Distribution and Effects of Bentazon in Crop Plants and Weeds

Bentazon, C 0 2-Assimilation, Inhibition of Photosynthesis, Photosystem 2-Herbicide, Shade-Type Adaptation of Chloroplasts The inhibition of photosynthetic C 0 2-assimilation and of the variable chlorophyll fluorescence as well as uptake and transport of 14C-labelled bentazon and the possibilities for a herbicideinduced shade-type modification of the photosynthetic apparatus were investigated in bentazonsensitive weeds (Galium, Sinapis, Raphanus) and in the tolerant crop plants wheat and maize. 1. In weeds the depression of photosynthetic C 0 2-assimilation is irreversible, whereas tolerant plants recover due to the metabolization of the active herbicide. 2. A lower rate of uptake and transport of bentazon associated with its fast metabolization is the reason for the tolerance of crop plants towards bentazon. 3. The transport of [l4C]bentazon proceeds in the tracheary elements of the xylem. Uptake and transport of bentazon in the weeds are light dependent. 4. The loss of variable fluorescence (Kautsky effect) in the leaves after root application of bentazon proceeds much faster at high-light than at low light conditions and confirms the lightdependency of the bentazon transport. 5. In the sensitive dicot weeds bentazon not only inhibits photosynthetic electron flow and depresses C 0 2-fixation but also induces the formation of shade-type chloroplasts which are less efficient in photosynthetic quantum conversion. This bentazon-induced modification of the photosynthetic apparatus (e.g. changes in ultrastructure, pigment ratios, and levels of chlorophyll-proteins) contributes to the effectiveness of bentazon as a herbicide.