Impact of compatible and incompatible barley-Blumeria graminis f.sp hordei interactions on chlorophyll fluorescence parameters

Interactions between different barley genotypes and the fungal pathogen Blumeria graminis f.sp. hordei (Bgh) have a specific impact on the crop physiology. Within the context of plant resistance phenotyping, it is relevant to investigate early host-pathogen interactions to avoid the crop infestation. Analyzing different parameters of the photosynthesis apparatus gives in-depth information of the plant's health status and can be used for a spatial and temporal assessment of interaction types during plant-pathogen infestation. In the present study, experiments were performed with a near-isogenic line of barley cv. Ingrid WT (susceptible), mlo3 (papilla-based resistance) as well as a near-isogenic line of cv. Pallas, containing the Mla1 (hypersensitive response-based resistance) gene. After inoculation with Bgh isolate K1, the leaves were measured daily using chlorophyll fluorescence imaging. Inoculated, susceptible wild-type leaves showed a reduced effective quantum yield of the photosystem II (Phi PSII) already 1 day after inoculation. In accordance with the quantum yield reduction, the non-photochemical quenching (NPQ) increased, indicating thermal dissipation of excess energy. The changes of UPSII and NPQ represent modifications of the leaf metabolism to aid the fungal nutrition uptake, which is influenced by Bgh. By analyzing these parameters, it was also possible to indicate resistance reactions of mlo3 and Mla1 barley genotypes against Bgh. During papilla formation in mlo3 leaves, UPSII revealed the lowest values. In contrast, inoculated Mla1 leaves showed the lowest NPQ. The present study proofs that chlorophyll fluorescence imaging is a valuable tool for investigating early plant-pathogen interaction noninvasively. Furthermore, this phytopathology study uses chlorophyll fluorescence imaging, chlorophyll extraction and microscopic observations to characterize the interaction response of different genotypes to an Bgh infection.