Adapted from an experiment for students authored by Klaus-J. Appenroth (see the 9th issue of Duckweed forum)
1. Introduction
Norflurazon belongs to the group of bleaching herbicides. Any other bleaching herbicide can also be used in this experiment. This experiment gives the opportunity to explain the complex effect of herbicides and their interaction with photosynthesis.
2. Material and methods
Materials and equipment:
- Erlenmeyer flasks (100 ml), 2 per group of students
- Cotton wool stoppers
- Inoculation needle
- Measure (50 ml)
- Autoclave
Plant material:
- Axenic culture of Lemna, e.g., Lemna minor, Lemna gibba, Lemna aequinoctialis, pre-cultivated for ca. 14 days. Other species of duckweed can also be used.
Chemicals:
- Autotrophic nutrient medium, e.g., according to Appenroth et al. (1996). Other nutrient media (Steinberg, Schenk-Hildebrandt or Hoagland) are also possible. Half of the nutrient medium should contain the solved herbicide.
- Norflurazon has to be pre-solved in a small amount of ethanol and stirred overnight in the nutrient medium.
3. Experimental procedure
50 ml nutrient medium with or without herbicide are filled into the Erlenmeyer flasks and some plants (e.g., 10 fronds, not 10 colonies!) are inoculated into the solutions. Take care that no traces of the herbicide-containing medium reach the herbicide-free medium. Do not use the same inoculation needle. The flasks were kept in a cultivation room or on a windowsill exposed to daylight. Control cultures can also be kept in complete darkness. After 2 weeks, the fronds have to be evaluated and the number of green and white fronds have to be counted and growth rate calculated.
4. Theoretical background
Bleaching is often assumed to be caused by inhibition of chlorophyll biosynthesis. This is not correct. Bleaching herbicides inhibit synthesis of carotenoids. Carotenoids are required to protect chlorophylls by taking over the surplus energy in light. Carotenoids quench electronically excited chlorophylls by different mechanisms. Carotenoids are relatively stable in plant cells. As a consequence already existing cells/fronds are not bleached in light as they have sufficient carotenoids. However, newly formed cells/fronds cannot synthesis carotenoids in the presence of the herbicide. Therefore, these fronds are bleached. Often it can be observed that one part of a frond (the old part) is still green, the younger part bleached. In darkness no bleaching affect is observed.
An experiment for students authored by Klaus-J. Appenroth from the 9th issue of Duckweed forum.