Comparison of Atrazine-Resistant and -Susceptible Biotypes ofSenecio vulgarisL.: Effects of High and Low Temperatures on thein vivoPhotosynthetic Electron Transfer in Intact Leaves

The effects of temperature on the yield of in vivo modulated chlorophyll fluorescence were measured in intact leaves of atrazineresistant and -susceptible biotypes of the weed Senecio vulgaris L. At 25 °C, the photochemical quenching (q_Q of steady-state chlorophyll fluorescence was reduced by around 30% in the atrazine-resistant mutant as compared to the susceptible wild type, indicating a higher reduction state of the primary electron acceptor Q_A of photosystem II in the former biotype. Moderately elevated temperatures (above 30 °C) further increased the steady-state concentration of reduced $QA-$ in the mutant. Analysis of the temperature dependence of both the photochemical quenching q_Q and the initial fluorescence level F_o clearly indicated a drastically enhanced heat-sensitivity of the photochemical apparatus in the atrazine-resistant Senecio leaves. The heat-induced changes in F₀ and q_Q were closely correlated, suggesting that the phenomenon responsible for the rise in F₀ was also involved in the inhibition of the photosynthetic electron flux. Low temperatures also affected $QA-$ reoxidation but, in contrast to heating, no apparent differences were observed in the behaviour of the two biotypes exposed to chilling stress.