Fluorescence and Polyphenols
Fluorescence, Chlorophyll and Polyphenols
Under UV excitation leaves display a specific and complex light emission, called fluorescence. There are two, fundamentaly different types of fluorescence: the blue-green fluorescence and the red fluorescence. Fluorescence imaging, reflecting the anatomy of the leaf, has shown that the blue-green fluorescence comes primarily from the epidermis, trichomes, vascular bundles and wall cells, whereas the red fluorescence predominantly comes from chloroplasts of the mesophyll cells.
UV-excited fluorescence emission of a transverse section of a wheat leaf (left) and of the surface (right).
A spectral analysis of the leaf fluorescence shows that the blue-green fluorescence is emitted in the 400- 630-nm range whereas the red fluorescence is emitted in the red and far-red region (630-800 nm) of the spectrum.
A typical UV-induced fluorescence spectrum of leaves has three maxima, at 440-450 nm, 687 nm and 735 nm and a shoulder at 530 nm, which can be more or less pronounced. With a large-band detection of the fluorescence these spectral features are usually named blue (BF), green (GF), or blue-green (BGF) when considered together, red (RF) and far-red (FRF) fluorescence.
These maxima have variable amplitude depending on intrinsic leaf properties and environmental factors: plant species, age, leaf side, developmental stage, past and present stresses. Therefore, leaf fluorescence can be considered as a signature of the physiological state of the plant.
Fluorescence emission spectrum of a wheat leaf under UV-excitation.
To understand plant fluorescence three questions need to be answered:
- the molecular origin
- the origin of its heterogeneity
- the origin of its variability
In vivo, red fluorescence is emitted only by chlorophyll a. For many years this chlorophyll a fluorescence was used as an accurate and non-destructive probe of photosynthetic efficiency, directly or indirectly related to many type of stresse of the plant. The two peaks of fluorescence (687 & 735) are due to the presence of two photosystems in the photosynthetic apparatus, PSI and PSII..
The blue-green fluorescence, rediscovered more recently, has a highly heterogeneous origin with a large number of candidate fluorophores, e. hydroxycinnamic acids (caffeic, ferulic), chromones, stilbenes (resveratrol), coumarins, isoflavones, , nicotinamides, flavins, pteridines, , alkaloids.
These molecules are mainly present in the cell vacuole when glycosilated or linked to the cell wall. In vivo, the latter, member of the larege family of plant phenolics seems to be the major emitters.
Domains of Plant Polyphenolics Research and Applications
The domain of plant polyphenolics research and applications is as large as the number of polyphenolics. In the following figure the most important ones are described, based and related to the role of polyphenolics in plants (the numbers come from the figure describing the role of polyphenolics in plants).