The overall role of precision agriculture is not restricted to those systems for in-field and in-season sensing of the impact of stresses. Much more, its contribution comprises the prevention of stresses, amongst others by supporting the selection of appropriate and stress-tolerant genotypes in breeding programs. In this context, the development, selection and use of cultivars which are tolerant to pathogens establish an essential tool for a more sustainable and environmental-friendly agriculture. In the last years the fluorescence spectroscopy has been established as important approach in many fields of basic and applied research in plant sciences. However, the quality and usefulness of the information produced by different techniques and equipments might vary considerably.
In our work we aimed to exploit the relation between the susceptibility degree of four barley cultivars (Belana, Marthe, Conchita, Tocada) to foliar diseases, and the pathogen-induced alteration of the characteristic fluorescence signature. For this purpose we recorded the fluorescence lifetime with a time-resolved fluoroscope (IOM Lambda 401, Berlin, Germany), the image-resolved multispectral fluorescence (Nuance CRI, Perkin-Elmer, USA) and the fluorescence intensity in the blue, red, and far-red spectral bands with a portable hand-held multiparametric fluorescence sensor (Multiplex, Force-A, France). The susceptibility of the selected cultivars, categorized in 2010 by the German Federal Plant Variety Office (1, low susceptibility; 9, high susceptibility) ranged from 2 to 7 for powdery mildew and 4-5 for leaf rust. The experimental plants were inoculated with either powdery mildew or leaf rust, and fluorescence recordings were taken at leaf level at three, six and nine days after inoculation. Non-inoculated leaves served as control.
The fluorescence-based sensing of leaf rust and powdery mildew could easily be accomplished with several fluorescence indices already at 3 days after inoculation. As expected, the impact of the pathogens on the fluorescence signature became stronger following the disease severity in the time course of the experiments. Moreover, we observed genotype-specific differences due to the occurrence of diseases. In particular, we detected higher mean fluorescence lifetimes in the spectral range from 410 to 560 nm (blue and green fluorescence) in the less susceptible cultivars already at three days after inoculation. Using the spectrally-resolved fluorescence pictures, we could exploit the pathogen-plant interaction even better. With this technique, and using the green fluorescence as indicative parameter, we quantified the increase of the leaf area which was affected by the diseases in the time-course of the experiment. Also, using this signal it was possible to distinguish between infections caused by powdery mildew and leaf rust. Alterations of the fluorescence were also confirmed by the more robust hand-held sensor. Here, noticeable pathogen-induced modifications were revealed by the indices ‘Blue-to-Far-red fluorescence ratio’ (BFRR_UV) and the ‘Simple Fluorescence Ratio’ (SFR). Particularly in susceptible cultivars, differences became more evident in the time-course of the pathogen development, indicating that these modifications might be used as additional information to characterize the susceptibility degree of genotypes to diseases. In the sum our results demonstrate the potential of specific fluorescence indices as diagnostic tool in plant breeding, selection, and precision farming.
