Time-resolved blue-green fluorescence of sugar beet leaves. Temperature-induced changes and consequences for the potential use of blue-green fluorescence as a signature for remote sensing of plants

Abstract

Sugar beet (Beta vulgaris L.) leaves emitted blue-green fluorescence (BGF) and red (chlorophyll a) fluorescence (RF) when excited with ultraviolet light. The leaf BGF increased when leaf temperature decreased (5% of BGF change per °C), unlike leaf RF that remained unchanged. Several lines of evidence indicate that the temperature-induced changes in the leaf BGF come from the epidermis: (i) the decrease from 33 to 3°C increased the leaf and epidermis BGF more than 2-fold, whereas the mesophyll BGF increased only 1.4-fold; (ii) the excitation and emission difference spectra 3 minus 33°C showed similar maxima in the leaf and in the epidermis at 340–350 nm and 420–450 nm respectively, whereas in the mesophyll they were shifted to 320 and 500 nm respectively; and (iii) time- resolved BGF measurements showed that the thermal sensitivities of different kinetic components found in leaves matched with those of the epidermis, unlike those of the mesophyll that were much lower. All these changes were fully reversible and paralleled the changes in temperature. Experiments in vitro with ferulic acid showed that the spectral properties of the BGF and its thermal sensitivity depend on the solvent in which it is dissolved, suggesting that the origin of these changes is in the surrounding or the micro-environment of the blue-green fluorophore. Discussion is focused on the consequences of these findings for the potential use of BGF as a signature for remote sensing of plants.