Variation of UV-induced fluorescence along wheat leaves

UV-excitation of green leaves induces a red fluorescence (RF) and a blue-green fluorescence (BGF) emitted by chlorophyll and hydroxycinnamic acids (HCA), respectively. The potential use of BGF for crop remote sensing requires the precise knowledge of molecular and anatomical origins of BGF during plant development. In this study, we investigated how cell age and the leaf stage of development influence the BGF signature. We measured the UV-induced fluorescence along the three leaves of 15-days old wheat plants (Triticum aestivum L.) using a UV-PAM fluorimeter. Then, the BGF was described and quantified along the leaves by microscopic fluorescence imaging. The HCA origin of BGF was verified by comparing alkali-induced changes of leaf BGF to pH-induced variation of ferulic acid fluorescence spectra. The decrease of RF from the base towards the tip of all leaves showed that UV-absorbers accumulated as cell aged. Except for the oldest leaf, there was no similar decrease in BGF, which therefore could emanates from leaf surface. We showed that BGF emanates from the wall of each epidermal cell type and from the trichome protoplast. We verified that all these structures contain HCA. Lignified cell wall of sclerenchyma bands contributed also to BGF. Among the three leaves, the oldest one emitted the lowest BGF which was related to the lowest density of trichomes. However, the decrease of BGF along this leaf can not be explained by a reduction in trichomes density. The significance of BGF signatures is discussed considering the spatial distribution of blue-green fluorophores and UV-absorbers