UV-induced blue-green and far-red fluorescence along wheat leaves: a potential signature of leaf ageing

Under UV-excitation, leaves emit red (RF) and far-red (FRF) fluorescence from chlorophyll and blue-green fluorescence (BGF) from hydroxycinnamic acids. In this study, the aim was to develop a fluorescence signature of wheat leaf ageing after the emergence of the lamina. FRF and BGF were examined in the first three leaves of 2-week-old wheat plants. It was investigated how FRF and BGF vary as leaf and tissue aged by spectroscopic measurements, time-resolved BGF analysis and microscopic imaging of the leaf surface. It was found that FRF decreased with leaf and tissue ageing because of an accumulation of UV-absorbers in the epidermis. BGF also decreased, but without changes either in the shape of excitation and emission spectra or in the fluorescence lifetime. So, BGF emanated from the leaf surface, without changes in fluorophore composition during leaf ageing. The shape of the BGF spectrum indicates that ferulic acid bound to the cell wall is the main blue-green fluorophore. The effects of pH and solvents on BGF from intact leaves and ferulic acid in solution were similar, confirming the hydroxycinnamic acid origin of BGF. UV-fluorescence microscopic imaging of the surface of intact leaves showed that different epidermis cell types and sclerenchyma bands emitted BGF. The decreasing gradient of BGF from the base to the apex of the lamina could be related to the decrease in the surface of the fluorescent sclerenchyma bands. The significance of FRF and BGF as potential signatures of wheat lamina growth are discussed.