no PLANT GROWTH AND TROPISMS 



Exercise XXI 



As you know, plants of all sizes and ages al- 

 ways retain meristematic tissue that is capable of 

 new growth. In a young shoot, growth in length 

 is confined to a rather narrow zone toward the 

 tip. This growth is controlled by a hormone 

 called auxin. The most prominent auxin is in- 

 dole-3-acetic acid (lAA), which has the following 

 formula : 



H 



C O 



/- \ ^ 



HC C C— CH2 • C 



HC 



\ 



CH 



OH 



C 



H 



N 

 H 



Indole 



Auxin acts by promoting cell elongation, 

 rather than cell division. It is synthesized in the 

 tip of the shoot, though small amounts of auxin 

 are also produced in roots, leaves, and fruit. The 

 auxins are distributed throughout the plant from 

 the apical buds via the phloem. The highest con- 

 centration of auxin is found nearest the apical 

 bud, and the concentration falls off rapidly 

 toward the basal portions of the plant. Auxin is 

 inactivated or destroyed during growth, and must 

 be continuously supplied from the apical bud. 

 Within the range of low auxin concentrations, if 

 one portion of a plant has more auxin than an- 

 other, it grows faster. The differential distribu- 

 tion of auxin accounts for much of the differen- 

 tial growth, and hence the tropisms of plants. 



Among today's experiments, you will have the 

 opportunity to examine the effects of auxin on 

 growth, the responses of plants to light and 

 gravity, and other aspects of the physiology of 

 plant growth. We shall use for these experiments 

 the classic oat shoot (Avena). The young shoot 

 consists of a colorless tubular sheath, the 

 coleoptile, which surrounds the yellow or green 

 primary leaf. It is the coleoptile that is princi- 

 pally responsible for the bending reactions. We 

 shall also take the opportunity to examine under 

 the microscope the tissues of a higher plant con- 

 cerned with growth and translocation. 



One of the most useful techniques developed 

 for investigating cellular metabolism depends 

 upon isotope-labeled molecules. Such molecules 

 have exactly the same chemical properties as 

 those lacking the label, and can be used to follow 

 the pathways and ultimate fates of metabolites 

 in the organism. Today, we shall offer CO2 

 labeled with the radioactive isotope of carbon, 

 C'^ (therefore C'"'02), to a bean leaf, and inves- 

 tigate its uptake in light and darkness, and the 

 subsequent translocation of the carbon com- 

 pounds newly synthesized from it. 



Radioactive compounds emit radiations that, 

 like light, affect a photographic film, producing a 

 latent image which darkens on development. 

 We will measure both the uptake and distribu- 

 tion of the radioactive carbon in the leaf by 

 exposing a film to it. 



Carbon-I4 is a relatively stable radioisotope 

 that emits /3-rays (electrons). This is not a very 

 penetrating radiation; one thickness of paper can 

 usually block it. For this reason €'■* compounds 

 are relatively safe to use; yet take care with them. 

 Be careful not to spill any radioactive materials. 

 Also place any contaminated materials as soon 

 as you are through with them into the special 

 containers which are provided. Wash your 

 hands thoroughly before leaving the laboratory. 



PLANT GROWTH AND TROPISMS 



First test the effects of auxin on the growth of 

 the stem. You will be supplied with 4 oat seed- 

 lings that are 3 days old (the first leaf should 

 not as yet have pushed through the coleoptile). 

 With a razor blade cut off the terminal 3 mm 

 of each tip, and discard it (why?). Then, using 

 a sharp razor blade, cut a segment exactly 10 mm 

 long from each plant. Place 2 such segments in 

 each of 2 small petri dishes. Fill one dish with a 

 2% sucrose solution, the other with 2% sucrose 

 containing 2 mg per liter of indole acetic acid, 

 brought to a slightly acid pH with KH2PO4. 

 Place the dishes in the dark (your desk drawer). 

 After at least 2 hours have passed, measure the 

 length of the seedlings to the nearest quarter 



