NUTRITION AND THE OZONE SENSITIVITY OF BIRCH (BETULA-PENDULA) .1. RESPONSES AT THE LEAF LEVEL

Cuttings of a single birch clone (Betula pendula) were grown in field fumigation chambers throughout the growing season in either filtered a ir (control) or 90/40 nl O-3 l(-1) (day/night). Both regimes were spli t into plants under high and low nutrient supply (macro-and micronutri ents). The stomatal density of leaves was increased by ozone but was l owered at high nutrition, while the inner air space was hardly affecte d by the treatments. Ozone induced macroscopic leaf injury regardless of nutrition, but leaf shedding was delayed in the low-fertilized plan ts, despite O-3 uptake being similar to that in high-fertilized plants . The :leaf turnover was enhanced in the O-3-exposed high-fertilized p lants, but length growth and leaf formation of stems were not affected by ozone in either nutrient regime. Leaves of high-fertilized plants showed Os-caused decline in photosynthetic capacity, water-use efficie ncy, apparent carbon uptake efficiency and quantum yield earlier as co mpared with low-fertilized plants, whereas chlorophyll fluorescence (F -V/F-M) and leaf nitrogen concentration were rather stable. CO2 uptake rate and rubisco activity of young leaves compensated for the O-3 inj ury in the ageing leaves of the low-fertilized plants. In 8-week-old l eaves, however, the O-3-induced decline in CO2 uptake did not differ b etween the nutrient regimes and was associated with increased dark res piration rather than changed photorespiration. The balance between CO2 supply and demand was lost, as was stomatal limitation on CO2 uptake. High nutrition did not help leaves to maintain a high photosynthetic capacity and life span under O-3 stress.