Herbicide with photosynthetic processes

Table III. Influence of herbicides that interfere with photosynthetic processes on growth and multiplication of Chlamydomonas reinhardii. The algae were kept in a synchronized 12/12 h light/dark cycle, with Oh at the beginning of a dark phase.

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. 

The first generation of urea herbicides is represented by types such as monuron and linuron (see Table 29.12). These compounds in general interfere with the photosynthetic process, as they inhibit electron transport in the photosystem II process of the plant. They have varying degrees of selectivity, depending upon the structure. Some owe their selectivity to physical properties such as water solubility and soil absorption. If the crop is a deep-rooted perennial, the shallow rooted annual weeds are controlled by using a herbicide that is not easily leached down into the root zone... 

Distinct differences in cells with regard to the presence or absence of target structures or metabolic processes also offer opportunities for selectivity. Herbicides such as phenylureas, simazine, and so on, block the Hill reaction in chloroplasts, thereby killing plants without harm to animals. This is not always the case because paraquat, which blocks photosynthetic reactions in plants, is a pulmonary toxicant in mammals, due apparently to analogous free-radical reactions (see Figure 18.4) involving enzymes different from those involved in photosynthesis. 

James Franck wrote in 1949 U) ... it is one of the miracles of photosynthesis that the plant can use a dye able to fluoresce in the presence of oxygen, predominantly for the purpose of reduction, and is able to hold the process of photooxidation in check so that damage is prevented or minimized even under severe conditions . It is evident that the chloroplast is endowed with protective devices that are able to limit damage except under extreme conditions. Such situations are promoted by the presence of photosynthetic inhibitor herbicides. 

Intensive use of the herbicide paraquat has resulted in the evolution of resistance in various weed species. Intensive research on the resistance mechanisms was mainly carried out with resistant biotypes from Hordeum spp. and Conyza spp., and altered distribution of the herbicide in the resistant weeds was suggested as the cause - or at least the partial cause - of resistance. In resistant Conyza canadensis it was supposed that a paraquat inducible protein may function by carrying paraquat to a metabolically inactive compartment, either the cell wall or the vacuole. This sequestration process would prevent the herbicide from getting in sufficient amounts into the chloroplasts as the cellular site of paraquat action. Inhibitors of membrane transport systems, e.g., N,N-dicyclohexylcarbodii-mide (DCCD), caused a delay in the recovery of photosynthetic functions of the paraquat-resistant biotype, when given after the herbicide. These transport inhibitor experiments supported the involvement of a membrane transporter in paraquat resistance [75]. 

The use of herbicides that inhibit or interact with the photosynthetic machinery is seen, superficially, to be advantageous because of the likelihood of fewer problems of animal toxicity. However, the identical nature of the process in both crop and weed species, apart from C4 plants, means that herbicide selectivity must be achieved by differential uptake, movement, or metabolism. Following the discovery of the herbicidal action of N (4-chlorophenyl)-N,N-dimethylurea (subsequently known as CMU or monuron) by Bucha and Todd in 1951, Wessels and Van der Veen and Cooke showed that this compound was a potent inhibitor of photosynthetic electron transport. This, the first so-called Hill reaction inhibitor, was followed subsequently by numerous other phenylureas, triazines, uracils,... 

---