Temperature Dependence of Photosynthetic Activities in Wheat Seedlings Grown in the Presence of BASF 13.338 (4-Chloro-5-Dimethylamino-2-Phenyl-3(2H)Pyridazinone)

When Triticum vulgare cv HD 2189 seedlings were grown in the presence of 125 micromolar BASF 13.338 (4-chloro-5-dimethylamino-2-phenyl-3(2H)pyridazinone), the rate of electron transport (H₂O → methyl viologen) in chloroplast thylakoids isolated from the treated seedlings was higher (by 50%) as compared to the control at assay temperatures above 30°C. Below 30°C, however, the rate with the treated seedlings was lower than the control rate. The temperature dependence of the rate of photosystem I electron transport (2-6-dichlorophenol indophenol-reduced → methyl viologen) in the treated system was similar to that in the control. At high temperatures (>30°C), with diphenyl carabazide as electron donor, the rates of electron transfer (diphenyl carbazide → methyl viologen) were similar in the treated and in the control thylakoids. Direct addition of BASF 13.338 to the assay mixture for the measurement of rate of electron transport (H₂O → methyl viologen) in the thylakoids isolated from the control plants did not cause any change in the temperature dependence of photosynthetic electron transport. These results suggested that the donor side of photosystem II became tolerant to heat in the treated plants. Chlorophyll a fluorescence emission was monitored continuously in the leaves of control and BASF 13.338 treated wheat seedlings during continuous increase in temperature (1°C per minute). The fluorescence-temperature profile showed a decrease in the fluorescence yield above 55°C; this decrease was biphasic in the control and monophasic in the treated plants.