M. Šprtová

Glass Domes with Adjustable Windows: A Novel Technique for Exposing Juvenile Forest Stands to Elevated CO2 Concentration

We present a new technological approach for in situ investigation of long-term impacts of elevated CO2 concentration (EC) on juvenile forests characterised by an intensive community level and canopy closure phase. Construction of the glass domes is based on the properties of earlier tested open-top chambers (OTCs). An air climatisation device together with an adjustable window system, that forms the shell cover of the domes, is able to keep the required [CO2] in both time and spatial scales with the relatively small consumption of supplied CO2. This is achieved by half-closing the windows on the windward side. We evidenced good coupling of treated trees to the atmosphere, including mutual interactions among trees. The semi-open design of the domes moderates the problems of strong wind, humidity, and temperature gradients associated with OTCs. The frequency distributions of the environmental variations within the domes indicate that: air temperature is maintained within the ambient range ±1.0 °C for ca. 80 % of the time, and changes in the relative air humidity vary from −15 to 0 % for ca. 82 % of the time. The most important chamber effect is associated with the penetration of solar irradiance, which is reduced by 26 % compared to the open condition outside the domes. The dimensions of the domes are 10×10 m in length and 7 m high in the central part. The experiment was done in three identical stands of twelve-year-old Norway spruce trees. The 56 trees are planted at two different spacings to estimate the impacts of stand spatial structure in relation to EC.

Seasonal changes of photosynthetic assimilation of Norway spruce under the impact of enhanced UV-B radiation

Abstract The cloned saplings of Norway spruce (7 years old) were exposed to enhanced UV-B irradiation (+25%) continuously over three growing seasons. An analysis of CO 2 assimilation and pigment composition was performed in the beginning (June) and the end (September) of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. The UV-B was administered at levels that were, at any moment, at 125% of the ambient UV-B radiation. The results of the long-term experiment supports an idea on cumulative exposure UV-B effect. Four months under the influence of enhanced UV-B radiation was responsible for the depression of A N over the whole interval of investigated PPFD/ C i . A significant decrease in A Nmax. (up to 9%) and α (up to 22%), and increase in Γ i (up to 57%) and of R D (up to 19%) was found. The long-term effect of enhanced UV-B radiation caused a significant decrease in A Nsat (up to 60%) and τ (up to 8%), and a significant increase in Γ C (up to 27%), compared to controls. The chlorophyll a + b content was significantly decreased in the UV-B exposed variant. This change was mainly caused by a decrease in the chlorophyll a content (up to 20%). Thus, the negative effect of the long-term 21% enhancement UV-B radiation on photosynthetic gas exchange and photosynthetic pigment is concluded. Investigations of the gas-exchange parameters showed that the main limitation of assimilation can be identified at the level of RuBP regeneration related to electron transport. This finding is supported by the analysis of photosynthetic pigment content that indicates a simultaneous decrease of the chlorophyll a content and the antenna size. Thus, the potential increased sensitivity to the photoinhibition of photosynthesis resulting from the long-term influence of enhanced UV-B radiation is suggested.

Photosynthetic UV-B Response of Beech (Fagus sylvatica L.) Saplings

Cloned saplings of beech (7-y-old) were exposed to enhanced UV-B irradiation (+25 %) continuously over three growing seasons (1999–2001). Analysis of CO2 assimilation, variable chlorophyll (Chl) a fluorescence, and pigment composition was performed in late summer of the third growing season to evaluate the influence of long-term elevated UV-B irradiation. This influence was responsible for the stimulation of the net assimilation rate (PN) over a range of irradiances. The increase in PN was partially connected to increase of the area leaf mass, and thus to the increased leaf thickness. Even a higher degree of UV-B induced stimulation was observed at the level of photosystem 2 (PS2) photochemistry as judged from the irradiance response of electron transport rate and photochemical quenching of Chl a. The remarkably low irradiance-induced non-photochemical quenching of maximum Chl a fluorescence (NPQ) in the UV-B plants over the entire range of applied irradiances was attributed both to the reduced demand on non-radiative dissipation processes and to the considerably reduced contribution of the quenching localised in the inactivated PS2 reaction centres. Neither the content of Chls and total carotenoids expressed per leaf area nor the contents of lutein, neoxanthin, and the pool of xanthophyll cycle pigments (VAZ) were affected under the elevated UV-B. However, the contributions of antheraxanthin (A) and zeaxanthin (Z) to the entire VAZ pool in the dark-adapted UV-B treated plants were 1.61 and 2.14 times higher than in control leaves. Surprisingly, the retained A+Z in UV-B treated plants was not accompanied with long-term down-regulation of the PS2 photochemical efficiency, but it facilitated the non-radiative dissipation of excitation energy within light-harvesting complexes (LHC) of PS2. Thus, in the beech leaves the accumulation of A+Z, induced by other factors than excess irradiance itself, supports the resistance of PS2 against combined effects of high irradiance and elevated UV-B.

