Lecture 10 Transport in Plants: Phloem Dr. Alan McElligott Chapters 34 & 35 Lecture 10 Transport in Plants: Phloem Dr. Alan McElligott

Aims: To introduce the structure of the phloem To study the translocation of substances in the phloem

Describe mechanisms for the life processes (LOC3). Phloem Aims: To introduce the structure of the phloem To study the translocation of substances in the phloem These lecture aims form part of the knowledge required for learning outcome 3: Describe mechanisms for the life processes (LOC3).

34.2 How Are Plant Cells Unique? (Phloem) Essential reading pages 751-752 pages 775-777

Figure 34.6 Three Tissue Systems Extend Throughout the Plant Body

34.2 How Are Plant Cells Unique? Phloem transport cells—sieve tube elements, are living. Cells meet end-to-end; transport carbohydrates.

34.2 How Are Plant Cells Unique? Plasmodesmata in end walls enlarge to form pores— form sieve plates. Membrane that encloses central vacuole (tonoplast) disappears; nucleus and some cytoplasmic components break down, thus pores in sieve plate are not clogged.

35.4 How Are Substances Translocated in the Phloem? 35 Transport in Plants 35.4 How Are Substances Translocated in the Phloem?

35.4 How Are Substances Translocated in the Phloem? Movement of carbohydrates and other solutes through the phloem is translocation. Substances are translocated from sources to sinks. Sources (e.g., leaves, produce more sugars than they require). A sink consumes sugars for growth or storage.

35.4 How Are Substances Translocated in the Phloem? In a classic experiment, a tree was girdled; a ring of bark containing the phloem was removed. Organic solutes collect in the phloem above the girdle, causing it to swell. Eventually the bark, then roots below, and whole tree die because sugars are not being translocated downwards.

Figure 35.11 Girdling Blocks Translocation in the Phloem

35.4 How Are Substances Translocated in the Phloem? Characteristics of translocation: Stops if phloem is killed. Proceeds in both directions simultaneously. Inhibited by compounds that inhibit respiration and limit ATP supply.

35.4 How Are Substances Translocated in the Phloem? Plant physiologists needed to sample pure phloem sap from individual sieve tube elements. Aphids feed on plants by drilling into sieve tubes and inserting their stylet. Pressure in the sieve tube forces sap through stylet and into aphid’s digestive tract.

Figure 35.12 Aphids Collect Sap

35.4 How Are Substances Translocated in the Phloem? Plant physiologists use aphids by cutting the body away from the stylet—phloem sap continues to flow for hours and can be collected and analyzed. Using radioactive tracers, they can infer how long it takes for translocation to occur. These and other experiments led to development of the pressure flow model.

35.4 How Are Substances Translocated in the Phloem? Two steps in translocation require energy: Transport of solutes from sources into sieve tubes: loading. Removal of solutes at sinks: unloading.

35.4 How Are Substances Translocated in the Phloem? Where loading is occurring, solute concentration in those sieve tube elements is greater than in surrounding cells. Water enters by osmosis, which makes a greater pressure potential, and water plus solutes is pushed toward the sink. At the sink, the solutes are unloaded by active transport, maintaining the pressure gradient.

Figure 35.13 The Pressure Flow Model

The Pressure Flow Model

Table 35.1 Mechanisms of Sap Flow in Plant Vascular Tissues

35.4 How Are Substances Translocated in the Phloem? For the pressure flow model to be valid, two requirements must be met: Sieve plates must be unobstructed so that bulk flow is possible. There must be effective methods for loading and unloading solutes.

35.4 How Are Substances Translocated in the Phloem? Early electron microscope studies of sieve tubes indicated the plates were blocked by fibrous proteins. These proteins were a response to damage when phloem was prepared for study. When tissues were undamaged, sieve plates were shown to be open.

35.4 How Are Substances Translocated in the Phloem? Secondary active transport loads sucrose into companion cells and sieve tubes by a sucrose–proton symport. The apoplast must have a high concentration of protons—supplied by the proton pump (active transport).

35.4 How Are Substances Translocated in the Phloem? At sinks, sucrose is actively transported out of sieve tubes and into the surrounding tissues. This maintains the pressure gradient, and promotes buildup of sugars and starches in storage areas, such as developing fruits and seeds.

35.4 How Are Substances Translocated in the Phloem? Many substances move through the symplast via the plasmodesmata, including at loading and unloading sites. In sink tissues, plasmodesmata are abundant and allow passage of large molecules.

35.4 How Are Substances Translocated in the Phloem? Plants (and viruses) produce “movement proteins” that change the permeability of plasmodesmata and allow large molecules to pass. Biologists study these proteins in hopes of modifying plasmodesmata—for example, to divert more photosynthetic output to seeds to increase crop yields.

PHLOEM Check out 35.4 Recap, page 777 35.4 Chapter summary, page 778, see WEB/CD activity 35.4 Self Quiz Page 779: Chapter 35, Questions 9 and 10 For Discussion Page 779: Chapter 35, Question 2

PHLOEM Key terms: aphids, companion cell, girdling, phloem, pressure flow model, sieve plate, sieve tube member, sink, source, stylet, translocation