Because siduron is absorbed by roots and transported by the transpiration stream, its phytotoxicity might be affected by environmental factors that influence transpiration. To test this, bermudagrass (Cynodon dactylon L. ‘Santa Ana’) and Kentucky blue‐grass (Poa pratensis L. ‘Merion’) were grown in controlled‐environment chambers at various temperatures and light intensities and treated with 1 and 5 ppm siduron. Untreated plants were also included. Bermudagrass was susceptible to 1 and 5 ppm siduron concentrations at all temperatures and light intensities. Shoot growth was reduced more than root growth. When air temperature was 30/20 C (day/night) the toxicity of the herbicides was greater than at a lower air temperature (22/15 C). Low light intensity decreased the toxicity of siduron to bermudagrass. Bluegrass was tolerant at all concentrations and all temperature and light treatments. Root temperature did not affect toxicity of the herbicide but maximum growth of bermudagrass was at 20 C and Kentucky blue‐grass at 15 C root temeprature. At 10 C, growth of both species was distinctly reduced. Differential rates of absorption are not the basis for selectivity.
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