力下降。糖浓度的降低是渗透压变化的主要原因。通过以分钟为时间分辨率观察这些变量，我们直接观察到了明希假说的预测结果。
