Chlorophyll Fluorescence Induction in Leaves of Phaseolus vulgaris Infected with Bean Rust (Uromyces appendiculatus).

To our knowledge, this report describes the first application of video imaging of Chl fluorescence to the study of light utilization in photosystem II of attached leaves of Phaseolus vulgaris infected with the obligate biotrophic fungus Uromyces appendiculatus (race 38). The video-based detection system produced a spatially resolved, quantifiable signal that was highly specific for chlorophyll fluorescence. Video images of spatial variation in the initial stage of the fluorescence induction (dark-light) transient revealed discreet regions of intense emission coinciding with centers of subsequent lesion development and accompanying chlorosis. Incipient lesions were visible by this procedure 3 d following inoculation, fully 3 to 4 d prior to visible symptoms. Fluorescence emission patterns in infected areas during the induction transient were heterogeneous with radial distance from the point of invasion and varied with the length of the time delay between re-illumination and image capture. During later ([greater than or equal to]1 min) stages of the induction transient, fluorescence emission in incipient lesions was quenched compared to surrounding tissue. These essential features of the induction transient observed in video images were also noted when individual lesions were examined using pulse modulation fluorimetry.