Physiological and biochemical action of carbamates

The carbamate herbicides discussed in the foregoing diifer considerably from one another, with respect to both the skeleton of the molecule and the substituent groups. 

On the basis of these differences it is reasonable to expect differences in physiological and biological action. Indeed, the members of the group show a wide variability in the mode of uptake (root, stem, leaf), the measure and rate of absorption and translocation, and their distribution and metabolism within the plant. All of these differences indicate that in spite of their chemical relationship, the biological mode of action of the individual active substances is different, as well as the detoxication mechanism of the herbicides in the plants of different sensitivity, the latter phenomenon being responsible for their selectivity. 

Almost all of the carbamate herbicides inhibit photosynthesis, as has been shown by the investigations of Moreland and Hill (1959). Asulam and terbutol do not inhibit photosynthetic electron transport in vitro, while the other carbamates do only in high concentrations not occurring in vivo. The conclusion of Corbett (1974), that the inhibition of photosynthesis is only a side-effect of these compounds, therefore seems justified. 

The general and primary action of carbamate herbicides is their inhibiting effect on mitosis, as has been shown by several workers in hundreds of experiments. Of the biochemical reactions of plant cells, carbamate herbicides inhibit oxidative phosphorylation and the synthesis of ATP, RNA and protein. As a result of these processes abnormal cells with anomalous nucleotides are formed in sensitive plants, which finally leads to the destruction of the plant. 

On the basis of numerous experiments, the selective action of carbamate herbicides can be attributed primarily to the fact that compounds taken up are rapidly metabolised by tolerant plants into harmless derivatives, while in sensitive plants their metabolism is slower, so that the plants are permanently injured by the unmetabolised herbicides. 

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