Hill reaction, inhibition

Figure 1. Optimizing both Transport and Hill Reaction Inhibition...

Mi sc. herbicides Hill reaction inhibition with mixture of bean leaves (phaseolus vulgaris) and redox indicator 2,6 dichlorophenol 0.01 mg L [94]... 

Gibbons et al. (j) reported on Quantitative Structure-Activity Relationships among selected Pyri-midinones and Hill Reaction Inhibition. The correlation equation (3) arrived at was based on 17 observa-... 

Moreland and Hill, in an earlier investigation (1963) of the photochemical activity of several polycyclic ureas, found that noruron has the strongest 50% Hill reaction inhibiting efiect, 4.0 10 mole/dm. The Hill reaction inhibition of diuron is 3.3 10 mole/dm, thus being somewhat higher than that of cyclic ureas. 

The 50% Hill reaction inhibiting concentration of fluoretoxuron on Chlorella algae is 1.4 10 mole/dm . In preemergence application it is a better weed-killer than linuron or fluometuron. 

The Hill reaction inhibiting effect of herbicides is given numerically as the molar concentration (1, ) or the logarithm of the reciprocal of the molar concentration (p/,o) causing 50% inhibition of the reaction. 

Moreland (1969) investigated the Hill reaction inhibiting effect of a few aryl and alicyclic dimethylurea herbicides (Table 6.8) and found a fairly good correlation of the p/jo values and the actual herbicidal activities. 

Moreland concluded with respect to the relationship between the structure and the action of Hill reaction inhibiting herbicides that they establish multipoint attachment with the active centre of chloroplasts by means of hydrogen bonds, thus... 

This degradation pathway makes it very likely that methazol is the precursor of the Hill reaction inhibiting l-(3,4-dichlorophenyl)-3-methylurea, which is the carrier of the herbicidal action. 

The Hill reaction inhibiting effect of herbicides of thiourea type is small compared to that of the C=0 analogues. Moreland (1969) presumes that a C==S-6C==0 conversion takes place within the plant, or that thioureas have a completely different mode of action. 

The Hill reaction inhibiting effect of triazine herbicides has been investigated by Exer (1958), while Gysin and Kniisli (1960) investigated this effect for a few triazine metabolites (Tables 13 and 14). The Hill reaction inhibiting values of several important triazines have been summarised by Moreland (1969) and Gabbot (1969). 

The Hill reaction can be monitored by measuring the oxygen produced with an oxygen electrode or a Warburg manometer, or by spectrophotometry. The ferricyanide - ferrocyanide reduction can readily be measured by the last method ferrocyanide acceptor and spectrophotometry are therefore used to measure the Hill reaction inhibiting effect of herbicides (Hill, 1937, 1940, 1965). 

There is an approximate but not direct correlation between the Hill reaction inhibiting effect and the actual herbicidal effect of herbicides inhibiting photosynthesis. 

Relevant investigations by Shimabukuro and Swanson (1969), Hansch (1969), Moreland (1969) and Buechel (1972), show that the mechanism of the Hill reaction inhibiting effect is still insufficiently known at the molecular level. This is due partly to the experimental techniques us, and partly to the varying sensitivities, which cannot be standardised, of the chloroplasts of the various plant species. The results obtained are highly dependent on the site of attachment of the inhibiting substance. 

The Hill reaction inhibiting effect of 2-halo-4,6-bis(alkylamino)-j-triazines and some known herbicides... 

The concentrations of the active herbicides of the triazine group giving 50% Hill reaction inhibition varied from 10 to 10" mole/dm. The discrepancy between the actual herbicidal effect and the Hill reaction inhibiting effect can be ascribed to the solubility, translocation ability and soil adsorption of the herbicide. 

Consideration of the Hill reaction inhibiting concentrations of urea, carbamate, acylanilide and triazine herbicides, the dependence of their efficiency on light intensity, and so on, lead to the conclusion that these herbicides exert their effect according to a similar mechanism of the molecular level. 

Hilton et al. (1969) found that the 50% Hill reaction inhibiting concentration of metflurazon is 4 mole/dm, a low value, but according to the investigations of Bartels and Hyde (1970) the primary cause of herbicidal action is in fact not the inhibition of photosynthesis, but the inhibition of carotenoid synthesis or of carotenoid accumulation (Bartels and Hyde, 1970). Practically, metflurazon is not toxic, its acute oral lDj being 9100 mg/kg for rats. 

The final detection method determines the sample preparation procedures. Highly selective detection methods need less thorough cleanup. That is the case when photosynthesis-inhibiting herbicides are detected on TLC plates by Hill reaction inhibition (3,4). If the final detection is less specific for the pesticides, a greater degree of sample preparation is required. 

Synergic in vitro effects of some mixtures can be determined by Hill reaction inhibition. This method is useful only when the components have quite different herbicidal activities and one of them is not a Hill reaction inhibitor. With the help of paper chromatography, a 40% synergic effect of an atrazine-defenuron (3 2) mixture was detected (166). 

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