Photosynthetic Assimilation of Sun versus Shade Norway Spruce [Picea abies (L.) Karst] Needles Under the Long-Term Impact of Elevated CO2 Concentration

The long-term impact of elevated concentration of CO2 on assimilation activity of sun-exposed (E) versus shaded (S) foliage was investigated in a Norway spruce stand [Picea abies (L.) Karst, age 14 years] after three years of cultivation in two domes with adjustable windows (DAW). One DAW was supplied with ambient air [AC, ca. 350 µmol(CO2) mol−1) and the second with elevated CO2 concentration [EC = AC plus 350 µmol(CO2) mol−1]. The pronounced vertical profile of the photosynthetic photon flux density (PPFD) led to the typical differentiation of the photosynthetic apparatus between the shaded and sun needles. Namely, photon-saturated values of maximal net photosynthetic rate (PNmax) and apparent quantum yield (α) were significantly higher/lower for E-needles as compared with the S-ones. The prolonged exposure to EC was responsible for the apparent assimilatory activity stimulation observed mainly in deeply shaded needles. The degree of this stimulation decreases in the order: S-needles dense part > S-needles sparse part > E-needles dense part > E-needles sparse part. In exposed needles some signals on a manifestation of the acclimation depression of the photosynthetic activity were found. The long-term effect of EC was responsible for the decrease of nitrogen content of needles and for its smoother gradient between E- and S-needles. The obtained results indicate that the E- and S-foliage respond differently to the long-term impact of EC.

The photosynthetic irradiance-response of Norway spruce exposed to a long-term elevation of CO2 concentration

During an open-top chamber experiment performed in a mountain stand of young (12-year-old) Norway spruce (Picea abies [L.] Karst.), the trees were exposed to one of two CO2 concentrations (ambient CO2, AC, or AC + 350 µmol mol-1 = elevated CO2, EC) continuously over three growing seasons. To evaluate the EC influence, measurements of the relations between the rate of net CO2 uptake (PN) and incidental photosynthetically active photon flux density (PPFD), as well as the content of photosynthetic pigments and chlorophyll (Chl) a fluorescence were taken in the third growing season. The short-term response to EC was evident mainly on ribulose-1,5-bisphosphate carboxylase/oxygenase kinetics without any significant change to the utilization of radiant energy. The long-term effect of EC was responsible for a decrease in PN, content of Chl a + b, Fv/Fm ratio, quantum yield of fluorescence, and photochemical quenching. Changes of stoichiometry between the electron transport, Calvin cycle and the end-product synthesis were confirmed for responses to the long-term import of EC and led to a definition of the photosynthetic acclimation to EC in Norway spruce.

Temperature dependences of carbon assimilation processes in four dominant species from mountain grassland ecosystem

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; Q10 = 2.0±0.5), maximum carboxylation rate (VCmax; Q10 = 2.2±0.6), and maximum electron transport rate (Jmax; Q10 = 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].

Impact of elevated CO2 concentration on dynamics of leaf photosynthesis in Fagus sylvatica is modulated by sky conditions.

It has been suggested that atmospheric CO2 concentration and frequency of cloud cover will increase in future. It remains unclear, however, how elevated CO2 influences photosynthesis under complex clear versus cloudy sky conditions. Accordingly, diurnal changes in photosynthetic responses among beech trees grown at ambient (AC) and doubled (EC) CO2 concentrations were studied under contrasting sky conditions. EC stimulated the daily sum of fixed CO2 and light use efficiency under clear sky. Meanwhile, both these parameters were reduced under cloudy sky as compared with AC treatment. Reduction in photosynthesis rate under cloudy sky was particularly associated with EC-stimulated, xanthophyll-dependent thermal dissipation of absorbed light energy. Under clear sky, a pronounced afternoon depression of CO2 assimilation rate was found in sun-adapted leaves under EC compared with AC conditions. This was caused in particular by stomata closure mediated by vapour pressure deficit.

