The effect of dust on photosynthesis and its significance for roadside plants

Dust scraped from a car exhaust and ground to a particle size range of 1–10 μm was applied at measured rates to leaves of Viburnum tinus. Its effects on photosynthesis and leaf diffusion resistance were measured over a range of light intensities. Photosynthesis was reduced and this appeared to be due to shading when the upper surfaces of leaves were dusted, and to impeded diffusion when lower surfaces were dusted. These effects were observed with 5 to 10 g dust per m2 leaf surface. The maximum dust load found on the leaves of shrubs on the central reserves of motorways was about 2 g m−2. Unless this deposit included much material too fine to be observed by light microscopy (i.e. < ca 0·5 μm) its effects on photosynthesis, mediated either by shading or by interference with diffusion, are likely to be small.