Gould Nick, Minchin Peter E H, Thorpe Michael R
University of Waikato, Department of Biology, Hamilton, NZ.
Funct Plant Biol. 2004 Nov;31(10):987-993. doi: 10.1071/FP04058.
According to the Münch hypothesis, solution flow through the phloem is driven by a hydrostatic pressure gradient. At the source, a high hydrostatic pressure is generated in the collection phloem by active loading of solutes, which causes a concomitant passive flow of water, generating a high turgor pressure. At the sink, solute unloading from the phloem keeps the turgor pressure low, generating a source-to-sink hydrostatic pressure gradient. Localised changes in loading and unloading of solutes along the length of the transport phloem can compensate for small, short-term changes in phloem loading at the source, and thus, maintain phloem flow to the sink tissue. We tested directly the hydrostatic pressure regulation of the sieve tube by relating changes in sieve tube hydrostatic pressure to changes in solute flow through the sieve tube. A sudden phloem blockage was induced (by localised chilling of a 1-cm length of stem tissue) while sieve-tube-sap osmotic pressure, sucrose concentration, hydrostatic pressure and flow of recent photosynthate were observed in vivo both upstream and downstream of the block. The results are discussed in relation to the Münch hypothesis of solution flow, sieve tube hydrostatic pressure regulation and the mechanism behind the cold-block phenomenon.
根据明希假说，溶液在韧皮部中的流动是由静水压力梯度驱动的。在源端，通过溶质的主动装载在收集韧皮部中产生高静水压力，这会导致水的伴随被动流动，从而产生高膨压。在库端，从韧皮部卸载溶质会使膨压保持较低水平，从而产生从源到库的静水压力梯度。沿着运输韧皮部长度方向溶质装载和卸载的局部变化可以补偿源端韧皮部装载的微小短期变化，进而维持向库组织的韧皮部流动。我们通过将筛管静水压力的变化与通过筛管的溶质流动变化联系起来，直接测试了筛管的静水压力调节。诱导韧皮部突然堵塞（通过对1厘米长的茎组织进行局部冷却），同时在体内观察堵塞部位上游和下游的筛管汁液渗透压、蔗糖浓度、静水压力和近期光合产物的流动。结合溶液流动的明希假说、筛管静水压力调节以及冷堵塞现象背后的机制对结果进行了讨论。
