A fluorescence spectroscopic study of light transmission and adaxial-abaxial distribution of emitting compounds in leaves of Christmas star (Euphorbia pulcherrima)

Abstract

In situ fluorescence measurements have been used to investigate relative amounts of blue-green pigments and their distributions in plant leaves from Euphorbia pulcherrima. Advantage was taken from the fact that this species has white leaves on the top, with low pigment concentrations, and green leaves on the stem with ordinary pigment concentrations. Excitation- and emission spectra below 410 nm from white leaves, where pigment absorption is low, are not distorted by self-absorption. Absorption- and reflection spectra from white and green leaves were measured using a spectrophotometer equipped with an integrating sphere. The absorption spectra were used to correct recorded fluorescence spectra for self-absorption. Self-absorption corrected photosystem fluorescence from green leaves, modeling light transmission in leaf tissue exponentially, matches to the excitation/emission spectra from white leaves, apart from small differences due to the pigment concentrations and selective scattering. The introduced exponentially decaying transmission relation also predicts that the ratio of excitation spectra from a white and green leaf is in proportion to the absorption spectrum of the green leaf, which was validated for Photosystem II particle fluorescence. This relation was also used to find a scaled absorption spectrum responsible for blue-green emission, which was assumed to originate from lignin. Excitation/emission spectra of the blue-green fluorescence were decomposed into five components and their relative amounts from adaxial and abaxial sides of the leaves have been quantified. Fluorescence lifetime measurements of the leaves, upon 403 nm excitation, revealed three decay times corresponding to the lignin fluorophores emitting in blue and green spectral region, and indicated that emissions at 500 and 550 nm may originate from the same fluorophore residing in the two physically different environments.

Keywords: Absorption spectroscopy; Euphorbia pulcherrima; Fluorescence spectroscopy; Leaf pigments; Light transmission; Lignin.