Photosynthesis herbicides interfering with

Research had confirmed that no parent simazine residues were found in treated com plants, and additional data on the dissipation pathway of simazine needed to be developed. Research also indicated that triazines interfered with the photosynthetic process on susceptible growing weeds, as evidenced by the appearance of chlorotic leaves. Steps were undertaken to elucidate simazine s dissipation pathway and herbicidal mode of action. In Basel, Dr. Gast (1958) showed that the accumulation of starch by common coleus (Coleus blumei Benth.) plants was inhibited from treatment with 2-chloro-4,6-bis-(alkyl-amino)-triazines due to the inhibition of sugar synthesis. At the same time, Moreland et al. (1958) found weed control activity could be reduced by supplying carbohydrates to the plants through their leaves and that simazine was a strong inhibitor of the Hill reaction in photosynthesis. Exer (1958) found that triazines inhibited the Hill reaction as strongly as urea of the CMU (monuron) type. 

This group of herbicides has many diverse modes of action. The mode of action of most of the members of this group is by interfering with photosynthesis, or by interfering with cell division in meristematic tissues (growing termini). This later action is effected by impairing nucleic acid metabolism and protein synthesis. 

Triazines act by interfering with photosynthesis and it seems clear that, like the substituted urea herbicides, the primary site of action is inhibition of the Hill reaction of photosynthetic electron transport. 

The triazines, such as simazine (4.62), are the most used of all soil-applied herbicides small changes in the substituents give excellent control over selectivity and persistence. These substances [and the phenylureas, such as diuron (4.63), which are also soil-types] act by interfering with the Hill reaction in photosynthesis (see Section 4.6). Both of these parent substances, simazine and diuron, have undergone extensive modification for particular purposes, but each is still extensively used. A newcomer, chlorsulfuron (6.73), combines molecular features from both parents, but is notable for the greater dilution at which it acts, and also for the speed (all cell division is halted within 1 hour). This substance, l-(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-l, 3,5-tri-azin-2-yl)urea, is taken up by wheat, oats, and barley plants which rapidly inactivate it. It is harmless to Man (Campion and Tichon, 1981). 

In the 1960s various heterocycles, anilides, and phenyl compounds substituted with urea or carbamate were known to exhibit herbicidal activity by interfering with photosynthesis in plant cells. Draber and co-workers at Bayer AG (Leverkusen, Federal Republic of Germany) investigated a new series of... 

Inhibitors not primarily involving photosynthesis.—Of the herbicides which do not inhibit photosynthesis, the phenoxycarboxylic herbicides have received the most attention with respect to their influence on the soil algae. These herbicides act on higher plants through interfering with nucleic acid metabolism and disrupting translocation systems. 

---