The effect of agronomic photosystem-II herbicides on lichens

The sensitivity of Hypogymnia physodes, Lobaria pulmonaria and Peltigera ap hthosa to six photosystem II herbicides and to DBMIB was tested in the labo ratory by chlorophyll fluorescence and oxygen-exchange measurements. In add ition, experiments with freshly isolated photobiont cells from H. physodes and L. pulmonaria were performed. Generally, the lichens were most sensitiv e to the urea herbicides diuron and isoproturon, whereas the triazines atra zine, terbuthylazine, and simazine and the triazinone metamitron were less inhibitory. Among the three lichen species investigated, H. physodes was th e most sensitive to the urea herbicides. For the other agents, no significa nt differences between lichen species could be found. The highest pI(50) va lues obtained from dose response curves were around 6.5 for isolated photob ionts, but most values for lichen thalli were in the range 5-6. Thus, there is no particular sensitivity of green algal lichen photobionts to photosys tem II herbicides as compared to other algae, higher plant chloroplasts or protoplasts. In nature, we observed recovery from (damaging) treatment with 10(-5) mol diuron 1(-1) for H. physodes within weeks. Therefore, damage to lichens from the use of photosystem-II herbicides in agriculture is probab ly only of very local occurrence. (C) 1999 The British Lichen Society.