OZONE-INDUCED ACCUMULATION OF CARBOHYDRATES CHANGES ENZYME-ACTIVITIESOF CARBOHYDRATE-METABOLISM IN BIRCH LEAVES

Birch cuttings (Betula pendula Roth) were grown in a sand-culture syst em with two concentrations (0.05, HF and 0.005%, LF) of fertilizer con taining macronutrients and micronutrients, and were exposed to 90/40 n l l(-1) O-3 (day/night) and < 3 nl l(-1) O-3 (control) for one growing season in the field fumigation chambers at Birmensdorf (Switzerland). Leaves of different ages were analysed for gas exchange, contents of chlorophyll, protein, and for metabolites as well as enzyme activities of carbohydrate metabolism. Ozone reduced net photosynthesis and chlo rophyll contents in mature leaves of both fertilization treatments, wh ereas that of protein was only reduced in high-fertilized plants (HF). However, net photosynthesis, chlorophyll, and protein increased in yo ung leaves of low-fertilized plants (LF). The effects of ozone on enzy me activities of carbohydrate metabolism were most pronounced in leave s of LF plants. Specific activities of the sucrose-cleaving enzymes, s ucrose synthase and alkaline invertase, were induced, whereas acid inv ertase was unchanged. Extractable activity of sucrose phosphate syntha se, which is a key enzyme of sucrose synthesis, was reduced. Levels of fructose 2,6-bisphosphate, an inhibitor of sucrose synthesis, were in creased in leaves of O-3/LF plants, but reduced in O-3/HF plants. In a ddition, activities of enzymes involved in starch metabolism, ADP-gluc ose pyrophosphorylase and starch phosphorylase, were lowered in ozone- treated samples and the ratio of ATP:ADP was increased. It is conclude d that chronic ozone exposure leads to an inhibition of sucrose synthe sis and favours sucrose degradation. This effect is modulated by the n utrient status of the plants, indicating higher O-3 tolerance in HF pl ants. Furthermore, as the metabolic responses in the ozone-treated sam ples resemble very closely those observed under end-product inhibition of photosynthesis, we assume that the O-3 effect is mainly due to red uced photosynthate export.