7.9 Investigating Water Loss in Plants

Question 1

In a Ringing Experiment, what happens as a protective layer and phloem is removed?

Answer 1

The area above of the removed circumference begins to swell with liquid rich in sugars and other dissolved organic substances. Non-photosynthetic tissue below begins to wither and die as the top keeps growing

Question 2

What do observations in the Ringing experiment suggest?

Answer 2

1. Sugars of the phloem accumulate above the ring, leading to swelling
2. interruption to flow of sugars to region below the ring leading to death of tissues
   Conclusion is that phloem, rather than xylem translocates sugars

Question 3

What are radioactive isotopes useful for?

Answer 3

Tracing movement of substances in plants. E.g. 14C can be used to makes radioactive carbon dioxide and this will be incorporated into plant sugars if they are grown in an atmosphere containing this. the sugars can then be traced using autoradiography

Question 4

What are the main steps for the Tracer Experiment?

Answer 4

1. Take a thin cross section of the plant stem
2. Place on X ray film
   The film becomes blackened where radioactive sugar touches it. These blackened regions correspond to where the phloem is. As other tissues re not blackened, we can conclude that the phloem alone is responsible for translocating sugars

Question 5

Evidence that translocation of organic molecules happens in the phloem:

Answer 5

1. When phloem is cut, a solution of organic molecules flows out
2. Plants grown in radioactive CO2 have radioactively labelled carbon in the phloem
3. Aphids, which feed on plants. penetrate the phloem and extract the contents of the sieve tube. Whichever variation these contents show in sucrose levels are mimicked by the phloem
4. Removal of the ring from the phloem leads to accumulation of sugars below, and death of tissues below it.