Young (12 years old) Norway spruce {Picea abies [L.] Karst.) trees were exposed to ambient CO2 or ambient + 350 |niiol(C02) moL' continuously over 2 growing seasons in open-top chambers, under field conditions of a mountain stand. Comprehesive analysis of CO2 assimílation was performed after 4 and 22 weeks of the second growing season to evaluate the influence of elevated atmospheric CO2. A combination of gas exchange and a mathematical model of ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBPCO) activity was ušed. After 4 weeks of exposure no statistically significant stimulation of the radiant energy and CO2 saturated rate of CO2 uptake (ENsat) by the elevated CO2 concentration was found. Yet after 24 weeks a statistically significant depression of Ejvjsa, (38 %) and carboxylation efficiency (32 %) was observed. Depression of photosynthetic activity by elevated CO2 resulted from a decrease in the RuBPCO carboxylation rate. The electron transport rate was also modified similarly to the rate of RuBP formation. An accompanying decrease in nitrogen content of the needles (by 12 %) together with an increase in total saccharides (by 34 %) was observed after 24 weeks of exposure to enhanced CO2.
o reveal the dynamics of short-term photosynthetic acclimation to increased irradiance, the light response of photochemical (qp), non-photochemical (q^) and Fo (qo) quenchings of chlorophyll (Chl) fluorescence and Chl and carotenoids compositíon in Norway spruce needles were monitored within three days after transfer of saplings ffom low diffuse irradiance (maximum photosynthetic photon fluence density PPFD 50 pmol m'^ s'i) to direct sun radiation (maximum PPFD 2000 pmol m‘2 s'*). Irradiance responses of fluorescence quenching coefficients revealed the occurrence of substantíal changes in partítioning of excitation energy between photochemical reactions and radiatíonless dissipation within two days. The saturating irradiance for qj,j and the capacity of non-radiatíve dissipation processes was shifted from about 450 pmol m'^ s** to 1620 pmol m-2 s'L Whereas immediately after exposure to ťull sunlight was completely reduced at 1620 pmol m'^ s'*, two days later 40 % of was stíll present in oxidized form at this irradiance. A fast pigment photobleaching at noon prevented the over-reduction of and thus it was one of the possible short-term acclimation processes. No severe photoinhibition of photosystem 2 (PS 2) photochemistry occurred within the period of investigation as can be judged from the high F^/F^ value.
Temperature responses of carbon assimilation processes were studied in four dominant species from mountain grassland ecosystem, i.e. Holcus mollis (L.), Hypericum maculatum (Cr.), Festuca rubra (L.), and Nardus stricta (L.), using the gas exchange technique. Leaf temperature (TL) of all species was adjusted within the range 13-30 °C using the Peltier thermoelectric cooler. The temperature responses of metabolic processes were subsequently modelled using the Arrhenius exponential function involving the temperature coefficient Q10. The expected increase of global temperature led to a significant increase of dark respiration rate
(RD; Q 10 = 2.0±0.5), maximum carboxylation rate (VCmax; Q10 = 2.2±0.6), and maximum electron transport rate (J max; Q 10 = 1.6±0.4) in dominant species of mountain grassland ecosystems. Contrariwise, the ratio between Jmax and VCmax linearly decreased with TL [y = -0.884 TL + 5.24; r2 = 0.78]. Hence temperature did not control the ratio between intercellular and ambient CO2 concentration, apparent quantum efficiency, and photon-saturated CO2 assimilation rate (Pmax). Pmax primarily correlated with maximum stomatal conductance irrespective of TL. Water use efficiency tended to decrease with TL [y = -0.21 TL + 8.1;
r2 = 0.87]. and O. Urban ... [et al.].