Dual-excitation FLIDAR for the estimation of epidermal UV absorption in leaves and canopies

A new FLIDAR was designed for remote quantitative assessment of epidermal UV absorption of leaves and canopies from chlorophyll (Chl) fluorescence (ChlF) measurements. The dual-excitation fluorescence light detection and ranging (DE-FLIDAR) performs a dual excitation of the Chl present in leaves, in the UV (355 nm) and visible (532 nm) part of the spectrum, the latter being used as a reference excitation not absorbed by the epidermis. Therefore, the epidermal UV absorption of vegetation can be estimated from the Chl fluorescence excitation ratio (FER), ΦF(532)/ΦF(355). Thanks to the alternated excitation by the DE-FLIDAR, the FER is immune to natural conditions in field, such as light-induced variable ChlF and leaf movement (variation of the angle of excitation). The DE-FLIDAR was used to investigate the presence of UV-absorbing compounds in individual leaves and canopies of different plant species, tobacco, pea, barley and wheat. The FER was much larger in outdoor-grown plants, indicating an accumulation of UV-absorbing compounds. We also analysed the epidermal UV absorption of the adaxial and abaxial side of tobacco leaves of different age. The logarithm of the FER showed a good agreement with the absorbance of methanolic extracts obtained from the same leaves. The presented DE-FLIDAR can perform up to three simultaneous fluorescence measurements; therefore, we could compare blue fluorescence (BR) to the epidermal UV absorption. In addition, a dual ratio, the red fluorescence (RF) to far-red fluorescence (FRF) emission ratio, excited at 355 and 532 nm, was shown to be linearly dependent on the Chl content. A mathematical model of leaf absorption and fluorescence, based on the Beer–Lambertʹs law, was developed to describe and analyse the fluorescence signatures obtained with the DE-FLIDAR.