STUDENT ABSTRACTS - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 275

Screening differences in photosynthesis and fluorescence characteristics of plant
leaves by UV-A and blue light fluorescence imaging
FATBARDHA BABANI 1 , MARTIN KNAPP 2 , HARTMUT K. LICHTENTHALER 2
1 Biological Research Institute, Academy of Sciences, Tirana, Albania
2 Botanical Institute II, University of Karlsruhe, Karlsruhe, Germany
Green plants exited by UV-radiation emit blue (F440) and green fluorescence (F520) as
well as red (F690) and far-red (740) chlorophyll (Chl) fluorescence. The blue-green fluorescence
is primarily emitted by ferulic acid covalently bound to cell wall carbohydrates 1 . The red and far-
red fluorescence is emitted by chlorophyll a in the chloroplasts of the green mesophyll cells. High
resolution multi-colour fluorescence imaging techniques, developed over the past seven years,
offer the new possibility to study the distribution and gradients of fluorescence signatures over the
whole leaf area 2,3,4 . We have described the differences in the UV-A induced fluorescence imaging
of sun and shade leaves in the fluorescence bands blue, green, red and far-red including the
fluorescence ratio images blue/green, blue/red, blue/far-red and red/far-red. The blue light induced
images of the Chl fluorescence decrease ratio (R Fd = (Fm-Fs)/Fs) are used to show the differences
and gradients in photosynthetic activity.
Fluorescence images of sun and shade beech leaves showed that the blue (F440) and green
(F520) fluorescence emitted from the upper leaf side in both leaf types were almost the same and
much lower than that of the lower leaf side whereas the lower leaf side of sun leaves showed
considerably lower fluorescence yield than in shade leaves. The lower intensity of the red and far-
red Chl fluorescence of sun leaves as compare to shade leaves is due to the fact that they possess
in their epidermis cells many soluble flavonols and other phenolic substances, which absorb the
UV-A used for fluorescence excitation but do not fluoresce themselves 1 . Thus, less UV-A reaches
the mesophyll chloroplasts for Chl fluorescence excitation. Also thicker epidermis cell walls of
sun leaves, a considerable wax layer of sun leaves and an enhanced scattering can influence the
amount of UV-radiation entering the leaf. A second cause for the lower Chl fluorescence yield is
that in sun leaves with their higher Chl content the red Chl fluorescence is reabsorbed to a higher
degree than in shade leaves. Shade leaves are characterized by significantly lower values of the
fluorescence ratios blue/red (F440/F690) and blue/far-red (F440/F740) at each leaf side than sun
leaves. The Chl fluorescence ratios red/far-red are lower in the upper side of both leaf types as
compared to the lower side. Thus, the fluorescence ratio F690/F740, as indicator of the in vivo Chl
content of leaves 2 , showed that Chl content is highest in the upper side of sun leaves and is higher
in the upper than in the lower side of both leaf types. A higher Chl fluorescence yield at Fp (1 s
after onset of illumination) and at Fs (reached after 5 min of illumination) of shade leaves as
compared to sun leaves was observed. The Chl fluorescence ratios, R Fd and Fp/Fs, which are
indicators of the photosynthetic quantum conversion and photosynthetic activity of leaves 3 ,
showed the expected higher values in sun than shade leaves of beech.
The much lower Chl fluorescence yield of sun leaves by UV-A excitation demonstrates
that they are better protected against UV-radiation than shade leaves. The photosynthetic
apparatus of sun leaves has a higher capacity for photosynthetic quantum conversion than shade
leaves as seen in the R Fd -images.
References
1. Lichtenthaler HK, Schweiger J (1998) J Plant Physiol 152: 272-282.
2. Buschmann C, Lichtenthaler HK (1998) J Plant Physiol 152: 297-314.
3. Lichtenthaler HK, Babani F (2000) Plant Physiology Biochemistry 38: 889-895.
4. Lichtenthaler H K, Babani F, Langsdorf G, Buschmann C (2000) Photosynthetica 38: 521-529.