GAS-EXCHANGE CHARACTERISTICS IN LEAVES OF THE EUPHORBIACEA ALEURITES-MONTANA AS CONSEQUENCE OF GROWTH UNDER 700 PPM CO2 IN AIR - A STUDY ONPHOTOSYNTHESIS AND PHOTORESPIRATION IN THE CHINESE TUNG-OIL TREE

Three months old plants of the Chinese tung-oil tree Aleurites montana (Euphorbiaceae) were cultivated for 4 months in air containing 700 pp m CO2. These plants, which grow substantially better in the CO2-enrich ed atmosphere, were analyzed by mass spectrometry for photosynthesis a nd photorespiration together with control plants grown all the lime in normal (350 ppm CO2) air. Thereafter part of the plants was subjected for two weeks to 0.3 ppm SO2 in the atmosphere and again analyzed for photosynthesis and photorespiration. Aleurites montana exhibits a str ongly CO2-dependent photosynthesis which partially explains the observ ed stimulatory effect of 700 ppm CO2 on growth of the plant. In contro l plants grown in normal air, photorespiration measured simultaneously with photosynthesis via the uptake of O-18(2) in the light, is much l ower than in C-3-plants like tobacco (He et at, 1995, Z. Naturforsch. 50c, 781-788). In Aleurites grown in 700 ppm CO2, however, photorespir ation is completely absent in contrast to tobacco when grown under 700 ppm CO2. In tobacco, photorespiration is not inhibited to the extent of the in vitro experiments in which plants grown at 350 ppm CO2 are m easured under the increased CO2 content of 700 ppm. Gas exchange measu rements carried out by mass spectrometry show that the ratio of O-2 ev olved to CO2 fixed is about 0.5. Apparently, part of the CO2 fixed is channelled into a metabolic path without concomitant O-2-evolution. Al though the plant has no succulent appearance (its leaves somehow resem ble maple leaves) apparently a Crassulacean type metabolism is perform ed. When Aleurites plants grown all the time in normal air with 350 pp m, are exposed for two weeks to 0.3 ppm SO2 the treatment completely i nhibits this CO2-fixing portion which is tentatively attributed to a C rassulacean type of metabolism. This is demonstrated by a normal C-3-t ype ratio O-2 evolved/CO2 fixed of 1. When Aleurites plants, grown for 4 months in a CO2-enriched atmosphere of 700 ppm CO2, are subjected f or two weeks to 0.3 ppm SO2, the features of control plants show up ag ain. When these plants are tested under 350 ppm CO2 the Crassulacean t ype CO2-fixation apparently is not inhibited by SO2. Photorespiration, although low is present in the same activity as in the controls. Seem ingly, an increased level of CO2 in air tends to alleviate the impact of the SO2 at least in the Chinese lung-oil tree.