[7.9] investigating transport in plants

Question 1

describe a ringing experiment

Answer 1

• remove outer layers (protective layer and phloem) from circumference of a woody stem while it is still attached to the rest of the plant
• after a period of time, the region of the stem immediately above the missing ring of tissue is seen to swell

Question 2

what is found be accumulated in samples of the liquid from the swollen region?

Answer 2

rich in sugars and other dissolved organic substances

Question 3

what happens to the non-photosynthetic tissues in a ringing experiment?

Answer 3

• in region below ring (towards roots), they wither and die
• above region they continue to grow

Question 4

what do these observations suggest that removing the phloem around the stem leads to?

Answer 4

• sugars of the phloem accumulating above the ring, leading to swelling in this region
• interruption of flow of sugars to region below the ring and death of tissues in this region

Question 5

what conclusions can be drawn from observations of the ringing experiement?

Answer 5

• phloem, rather than xylem, is the tissue responsible for translocating sugars in plants
• as the ring of tissue removed had not extended into the xylem, its continuity had not been removed
• if it were the xylem responsible for translocating sugars you would not have expected sugars to accumulate above the ring nor tissues below it to die

Question 6

how can radioactive isotopes be useful for tracing the movement of substances in plants?

Answer 6

• ¹⁴C isotope can be used to make radioactively labelled carbon dioxide (¹⁴CO₂)
• if a plant is grown in an atmosphere containing ¹⁴CO₂, the ¹⁴C isotope will be incorporated into the sugars produced during photosynthesis
• these radioactive sugars can then be traced as they move within the plant using autoradiography

Question 7

describe how autoradiography can be used to track the movement of sugars in plants

Answer 7

• take thin cross-sections of the plant stem and place them on a piece of X-ray film
• the film becomes blackened where it has been exposed to the radiation produced by the ¹⁴C in the sugars
• blackened regions are found to correspond to where phloem tissue is in the stem

Question 8

what conclusion can be drawn from autoradiography?

Answer 8

• as other tissues do not blacken the film, it follows that they do not carry sugars
• phloem alone is responsible for their translocation

Question 9

describe a tracer and ringing experiment with ¹⁴C and ³²P

Answer 9

• phloem tissue removed by ringing
• plant is submerged in water containing radioactively labelled ³²P
• leaf if supplied with radioactively labelled ¹⁴C
• ³²P radioactivity found all over plant
• ¹⁴C accumulates in leaf and before ringing and some at top leaves

Question 10

how does the tracer and ringing experiment with ¹⁴C and ³²P demonstrate that water moves through the xylem?

Answer 10

• ³²P is transported faster than ¹⁴C because xylem transports mineral ions
• no conversion is needed
• ¹⁴CO₂ is photosynthesised then converted into sucrose, which takes longer

Question 11

evidence that translocation of organic molecules occurs in phleom

Answer 11

• when phloem is cut, a solution of organic molecules flow out
• plants provided with radioactive CO₂ can be shwon to have radioactively labelled C in phloem after a short time
• aphids have needle-like mouthparts which penerate phloem so can extract contents of sieve tubes. these contents show daily variations in the sucrose content of leaves that are mirrored a little later by identical changes in the sucrose content of the phloem
• removal of a ring of phloem from around the whole circumference of a stem leads to the accumulation of sugars above the ring and their disappearance from below it