Fluorescence Imaging of Water and Temperature Stress in Plant Leaves

Summary Fluorescence images of leaves from tobacco plants (green wild type and aurea mutant) were determinedin the blue (F440), green (F520), red (F690) and far-red region (F740), and also expressed as fluorescence ratio images. Under long-term water stress tobacco plants initially showed constant ratios of the blue to red fluorescence (F440/F690) and the blue to far-red fluorescence (F440/F740). Below a distinct threshold in water content (84° in green and 88° in aurea tobacco), however, a linear increase of the fluorescence ratios blue/red and blue/far-red was observed. This was due to a distinct increase in the bluegreen fluorescence emission, whereas the red and far-red chlorophyll fluorescence increased to a lower proportion. These changes in fluorescence ratios could easily be monitored by high resolution fluorescence imaging of whole leaves. For each point of the leaf, the fluorescence ratio can be read from the fluorescence ratio images of the leaves. In contrast, a short-term heat plus water stress in green tobacco plants was very fast detected via fluorescence imaging as a significant increase of red and far-red chlorophyll fluorescence emission (F690 and F740) on the leaf rim, whereas the central part of the leaf still exhibited the regular fluorescence signatures of photosynthetically active leaves. A combined outdoor stress (light, heat and water stress) at a dry sunny summer period was detected in Rhododendron by fluorescence imaging due to a much reduced red and far-red chlorophyll fluorescence. The latter was caused by UV-absorbing substances (e.g. flavonols) which accumulated primarily in the epidermis of these stressed leaves. These compounds seemed to act as UV-radiation filter, thus reducing the amount of the UV-excitation radiation, which could penetrate the mesophyll and which resulted in a reduced chlorophyll fluorescence excitation and emission. These results demonstrate that fluorescence imaging of leaves in the blue, green, red and far-red emission bands is an excellent tool for an early stress detection in plants, which is much superior to the hitherto applied spectral point data measurements.