INQUIMAE   12526
Unidad Ejecutora - UE
congresos y reuniones científicas
Photosystems ratio in Schefflera arboricola variegata leaves. Fluorescence spectra and optical properties
San Pablo. Brasil
Congreso; IX Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2008
Institución organizadora:
Sociedad Latinoamericana de Fotoquímica y Fotobiología
Variegation is the appearance of differently colored areas in the leaves. White or yellow colored zones are usually connected with a strong decrease of chlorophyll in plastids. In the present work we used biospectroscopical methodologies to study comparatively the optical properties and fluorescence emission of green and white leaves of Schefflera arboricola variegata, covering an important shortage in literature on this field. Non-destructive optical methods are relevant nowadays due to field applications and remote sensing to assess plant health. In particular, reflectance, transmittance and emission spectroscopies are useful tools to develop such methodologies. From reflectance and transmittance spectra, important optical parameters as the absorption (k) and the scattering coefficient (s) may be obtained. Determination of these parameters is significant in heterogeneous media like plant tissues, being connected with photons transport and photosynthesis inside the leaves (1). On the other hand, fluorescence emission is a sensitive tool giving information on plant physiology and stress situations (2).   In the present research, reflectance and transmittance spectra were obtained as a function of wavelength with a spectrophotometer equipped with an integrating sphere. On the other hand, fluorescence emission was obtained for both intact leaves at 298K and for diluted suspensions of the same powdered leaves at 77K. As complementary work, photosynthetic pigments were extracted and quantitatively evaluated (3).   The remission function (F(R)), according the Kubelka-Munk theory, relates the absorption coefficient and the scattering coefficient (F(R) = k/s). Obtained values for F(R) and for k were higher for green leaves than for white ones. On the contrary, s values were higher for white leaves, but differences were not as high as for k. Chlorophyll fluorescence spectra is characterized by two emission maxima, one at 685 nm (Fred), due to photosystem II (PSII) emission and  other at 737 nm (Ffar-red) due to both photosystems I (PSI) and II emission. The fluorescence ratio F685/F737 is related in literature to the plant response to stress situations, to the photosynthetic activity and to the photosystems ratio (4). However, spectra from intact leaves are normally distorted by light re-absorption processes and only corrected spectra should be related with the plant physiological state. For white leaves, the fluorescence ratio (F685/F737) corrected by a physical model (4) was 4.1 while for green leaves it decreased to 1.79. This result would indicate a lower PSII content in green leaves. To prove this hypothesis, fluorescence measurements on dilute suspensions of leaves, at 77K, were performed. Under these conditions, contributions from both photosystems are well separated, belonging Fred to PSII and Ffar-red to PSI (5). Values for the fluorescence ratio under these conditions were 0.99 and 0.59 for white and green leaves-suspensions respectively, proving that effectively, white leaves shows higher PSII/PSI ratio. Chlorophyll a and chlorophyll b contents were higher for green leaves (0.065 and 0.033 mg/cm2 respectively) than for white leaves (0.006 and 0.098 mg/cm2 respectively), as expected. Additionally, the ratio Chlorophyll-a/b was also higher for green leaves than for white leaves (1.95 and 0.63).   In conclusion, the results show not only that PSII/PSI is higher for white leaves in Schefflera arboricola plants but also that this fact may be deduced from room temperature fluorescence emission spectra of intact leaves, when physical models correcting light re-absorption processes are applied.   Bibliography 1) Cordon G. and Lagorio M. G., J. Chem. Educ., 2007, 85, 1167-1170 2) Subhash, N.; Wenzel, O; Lichtenthaler, H. K., Remote Sens. Environ., 1999, 69, 215-223 3) Sims, D. A.; Gamon, J. A., Remote Sens. Environ., 2002, 81, 337-354 4) Cordon G. and Lagorio M. G., Photochem. Photobiol. Sci., 2007, 6, 873-882 5) Pfundel, E., Photosynth. Res., 1998, 56, 185-195