Adapted from an experiment for students authored by Klaus-J. Appenroth (see the 12th issue of Duckweed forum)
Isolating chlorophyll and carotenoids from plant tissues is very often carried out in basic courses of Plant Physiology. Beside determining the concentration by absorbance measurements in a spectrometer or separating the different compounds by thin layer chromatography, chlorophyll fluorescence can be investigated. After homogenization of green plant tissue with a suitable organic solvent and perhaps after filtration or centrifugation, fluorescence can be observed directly under a UV-B lamp. This can be done with practically all green plants. Duckweed offer an additional opportunity. As the cuticula is almost non-existent, substances are therefore easily taken up from liquid medium. This holds true e.g. for herbicides that uncouple the photosynthetic electron transport. Then, the energy cannot be transported away from photosystem II and the probability for fluorescence is increased. Theoretically, this can be investigated by pulse-amplitude modulated fluorescence (PAM) or by direct fluorescence measurements (OJIP method). However, equipment available for that are normally not available for student courses.
We used the following experiment to demonstrate inhibition of the photosynthetic electron transport in duckweed, e.g. Lemna minor:
- Fronds of duckweed were put in a petri dish and a few droplets of a solution of DCMU ((3-(3,4- dichlorophenyl)-1,1-dimethylurea; Diuron) on the surface is applied.
- Using a UV-B lamp (eye protecting specs are required) the fronds are irradiated.
- Whereas untreated areas show no fluorescence, the treated area shows red fluorescence after a few minutes. The lack of this effect when other leafs are used (e.g. garden bean) demonstrates the importance of the cuticula for terrestrial plants in contrast to water plants.
DCMU solution: the final concentration should be 1 mM (molecular mass 233 g mol-1). The solid substance is pre-solved with as less as possible acetone. Then the solution is diluted to 1 mM with water.
Mechanism: The herbicide DCMU blocks the plastoquinone binding site of photosystem II interrupting this way the photosynthetic electron transport from PSII to PSI. As a consequence fluorescence can be observed even with the naked eye.
Adapted from an experiment for students authored by Klaus-J. Appenroth (see the 12th issue of Duckweed forum).