韧皮部运输

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phloem transport.PPT(下载附件 3.45 MB)

Translocation

The solutes not only need to get into or out of cells, but they need  to be moved from one area of a plant to another. We have already seen how water  moves through the plant in the xylem: by transpiration! For soil mineral  solutes, this is the pathway to get into the plant and up the xylem too. These  solutes are dissolved in the water that we have seen passing through that  pathway.

Organic solutes are translocated in phloem

No, indeed there must be ways for the solutes from photosynthesis  and other biochemical pathways to depart from the leaf and go both down to the  root and up to flowers, fruits, and apical meristems to provide fuel for  respiration carried out in these non-photosynthetic areas. So leaves are the  likely source of small organic molecules, and the rest of the plant  organs are sinks for those molecules. The flow of these solutes then must  be able to be both upward and downward in the plant. The solutes could be any of  the subunits of the macromolecules; examples include sugars, amino acids,  nucleotides, and fatty acids.

The flow of these organic molecules is called translocation and this process occurs mostly through the phloem. While phloem lies  alongside the xylem in veins in leaves, vascular bundles in stems, and the  vascular cylinder of roots, it is a completely different tissue conducting in  different directions and by different mechanisms!

How do solutes get into the phloem?

THE PRESSURE-FLOW MODEL OF PHLOEM TRANSLOCATION

Theory of pressure flow, alsocalled mass flow theory, was put forward by Münch in 1930，suggests the mass flow is driven bypressure gradient.

In the phloem of a source leaf, the water potential is typically more negativethan it is in the phloem of the sink. In this case, water transport is drivenby a difference in hydrostatic pressure, and water moves up a gradient in waterpotential.

Phloem unloading

symplastic pathway: To young growing tissues such as young root. Sufficient plasmodesmata exist in these pathways to support symplastic unloading.

apoplatic pathway: Mainly to story organs such asearthnut and tuber. Also to developing seeds which is symplastically isolated (An apoplastic step is required in developing seeds because there are no symplastic connections between the maternal tissues and the tissues of the embryo).