Summary1. The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34 -36 nL L − 1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3. After 3 years of exposure to 560 µ mol mol − 1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.

No abstract

Photosynthesis is a complex process whose rate is affected by many biochemical and biophysical factors. Fortunately, it is possible to determine, or at least estimate, many of the most important parameters using a combination of optical methods and gas transient analyses. We describe here a computer-operated routine that has been developed to make detailed assessments of photosynthesis at a comprehensive level. The routine comprised the following measurements: steady-state light and CO 2 response curves of net CO 2 assimilation at 21 and 2 kPa O 2 ; transients from limiting to different saturating CO 2 concentrations at 2 kPa O 2 ; post-illumination CO 2 fixation transient; dark-light induction of O 2 evolution; O 2 yield from one saturating single-turnover flash; chlorophyll fluorescence F 0 , F s and F m during the light and CO 2 response curves; leaf transmission at 820 nm (P700 + ) during the light and CO 2 response curves; post-illumination re-reduction time of P700 + . The routine was executed on a two-channel fast-response gas exchange measurement system (A. Laisk and V. Oja: Dynamic Gas Exchange of Leaf Photosynthesis. CSIRO, Canberra, Australia). Thirty-six intrinsic characteristics of the photosynthetic machinery were derived, including quantum yield of CO 2 fixation ( Y CO2 Key-words : Betula pendula Roth; leaf; methods; photosynthesis.)Abbreviations : A , net CO 2 assimilation rate; AC, assimilatory charge; a II , a I , a 0 , relative optical cross-sections of PSII and PSI antenna and of non-photosynthetic absorption; C a , C i , C w , C c , CO 2 concentrations: ambient, intercellular space, cell wall liquid and carboxylation site, respectively; CRC, carbon reduction cycle; Cyt, b 6 f, cytochrome b 6 f; DCMU, dichlorophenol-dimethyl urea; ETR, and J , electron transport rate; F 0 , F s , F m , F md , fluorescence yields, minimum, steady state, maximum (all in the light) and maximum F m in the dark, respectively; FRL, far-red light; Γ , CO 2 compensation point; K s , Rubisco CO 2 /O 2 specificity; k N , relative rate constant for regulatory non-photochemical excitation quenching; k P0 , relative rate constant for photochemical excitation quenching at open PSII centres; P s , P m , P o , 820 nm signal difference from the dark level, steady state, maximum and corresponding to oxidizable P700; PAD, ( I in equations), PFD, photon flux density, absorbed and incident, respectively; PC, plastocyanin; PGA, 3-phosphoglyceric acid; P i , inorganic phosphate; PSI, PSII, photosystems I and II; PQ, plastoquinone; P700, donor pigment of PSI; q E , q I , non-photochemical quenching, energy dependent and inhibitory; R d , R K , Krebs cycle CO 2 evolution rate in the dark and in the light, respectively; r gw , r m , r md , leaf diffusion resistances, in gas phase, mesophyll total and mesophyll diffusional; RuBP, ribulose 1,5-bisphosphate; Rubisco, ribulose 1,5-bisphosphate carboxylase-oxygenase; SCE, specific carboxylation efficiency; VPD, water

), at elevated temperature (ambient + 2·8-6·2 ∞ ∞ ∞ ∞ C depending on the time of the year) and in a combination of elevated CO 2 and temperature in closed-top chambers. The treatments were started in August 1996. At elevated temperature, the needles that were grown in the first year (i.e. the 1997 cohort) were thinner, had thinner mesophyll in the abaxial side, thinner vascular cylinder and lower stomatal density than those grown at ambient temperature. The proportion of mesophyll area occupied by vascular cylinder or intercellular spaces were not changed. Lower stomatal density apparently did not lead to decreased use of water, as these needles had higher concentrations of less mobile nutrients (Ca, Mg, B, Zn and Mn), which could indicate increased total transpiration. In the 1997 and 1998 cohorts, elevation of temperature decreased concentrations of N, P, K, S and Cu. In the 1999 cohort, contradictory, higher concentrations of N and S at elevated temperature may be related to increased nutrient mineralization in the soil. Elevation of CO 2 did not affect stomatal density, needle thickness, thickness of epidermis or hypodermis, vascular cylinder or intercellular spaces. Concentrations of N, P, S and Cu decreased at elevated CO 2 . Reductions were transient and most distinct in the 1997 cohort. The effects of CO 2 and temperature were in some cases interactive, which meant that in the combined treatment stomatal density decreased less than at elevated temperature, and concentrations of nutrients decreased less than expected on the basis of separate treatments, whereas the thickness of the epidermis and hypodermis decreased more than in the separate treatments. In conclusion, alterations in the anatomy and stomatal density of Scots pine needles were more distinct at elevated temperature than at elevated CO 2 . Both elevated CO 2 and temperature-induced changes in nutrient concentrations that partly corresponded to the biochemical and photosynthetic alterations in the same cohorts (

Impacts of ozone and CO 2 enrichment, alone and in combination, on leaf anatomical and ultrastructural characteristics, nutrient status and cell wall chemistry in two European silver birch (Betula pendula Roth) clones were studied. The young soil-growing trees were exposed in open-top chambers over three growing seasons to 2 Â ambient CO 2 and/ or ozone concentrations in central Finland. The trees were measured for changes in altogether 35 variables of leaf structure, nutrients and cell wall chemistry of green leaves, and 20 of the measured variables were affected by CO 2 and/or O 3 . Elevated CO 2 increased the size of chloroplasts and starch grains, number of mitochondria, P : N ratio, and contents of cell wall hemicellulose. Elevated CO 2 decreased the total leaf thickness, specific leaf area, concentrations of N, K, Cu, S and Fe, and contents of cell wall a-cellulose, uronic acids, acid-soluble lignin and acetone-soluble extractives. Elevated ozone led to thinner leaves, higher palisade to spongy ratio, increased number of peroxisomes and mitochondria, reduced content of Mn, Zn, Cu, hemicellulose and uronic acids, and lower Mn : N and Zn : N ratios. In the combined exposure, interactions were antagonistic. Ultrastructural changes became more evident towards the end of the exposure. Young leaves were tolerant against ozone-caused oxidative stress, whereas oxidative H 2 O 2 accumulation was found in older leaves. CO 2 enrichment improved ozone tolerance not only through increased photosynthesis rates, but also through changes in cell wall chemistry (hemicellulose, in particular). However, nutrient imbalances due to ozone and/or CO 2 may predispose the trees to other biotic and abiotic stresses. Down-regulation and up-regulation of photosynthesis under elevated CO 2 through anatomical changes is discussed.