Leaf traits and photosynthetic responses of Betula pendula saplings to a range of ground-level ozone concentrations at a range of nitrogen loads.

Ground-level ozone (O 3 ) concentrations and atmospheric nitrogen (N) deposition rates have increased strongly since the 1950s. Rising ground-level O 3 concentrations and atmospheric N deposition both affect plant physiology and growth, however, impacts have often been studied in isolation rather than in combination. In addition, studies are often limited to a control treatment and one or two elevated levels of ozone and/or nitrogen supply. In the current study, three-year old Betula pendula saplings were exposed to seven different O 3 profiles (24 h mean O 3 concentration of 36–68 ppb in 2013, with peaks up to an average of 105 ppb) in precision-controlled hemispherical glasshouses (solardomes) and four different N loads (10, 30, 50 or 70 kg N ha −1 y −1 ) in 2012 and 2013. Here we report on the effects of enhanced O 3 concentrations and N load on leaf traits and gas exchange in leaves of varying age and developmental stage in 2013. The response of leaf traits to O 3 (but not N) vary with leaf developmental stage. For example, elevated O 3 did not affect the chlorophyll content of the youngest fully expanded leaf, but it reduced the chlorophyll content and photosynthetic parameters in aging leaves, relatively more so later than earlier in the growing season. Elevated O 3 enhanced the N content of senesced leaves prior to leaf fall, potentially affecting subsequent N cycling in the soil. Enhanced N generally stimulated the chlorophyll content and photosynthetic capacity. Whilst elevated O 3 reduced the light-saturated rate of photosynthesis (A sat ) in aging leaves, it did not affect stomatal conductance (g s ). This suggests that photosynthesis and g s are not closely coupled at elevated O 3 under-light saturating conditions. We did not observe any interactions between O 3 and N regarding photosynthetic parameters (V c,max , J max , A sat ), chlorophyll content, g s , N content in senesced leaves and leaf number. Hence, the sensitivity of these leaf traits to O 3 in young silver birch trees is neither reduced nor enhanced by N load. [ABSTRACT FROM AUTHOR]