The Effect of PS II Herbicides, Amitrol and SAN 6706 on the Activity of 3-Hydroxy-3-methylglutaryl-coenzyme-A-reductase and the Incorporation of [2-14C]Acetate and [2-3H]Mevalonate into Chloroplast Pigments of Radish Seedlings

The influence of photosystem II herbicides (DCMU, bentazone) and herbicides that block chloroplast-pigment formation (amitrol, SA N 6706) on the incorporation of [14C] acetate and [3H ]- mevalonate in chlorophylls and carotenoids was studied in radish seedlings (Raphanus sativus L .). 1. Though the pigment content in SAN 6706-treated plants is very low, they exhibit the highest specific incorporation rate as compared to controls and to the other herbicide-treated plants. This high metabolic activity coincides with a high specific activity of HMG-CoA-reductase in the plastidic and cytoplasmic fraction. 2. [2-3H] mevalonic acid was incorporated in chlorophylls and carotenoids of controls and all herbicide-treated plants. 3. [2-14C] acetate was only incorporated in β-carotene and lycopene, but not in α-carotene or xanthophylls. From this it is concluded that β-carotene is synthesized by two different biosynthetic pathways. 4. The incorporation of [14C ]acetate in DCMU- and bentazone-treated plants proceeded at a much lower rate (chlorophyll a) - or not at all (chlorophyll b) - than in controls of plants treated with amitrol or SAN 6706.