Girdling Blocks Translocation In The Phloem Figure

Remove bark to girdle the tree.

Organic solutes accumulate in the phloem above the girdle, causing swelling.

Remove bark to girdle the tree.

Organic solutes accumulate in the phloem above the girdle, causing swelling.

Phloem Malpighi Ring Tree
Conclusion: Organic solutes are translocated in the phloem, not in the xylem.

36.12 Girdling Blocks Translocation in the Phloem By removing a ring of bark (containing the phloem), Malpighi blocked the translocation of organic solutes in a tree.

minerals, and a variety of other solutes are translocated between sources and sinks in the phloem.

How do we know that such organic solutes are translocated in the phloem, rather than in the xylem? Just over 300 years ago, the Italian scientist Marcello Malpighi performed a classic experiment in which he removed a ring of bark (containing the phloem) from the trunk of a tree—that is, he girdled the tree (Figure 36.12). The bark in the region above the girdle swelled over time. We now know that the swelling resulted from the accumulation of organic solutes that came from higher up the tree and could no longer continue downward because of the disruption of the phloem. Later, the bark below the girdle died because it no longer received sugars from the leaves. Eventually the roots, and then the entire tree, died.

Any explanation of the translocation of organic solutes must account for a few important observations:

Translocation stops if the phloem tissue is killed by heating or other methods; thus the mechanism must be different from that of transport in the xylem.

Translocation often proceeds in both direc-tions—up the stem and down the stem— simultaneously.

Translocation is inhibited by compounds that inhibit respiration and thus limit the ATP supply in the source.

To investigate translocation, plant physiologists needed to obtain samples of pure sieve tube sap from individual sieve tube elements. This difficult task was simplified when it was discovered that a common garden pest, the aphid, feeds on plants by drilling into a sieve tube. An aphid inserts its specialized feeding organ, called a stylet, into a stem until the stylet enters a sieve tube (Figure 36.13a). The pressure within the sieve tube is greater than that in the surrounding plant tissues or outside the plant, so the nutritious sieve tube sap is forced through the stylet and into the aphid's digestive tract. So great is the pressure that sugary liquid is forced through the insect's body and out the anus (Figure 36.13b).

Plant physiologists use aphids to collect sieve tube sap. When liquid appears on the aphid's abdomen, indicating that the insect has connected with a sieve tube, the physiologist quickly freezes the aphid and cuts its body away from the stylet, which remains in the sieve tube element. For hours, sieve tube sap continues to exude from the cut stylet, where it may be collected for analysis. Chemical analysis of sieve tube sap collected in this manner reveals the contents of a single sieve tube element over time. Physiologists can also infer the rates at which different substances are translocated by measuring how long it takes for radioactive tracers administered to a leaf to appear at stylets at different distances from the leaf.

These methods have allowed us to understand how, at times, different substances might move in opposite directions in the phloem of a stem. Experiments with aphid stylets have shown that all the contents of any given sieve tube element move in the same direction. Thus, bidirectional translocation can be understood in terms of different sieve tubes conducting sap in opposite directions. These and other experiments

Longistigma caryae

Sieve tube element

The aphid's stylet has successfully penetrated the sieve tube.

The aphid's stylet has successfully penetrated the sieve tube.

Image Aphid Stylet Penetrating

Longistigma caryae

Longistigma Caryae
Sap droplet

36.13 Aphids Collect Sieve Tube Sap (a) Aphids feed on sap drawn from a sieve tube, which they penetrate with a modified feeding organ, the stylet. (b) Pressure inside the sieve tube forces sap through the aphids digestive tract, from which it can be harvested.

led to the general adoption of the pressure flow model as an explanation for translocation in the phloem.

Phloem sieve Source

M Transpiration | pulls water up xylem vessels. I

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Responses

  • primrose
    When phloem is removed phloem swells?
    8 years ago

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