CHLOROPLASTIC CARBON DIOXIDE CONCENTRATION IN NORWAY SPRUCE (PICEA ABIES L. KARST.) NEEDLES RELATES TO THE POSITION WITHIN THE CROWN

Differences between sun (E) and shaded (S) foliage were studied in a Norway spruce (Picea abies [L.] Karst.) stand. Response curves describing the dependence of the CO2 assimilation rate (PN) on the CO2 concentration at the catalytic site of ribulose-1,5-bisphosphate carboxylase/oxygenase, RuBPCO (PN-Cc) were estimated using the simultaneous measurements of chlorophyll fluorescence and leaf gas exchange. Higher PN, higher electron transport (Ja), higher carboxylation capacity (Vc), and higher RuBPCO activity (τ) for sun acclimated needles was found. The S-needles had higher portion of internal limitation and higher CO2 compensation concentration (Γ) than the E-needles. Because higher degree of limitation of photosynthesis by carboxylation was ascertained, it can be assumed that photosynthesis in shade foliage is limited mainly by lower carboxylation capacity and by low chloroplastic CO2 concentration

Response of Photosynthesis to Radiation and Intercellular CO2 Concentration in Sun and Shade Shoots of Norway Spruce

Functional differentiation of assimilation activity of sun versus shade foliage was analysed in a Norway spruce monoculture stand (age 15 years). The investigated stand density (leaf area index 8.6) and crown structure led to variation in the photosynthetically active photon flux density (PPFD) within the crowns of the sampled trees. At the saturating PPFD, the maximum rate of CO2 uptake (PNmax) of exposed shoots (E-shoots) was 1.7 times that of the shaded shoots (S-shoots). The apparent quantum yield (α) of E-shoots was 0.9 times that of the S-shoots. A lower ability to use excess energy at high PPFD in photosynthesis was observed in the S-layer. The CO2- and PPFD-saturated rate of CO2 uptake (PNsat) of the E-shoots was 1.12 times and the carboxylation efficiency (τ) 1.6 times that of the S-shoots. The CO2-saturated rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) carboxylation (VCmax) and of actual electron transport (Jamax) in the S-needles amounted to 89 and 95 % of VCmax and Jamax in the E-needles. Thus, in addition to the irradiation conditions and thus limitation by low Ja, the important limitation of photosynthesis in shade needles is due to carboxylation. This limitation of photosynthesis is accompanied by lower stomatal conductance.

Effect of enhanced UV-B radiation on chlorophyll a fluorescence parameters in Norway spruce needles.

Summary Cloned saplings of Norway spruce (7 years old) were continuously exposed to enhanced UV-B irradiation (+ 25 %) over three growing seasons. An analysis of variable chlorophyll a fluorescence was performed at the beginning Gune) and the end (September) of the third growing season in order to evaluate the longterm effects of elevated UV-B irradiation on the primary photosynthetic reactions. A time-dependent response to elevated UV-B radiation was observed. The saturated PPFD electron transport rate (ETR) in the control (C-variant) was 1.92 times that of the exposed plants (E-variant). The midday depression of the maximal photochemical yield of PSII (F V /F M ) was deeper in the E-variants, and the extent of this depression, as compared with the C-variants, increased at the end of the growing season. A light-induced decline in the F V /F M ratio was followed by a bi-phasic recovery phase upon return to shade conditions. The rapid recovery phase, possibly related to epoxidation of zeaxanthin, showed a sharper slope in the UV-B exposed E-variants. Thus, the long-term exposure to the elevated UV-B radiation was related to induction of the interconversion between violaxanthin and zeaxanthin. The slow phase of recovery of the Fy/FMratio possibly represents the reactivation of PSII by means of 0 1 protein turnover. The slope of the slow phase of the recovery in E-variants was 0,58 time that of the C-variants at the end of the growing season (i.e. 16 weeks of UV-B illumination). This indicates impairment of the 01 protein turnover as a result of the long-term influence ofUV-B radiation enhancement. The results show that in Norway spruce a long-term exposure to enhanced UV-B radiation under field conditions can be a reason for negative changes at the level of primary photosynthetic reactions.