Effects of dimethipin, a defoliant and desiccant, on stomatal behavior and protein synthesis
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The first visible macroscopic effect of a foliar spray of dimethipin (2,3-dihydro-5,6-dimethyl-1,4-dithiin 1,1,4,4-tetraoxide) on kidney beans (Phaseolus vulgaris L. cv. Back Valentine) was a severe loss of leaf turgor followed by desiccation and, ultimately, abscission. Dimethipin-treated leaves had higher rates of transpiration than control leaves when the leaves received treatments that cause stomatal closure (e.g., darkness, water stress, or exogenous abscisic acid). The higher rates of water loss from the dimethipin-treated leaves were not due to a massive nonspecific disruption of leaf cells, since dimethipin-treated leaves maintained turgor for 24 h if the plants were placed in a chamber of 100% relative humidity. These results indicate that the dimethipin-induced loss of leaf turgor is due, at least in part, to a loss in stomatal control.
The earliest detectable biochemical effect of dimethipin was an inhibition of the incorporation of14C-leucine into protein. In both kidney bean leaf discs and oat (Avena sativa L. cv. Garry) coleoptiles, greater than 50% inhibition of14C-leucine incorporation into protein was observed 1 h after the start of incubation in 1 mM dimethipin. Dimethipin had a substantially smaller effect, however, on the incorporation of3H-uridine into RNA, suggesting that dimethipin acts primarily on the processes associated with translation rather than transcription. Cycloheximide also caused a loss of stomatal control, and both dimethipin and cycloheximide retarded the degradation of chlorophyll in senescing oat leaf segments in the dark, indicating similar mechanisms of action for the two compounds. In summary, the evidence suggests that an initial inhibition of protein synthesis is responsible for the loss of stomatal control associated with high rates of transpiration and loss of leaf turgor. The possible role for dimethipin-induced loss of turgor in abscission is discussed.
KeywordsGuard Cell Cycloheximide Stomatal Closure Kidney Bean Cordycepin
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