Light-induced susceptibility of Phaseolus vulgaris L. to tobacco mosaic virus infection II. Daily variation in susceptibility

Abstract

Pinto bean plants grown under a photoperiod of 8 hours of light and 16 hours of darkness, showed a daily variation in susceptibility to TMV. Fewer lesions formed in plants inoculated at the beginning of the light period (zero time) than in plants exposed to light for 6 minutes before they were inoculated. The number of lesions reached a maximum when plants received light of 4000 ft-c for 54 minutes before inoculation, or light of 640 ft-c for 105 minutes. With further exposure of plants to preinoculation light, the number of lesions decreased, and when plants received 6–8 hours of light before they were inoculated, lesion number did not differ significantly from that in plants inoculated at zero time. In plants inoculated during the 16 hours of darkness that followed the 8-hour photoperiod, the number of lesions remained essentially constant if the light intensity during the preceding light period was 4000 ft-c, but decreased if the light intensity during the preceding light period was 640 ft-c. In plants inoculated after 24 hours of continuous light at either 4000 ft-c or 640 ft-c, the number of lesions did not differ significantly from that in plants inoculated at zero time.