Hill reaction and

Members of an extensive group of sym-triazine herbicides, usually having one or two secondary amine substituents, block the Hill reaction and inhibit photosynthesis in a manner quite similar to that of the urea herbicides. The most widely used, 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), (10), is one of several hundred herbicidal analogs... 

In a study designed to determine the mode of action of atrazine in higher plants, Shimabukuro and Swanson (1969) concluded that atrazine inhibits the Hill reaction and its noncyclic phosphorylation, while being ineffective against cyclic photophosphorylation. Atrazine readily penetrated the chloroplast of resistant as well as susceptible plants. In tolerant plants such as sorghum, the metabolism of atrazine was postulated to occur outside the chloroplasts to form water-soluble and insoluble residues that reduced the concentration of photosynthetic inhibitors in the chloroplasts. 

Shimabukuro et al. (1973) identified 2-chloro-4,6-diamino-.v-triazinc (G-28273), which represented complete dealkylation of the triazine ring, as an organosoluble metabolite in sorghum. This metabolite did not inhibit the Hill reaction and cyclic and noncyclic photophosphorylation in isolated pea chloroplasts. 

Artificial electron acceptors, such as ferricyanide, can be substituted for NADP these give rise to oxygen evolution but involve only a short segment of the oxidation chain. This partial reaction is known as the Hill reaction and compounds that disrupt it are known as Hill inhibitors. Herbicides that inhibit the Hill reaction, by blocking electron transport, prevent the production of ATP and NADPH required for carbon dioxide fixation. 

Diuron [3-(3,4-dichlorophenyl)-l,l-dimethylurea] is a potent PS II inhibitor causing necrosis, whereas the related urea Fluometuron (Chart I) induces albinism in growing plants. Pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone] inhibits the Hill reaction and photosynthetic CO fixation. meta-Trifluoromethyl substitution on... 

C Eyster, TE Brown, HA Tanner and SL Hood (1958) Manganese requirement with respect to growth, Hill reaction and photosynthesis. Plant Physiol 33 235-241... 

Metflurazon inhibits photosynthesis and prevents the development of chloro-plastids in sensitive plants (Hilton et al., 1969). The authors also report on their investigation of the mode of action of 4 pyridazinone herbicides on barley. Metflurazon and its phenyl- and unsubstituted amino analogues, structurally similar to pyrazon, also inhibited the Hill reaction and photosynthesis, but showed two further biological features they resisted metabolic oxidation and inhibited chloroplast formation. The latter effect is similar to that of amitrol and dichlormate, but 100-1000 times stronger. 

Isocil and bromacil are soil-applied herbicides. Both block the Hill reaction and interfere with a step in the photosynthetic pathway close to oxygen evolution. This blocking may cause the accumulation of a phytotoxic product, possibly a reactive free radical. Though this particular antiphotosynthetic action is not in itself sufficient to explain the total phytotoxic action (Hoffmann, 1972), it is certain that the herbicidal action of substituted uracils is based on the inhibition of photosynthesis. 

Buthidazole and methazole possess heterocyclic rings containing a cyclized urea function, and, like the ureas, they act by inhibiting the Hill reaction in chloroplasts. It is believed that methazole is metabolized to a less lipophilic phenylurea that is the active compound, for methazole itself is a poor inhibitor of the Hill reaction and also is too lipophilic to be well transported to leaves from roots. ... 

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