CO2-FIXATION AND ASSIMILATE DISTRIBUTION IN SPRUCE UNDER LONG-TERM FUMIGATION WITH OZONE

Four-year-old Picea abies supplied with proper nutrients including Mg++ were exposed during daylight in special glasshouse chambers to 100 or 200 ppb O3 for 6 weeks. (CO2)-C-14 was supplied to current-year needles of lower twigs, and segments of source and various sink tissues were analyzed after chase-periods of 2 to 30 days. In May, only slight reduction of net CO2-fixation was observed whereas in October 40 % depression of net photosynthesis by ozone was monitored, probably partially due to increased respiration. No alteration in chlorophyll content was found. Under 200 ppb O3, significantly less C-14-transport was observed in August and October in both absolute and relative amounts. Extract fractionation of the C-14-fed (source) region and of sink tissues revealed no principal differences (as compared with the control) for the lipophilic fraction, the cationic and anionic fraction. But decrease of soluble sugars and of starch in the root and the secondary phloem of the ozone-treated plants are as noteworthy as the corresponding data of the current-year needles: under O3-exposure, increase of both sucrose and starch is conspicuous in May. It is concluded that O3 affects assimilate allocation. Since net photosynthesis was reduced only later in the season, the observed changes in partitioning cannot be due solely to this depression: assimilate transfer within the mesophyll towards the phloem might be primarily affected, whereas CO2-fixation consecutively responds to the buildup of starch and sucrose in the needle mesophyll.