Differential Gas Exchange Responses of Two Biotypes of Redroot Pigweed to Atrazine

Differences were shown to exist in photosynthetic rate, transpiration rate, and carbon dioxide leaf diffusive resistance between atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)- s -triazine] susceptible (S) and resistant (R) plants of redroot pigweed ( Amaranthus retroflexus L.). Chlorbromuron [3-(4-bromo-3-chlorophenyl)-1-methoxy-1-methylurea] and diruon [3-(3,4-dichlorophenyl)-1,1-dimethylurea] were the only herbicides tested that controlled both biotypes, but all of the herbicides except norea [3-(hexahydro-4,7-methanoindan-5-yl)-1,1-dimethylurea] controlled the S biotype. Although photosynthetic activity and transpiration were reduced in both biotypes by atrazine at 50 and 70 ppm, the decline was much greater in the S biotype than in the R biotype and persisted a longer time in the S biotype. Leaf CO 2 diffusive resistances of the biotypes were increased by atrazine applications. Mesophyll resistance was increased to a greater extent than stomatal resistance suggesting that reduction of photosynthesis is due to a greater effect of atrazine on the mesophyll tissue than on the guard cells.