Characteristics of Chlorophyll a Fluorescence Induction in Cucumber Cotyledons Treated with Diuron, Norflurazon, and Sulcotrionem

Chlorophyll a fluorescence analysis was carried out to investigate the potential relationship between photosynthetic electron transport and herbicidal action mechanism of sulcotrione, norflurazon, diuron, and two experimental herbicides in cucumber cotyledons. The diuron treatment decreased quantum yield of electron transport and photochemical fluorescence quenching (qP) immediately but increased the steady-state fluorescence yield (Ft) greatly in the light, with a decreased magnitude upon dark adaptation. The profile of changes of qP and Ft in sulcotrione-treated tissues was similar to that of diuron-treated tissues; however, a decline in quantum yield was faster in diuron-treated tissues. Norflurazon-treated tissues also decreased quantum yield as in sulcotrione-treated tissues, but had a slight increase in Ft and a prominent increase in qN in contrast to those of diuron and sulcotrione. Experimental herbicides EK 3651, sulcotrione derivative, and KC 6361, phytoene desaturase inhibitor, also exhibited a similar profile of chlorophyll a fluorescence to that of sulcotrione and norflurazon, respectively. This demonstrates that chlorophyll a fluorescence induction profiles of the chemicals are related to their respective herbicidal activities and can be utilized as a simple way of differentiating the target sites of herbicides in vivo. Based on a correlation between quantum yield inhibition and injury, sulcotrione appears to be closer to diuron than norflurazon. In the combined treatment of herbicides, sulcotrione and diuron had synergistic effect to each other. Our results suggest that sulcotrione inhibits photosynthetic electron transport, in mature cotyledons, through a decrease in plastoquinone pool, which is essential for phytoene desaturase and involved in photosynthetic electron transport as a mediator.