Multi-colour fluorescence imaging of photosynthetic activity and plant stress

Abstract

Imaging the four fluorescence bands of leaves, the red (F690) and far-red (F740) chlorophyll (Chl) fluorescence as well as the blue (F440) and green (F520) fluorescence of leaves and the corresponding fluorescence ratios is a fast and excellent nondestructive technique to detect the photosynthetic activity and capacity of leaves, of gradients over the leaf area as well as the effect of various strain and stress parameters on plants. This review primarily deals with the first and pioneering multi-colour fluorescence imaging results obtained since the mid-1990s in a cooperation with French colleagues in Strasbourg and in my laboratory in Karlsruhe. Together we introduced not only the joint imaging of the red and far-red Chl fluorescence but also of the blue and green fluorescence of leaves. The two instrumental setups composed for this purpose were (1) the Karlsruhe-Strasbourg UV-Laser Fluorescence Imaging System (Laser-FIS) and (2) the Karlsruhe Flash-Light Fluorescence Imaging System (FL-FIS). Essential results obtained with these instruments are summarized as well as the basic principles and characteristics of multi-colour fluorescence imaging. The great advantage of fluorescence imaging is that the fluorescence yield in the four fluorescence bands is sensed of several thousand up to 200,000 pixels per leaf area in one image. The multi-colour FIS technique allows to sense many physiological parameters and stress effects in plants at an early stage before a damage of leaves is visually detectable. Various examples of plant stress detection by the multi-colour FIS technique are given. Via imaging the Chl fluorescence ratio F690/F740 it is even possible to determine the Chl content of leaves. The FIS technique also allows to follow the successive uptake of diuron and loss of photosynthetic function and to screen the ripening of apples during storage. Particularly meaningful and of high statistical relevance are the fluorescence ratio images red/far-red (F690/F740), blue/red (F440/F690), and blue/green (F440/F520) as well as images of the fluorescence decrease ratio RFd, which is an indicator of the net CO2 assimilation rates of leaves.