Herbicide Resistant Weeds

Inhibition of photosynthesis in intact plants of biotypes resistant or susceptible to atrazine and cross-resistant to other herbicides.

Oorschot, J. L. P. van, P. H. Leeuwen van, J. L. P. Van Oorschot, and P. H. Van Leeuwen. 1988. Inhibition of photosynthesis in intact plants of biotypes resistant or susceptible to atrazine and cross-resistant to other herbicides. Weed Research. 28: 223-230.

Atrazine at a concn of 0.1 mM/litre in the nutrient solution resulted in complete inhibition of photosynthesis in intact leaves of susceptible biotypes of Amaranthus retroflexus, Polygonum lapathifolium, Chenopodium album, Solanum nigrum, Poa annua, and Stellaria media within a few hours of treatment, whereas the inhibition of the resistant biotypes by the same concn varied from little to moderate. In contrast, diuron (10 or 20 æM/litre) produced only minor differences between resistant and susceptible biotypes. The influence of some other herbicides on photosynthesis of these resistant biotypes was also smaller than that on the susceptible biotypes. This cross-resistance was evaluated with the resistance factor for intact leaves of Brassica napus, A. retroflexus, and S. nigrum. This factor is equal to the ratio of the herbicide concn in leaves of the resistant biotype to that in leaves of the susceptible biotype with inhibition to half-max. rate of photosynthesis. This concn in the leaves was calculated from the concn in the nutrient solution, and the total transpiration divided by leaf area from the beginning of the herbicide treatment until the moment of half-max. of photosynthesis. The resistance factors for intact leaves of A. retroflexus, S. nigrum and B. napus were 26-30 for atraton, 3-7 for metamitron, 2-9 for bromacil, 3-5 for monolinuron, 1 for diuron and < 1 for bentazone. For isolated chloroplasts much hihger values have been reported. The reason for this discrepancy is not clear. A somewhat higher resistance factor (around 50-60) was derived after infiltration of detached leaves of these species with atraton solutions.