Circadian oscillations in chlorophyll fluorescence in a triazine-resistant chronomutant of Brassica napus.

The objectives of this study were to determine if there was variation in atrazine tolerance, measured by intensity of leaf-disc chlorophyll fluorescence, in triazine-resistant and -susceptible Brassica napus in terms of age of leaf on the plant, time of day, and time of assay after leaf disc removal and immersion in atrazine. In a growth room and field experiment, triazine-susceptible B. napus cv. "Tower" and triazine-resistant B. napus cv. "OAC Triton" were used. Chlorophyll fluorescence intensity measurements were made 15-30 min after disc removal. In both environments, two periods of reduced photosynthetic efficiency occurred in the circadian phase. The times that these periods occurred during the diurnal phase differed between biotypes. A phase shift in leaf chlorophyll fluorescence (LCF) maxima between resistant and susceptible biotypes resulted in two periods, early and late in the light phase, of increased LCF in resistant tissue. This differential pattern in LCF is support for the hypothesis that triazine-resistance chloroplast alterations imply an alteration in the temporal organization of chloroplast physiological function. Atrazine reduced the photosynthetic efficiency of resistant tissue in some instances. This could indicate that triazine resistance is not complete and that some metabolic role may be involved. The results reported here indicate that the most accurate estimation of the physiological state will be obtained from LCF measurements taken soon after leaf disc removal from the plant.