Response of subterranean clover (Trifolium subterraneum) to ozone in relation to plant age and light conditions during exposure

Potted subterranean clover (Trifolium subterraneum) plants of different age s were exposed to 70 nl l(-1) ozone for 6 h, either during the light or dur ing the dark period in a laboratory-based climate chamber. There was limite d visible leaf injury on plants which were 14-20 and 28-34 d old and no sig nificant decrease in biomass after daytime ozone exposure. The oldest leave s of 22-26 d old plants exhibited severe visible injury, which was associat ed with a significant reduction in biomass in 24-26 d old plants. Thus, ozo ne-induced visible injury of different magnitude developed in all plants, b ut was associated with biomass reduction only during a limited period of th e plant's lifespan. Apart from modifying ozone uptake by plants, climatic c onditions are important as growth modifiers. It is suggested that subterran ean clover plants of defined developmental stages should be used in bioindi cation of ozone. Night ozone exposure injured significantly fewer leaves th an day exposure. However, some leaves developed visible injury even after n ight ozone exposure. Night uptake of ozone may be of more importance in nor thern than in central and southern Europe, because summer nights are short and, for a certain period, never completely dark.