Mass Transport in Plants the Phloem What is

What is translocation? Translocation is the movement of nutrients around a plant. The term

The mass flow hypothesis The most widely accepted explanation of sap movement in plants

Translocation of sugars 5 of 26 © Boardworks Ltd 2008

Ringing Experiments • Evidence for water movement in xylem and sugar movement in phloem.

Ringing Experiments 9 of 26 • Stem above missing tissue ring swells with liquid

Ringing experiments Conclusions • Sugars of phloem accumulate above ring as they are unable

Tracer experiments • Radioactive carbon isotope 14 C is used to make 14 CO.

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Mass Transport in Plants: the Phloem

What is translocation? Translocation is the movement of nutrients around a plant. The term includes the movement of minerals, which can be dissolved in water and transported in the xylem, but usually refers to the transport of sugars, amino acids, and other organic molecules in the phloem. Translocation can occur in either direction in the phloem – it is bidirectional. It is an active process, requiring energy, unlike water transport in the xylem. 2 of 26 © Boardworks Ltd 2008

Phloem as seen under a light microscope

The mass flow hypothesis The most widely accepted explanation of sap movement in plants is the mass flow hypothesis (sometimes called the pressure flow hypothesis). According to theory, sap moves through phloem vessels due to differences in hydrostatic pressure. Evidence for this effect includes the excretion of sap, or honeydew, by an aphid when it taps a phloem vessel to feed. The sap is forced through the aphid’s body, demonstrating that the sap in the phloem is under pressure. 4 of 26 © Boardworks Ltd 2008

Translocation of sugars 5 of 26 © Boardworks Ltd 2008

6 of 26 © Boardworks Ltd 2008

Ringing Experiments • Evidence for water movement in xylem and sugar movement in phloem. • Woody stems have an outer protective layer of bark, with a ring of phloem inside and a ring of xylem inside that. • Outer layers of bark and phloem are removed from a section of stem. 8 of 26 © Boardworks Ltd 2008

Ringing Experiments 9 of 26 • Stem above missing tissue ring swells with liquid rich in sugars. • Non-photosynthetic tissues below the ring wither and die. © Boardworks Ltd 2008

Ringing experiments Conclusions • Sugars of phloem accumulate above ring as they are unable to move down the stem. • Sugars don’t pass below the ring so tissues die. Phloem, rather than xylem, is responsible for the translocation of sugars in the plant. 10 of 26 © Boardworks Ltd 2008

Tracer experiments • Radioactive carbon isotope 14 C is used to make 14 CO. 2 • The 14 C is incorporated into the sugars produced during photosynthesis. • Autoradiography allows tracking of these sugars. Radioactive areas correspond to areas of phloem, not xylem. 11 of 26 © Boardworks Ltd 2008