Detection of Vegetation Stress Via a New High Resolution Fluorescence Imaging System

Abstract

The UV-laser ( λ 355 nm) induced fluorescence emission spectra of green leaves comprise the blue(F440) and green (F520) fluorescence bands as well as the red (F690) and far-red (F740) chlorophyll fluorescence emission bands. Based on the four UV-laser induced fluorescence bands blue, green, red and farred a high resolution fluorescence imaging system was established, which allows a fast and large scale screening of fluorescence gradients and local disturbances in fluorescence emission over the whole leaf surface. The new imaging method not only permits to screen leaves by means of images in four fluorescence bands (LIF images) but, in addition, via images of the fluorescence ratios blue/red (F440/F690), blue/farred (F440/F740), the chlorophyll fluorescence ratio red/far-red (F690/F740) and the ratio blue/green (F440/F520) (LIF ratio images). By fluorescence imaging we could prove that in aurea tobacco the major part of the leaves' blue and green fluorescence is emitted from the main and side leave veins, whereas the major part of the leaves' red and far-red chlorophyll fluorescence is emitted from the vein-free leaf regions, which also have the highest chlorophyll content. A smaller proportion of the aurea tobacco leaves' blue-green fluorescence emission is derived from the cell walls of epidermis cells. The fluorescence ratios blue/red and blue/far-red are very sensitive to environmental changes, and thus permit early stress and strain detection in plants, and the evaluation of damage to the photosynthetic apparatus. Via monitoring the increase in chlorophyll fluorescence LIF images allow to detect differences in the time-dependent uptake of diuron and the progressing inhibition of photosynthetic electron transport in the treated leaf part. The novel fluorescence imaging technique sets a new dimension for early stress detection in the photosynthetic apparatus and in plants. It has many advantages over the previously applied point-data measurements of selected leaf points using conventional spectrofluorometers. The new fluorescence imaging system proved to be very suitable for remote sensing of plants in the near distance, and can be further developed for far distance remote sensing of the state of health of terrestrial vegetation. Some examples in the many possible ways of computer-aided fluorescence data processing (formation of different fluorescence ratios, screening of fluorescence profiles, histogramme plotting) are indicated.