NIGHTTIME EXPOSURE TO OZONE REDUCES WHOLE-PLANT PRODUCTION IN BETULA-PENDULA

For 20 weeks during the growing season, cuttings of one birch clone (B etula pendula Roth.) were exposed in the Birmensdorf fumigation chambe rs to O-3-free air (control) or 75 nl O-3 l(-1). Ozone was supplied ei ther from 1900 until 0700 h (nighttime regime), from 0700 until 1900 h (daylight regime), or all day (24-h regime). By autumn, reductions in whole-plant biomass production, root/shoot biomass and stem weight/le ngth ratios were evident in all three O-3 regimes. The reductions in c uttings receiving the 24-h O-3 treatment were about twofold larger tha n in cuttings receiving the daylight O-3 treatment. Stomata were open at night, and stomatal conductance was about 50% of its maximum daytim e value. We calculated that the rate of O-3 uptake into leaves in the dark approached 4 nmol m(-2) s(-1). Whole-plant production and carbon allocation were more sensitive to O-3 during the night than during the day; however, O-3 exposure caused similar visible leaf injury in both of the 12-h regimes, although the leaves exposed to O-3 at night exhi bited delayed O-3-induced shedding. Overall, changes in production and carbon allocation were determined by the external O-3 dose rather tha n by the kind of O-3 exposure, indicating that, at the seasonal scale, the internal dose of ozone that was physiologically effective was a c onstant fraction of the external O-3 dose. We conclude that nighttime O-3 exposures should be included in the daily time period for determin ing critical concentrations of O-3 causing injury in trees.