Plant stress detection by reflectance and fluorescence

During their life cycle, plants are exposed to various kinds of stress constraints that are caused by natural and anthropogenic as well as biotic or abiotic stressors. An early stress diagnosis is an option in order to take immediate countermeasures to protect the crop plants and forest trees against damage. In recent years noninvasive, optical methods have gained much attention in stress detection in plants. These are passive reflectance measurements (sunlight reflectance) and active laser-induced fluorescence measurements. The progress made in both fields is summarized in this report, which also provides some basic information. The main emphasis in the field of reflectance signatures was put on improved vegetation indices. In the field of fluorescence signals, the emphasis was on the screening of red and far-red chlorophyll fluorescence signatures and the sensing of the plants' blue-green fluorescence. A technological innovation was the presentation of the laser-induced fluorescence (LIF) imaging technique, which includes blue-green fluorescence and sets a new standard in an early stress detection in plants. The fluorescence ratios blue/red (F440/F690) and blue/far-red (F440/F735) proved to be very sensitive indicators of ongoing stress events. This has been demonstrated by fluorescence emission spectra and the novel fluorescence imaging technique. High-light, water, and temperature stress as well as nitrogen deficiency, herbicide application, and attacks by mites and other predators can easily be monitored via increasing or decreasing fluorescence ratios and ratio images. In addition, for the first time a method of evaluation of the true chlorophyll fluorescence from joint reflectance and fluorescence measurements has been established that opens further possibilities for stress detection. Several of the presented optical reflectance and fluorescence techniques are also applicable for remote sensing of the state of health of terrestrial vegetation.