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p The mode of action of some herbicides viz DCMU simetone and diquat was investigated by studying their effects upon oxygen evolution and cyclic photophosphorylation in the unicellular green alga em Scenedesmus em spec p Oxygen evolution was measured with the aid of the WARBURG technique the suspension medium mostly was WARBURG buffer no 9 the gas phase was air in general the temperature 25 C p Cyclic photophosphorylation was determined by measuring inorganic phosphate uptake by the algae during a 90 minutes illumination period under a nitrogen atmosphere p In Chapter III it is shown that phosphate uptake under the conditions described is saturated at much lower light intensity than oxygen evolution fig 4 In far-red light preferentially absorbed by PS I there is almost no oxygen evolution table 2 while the fixation of phosphate proceeds equally well as in white light fig 5 This demonstrates that the latter process represents cyclic photophosphorylation em in vivo em p In Chapter IV it is shown that DCMU at light saturation and at 5 mm sup 3 sup cells ml inhibits oxygen evolution for 50 at a concentration of 2 x 10 sup -7 sup M fig 6 This concentration has no effect on dark O sub 2 sub -uptake fig 11 The degree of inhibition of oxygen evolution by DCMU decreases with increasing suspension density section IV 1 2 Moreover the degree of inhibition depends on light intensity section IV 1 4 but not on temperature section IV 1 5 By washing the cells the inhibiting effect of DCMU can be removed section IV 1 3 DCMU has no effect on the photosynthetic quotient table 4 p Phosphate fixation in white light is much less sensitive to DCMU than oxygen evolution in far-red light phosphate fixation is more strongly inhibited than in white light but less than oxygen evolution fig 36 p In Chapter V the effect of simetone is studied Simetone has effects qualitatively similar to those of DCMU on oxygen evolution and phosphate fixation It differs from DCMU only in being less effective not accumulated by the algae and washed out more easily p In Chapter VI experiments on the influence of diquat on oxygen evolution and phosphate fixation are presented The inhibition of O sub 2 sub -evolution by diquat increases with time treatment in light for one hour with 2 x 10 sup -5 sup M gives an inhibition of about 50 fig 27 The degree of inhibition decreases with increasing suspension density fig 29 which points to an accumulation of diquat by the cells p The inhibiting effect cannot be washed out but when diquat is added in the absence of oxygen or in the presence of 10 sup -5 sup M simetone the inhibition after washing is decreased section VI 1 4 It is concluded that diquat can be removed to a large extent by washing and that oxygen is required to bring about the inhibiting effect p Diquat initially stimulates dark O sub 2 sub -uptake which subsequently changes into inhibition section VI 1 2 The degree of inhibition of O sub 2 sub -evolution increases with light intensity Inhibition was observed both in the light-limited and in the light-saturated part of the photosynthesis-light curve The degree of inhibition increases with temperature section VI 1 5 p During the first 45 minutes after addition of diquat with air as a gas phase the photosynthetic quotient is increased table 10 This shows that during this time diquat acts as a HILL-oxidant for O sub 2 sub -evolution while CO sub 2 sub -uptake is decreased by lack of reduced NADP p Diquat was found to inhibit phosphate fixation to about 50 in the complete absence of oxygen fig 35 p The results are consistent with the hypothesis that diquat is reduced to a free radical in the photosynthetic process and also to a smaller extent in respiration The reaction of this free radical with water and oxygen leads to the formation of toxic peroxide radicals or hydrogen peroxide fig 40 These peroxides are assumed to disrupt cellular organization structure and function p In Chapter VII the mode of action of the various herbicides studied and the implications of the results for the mechanism of photosynthesis are discussed The results obtained give rise to the following interpretation of the mode of action of DCMU and simetone both herbicides affect the oxidized state of a substance X which may represent or is very close to the primary electron acceptor of PS II Moreover X takes part in the cyclic electron transport chain fig 40 X might be plastoquinone the binding with the herbicides probably occurs via hydrogen bonds figs 38 39 p With respect to the mechanism of photosynthesis it is concluded that cyclic photophosphorylation occurs also em in vivo em The cyclic electron transport chain probably contacts the non-cyclic one at the level of plastoquinone Evidence suggests that there are two phosphorylation sites in the cyclic electron transport chain one between cytochrome 559 and cytochrome 553 and another one in the chain of PS I between Z and plastoquinone For more details regarding these considerations cf section VII 2 3

Een snelle chromatografische bepalingsmethode van aminozuren met Animex harsen als inonenwisselaars

Bio-assay of soil samples for content of photosynthesis inhibiting herbicides

Verslag van een bezoek aan de "Jahreshauptversammlung des Verbandes Deutscher Landwirtschaftlicher Untersuchungs - und F...

Action of some herbicides in photosynthesis of Scenedesmus as studied by their effects on oxygen evolution and cyclic ph...

Een onderzoek naar de zetmeelwaardebehoefte van drachtige ooien

Radio - ecologie

Bestrijding van kweekgras in bieten op klei- en zandgrond : verslag over 1968 t/m 1970

Influence of herbicides on photosynthetic activity and transpiration rate of intact plants

The determination of sugars in potatoes as studied in a ring - analysis

Verslag van een dienstreis door Australie en Nieuw Zeeland van begin maart tot midden mei

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