ALBERT

Notes: The present study was performed to investigate the adjustment of the constituents of the light and dark reactions of photosynthesis to the natural growth irradiance in the leaves of an overstorey species, Betula pendula Roth, a subcanopy species Tilia cordata P. Mill., and a herb Solidago virgaurea L. growing in a natural plant community in Järvselja, Estonia. Shoots were collected from the site and properties of individual leaves were measured in a laboratory, by applying a routine of kinetic gas exchange and optical measurements that revealed photosystem II (PSII), photosystem I (PSI), and cytochrome b6f densities per leaf area and the distribution of excitation (or chlorophyll, Chl) between the two photosystems. In parallel, N, Chl and ribulose-bisphosphate carboxylase-oxygenase (Rubisco) content was measured from the same leaves. The amount of N in photosynthetic proteins was calculated from the measured contents of the components of the photosynthetic machinery. Non-photosynthetic N was found as the residual of the budget. Growth in shade resulted in the decrease of leaf dry mass to a half of the DW in sun leaves in each species, but the total variation, from the top to the bottom of the canopy, was larger. Through the whole cross-section of the canopy, leaf dry weight (DW) and Rubisco content per area decreased by a factor of four, N content by a factor of three, but Chl content only by a factor of 1.7. PSII density decreased by a factor of 1.9, but PSI density by a factor of 3.2. The density of PSI adjusted to shade to a greater extent than the density of PSII. In shade, the distribution of N between the components of the photosynthetic machinery was shifted toward light-harvesting proteins at the expense of Rubisco. Non-photosynthetic N decreased the most substantially, from 54% in the sun leaves of B. pendula to 11% in the shade leaves of T. cordata. It is concluded that the redistribution of N toward light-harvesting Chl proteins in shade is not sufficient to keep the excitation rate of a PSII centre invariant. Contrary to PSII, the density of PSI – the photosystem that is in immediate contact with the carbon assimilation system – shade-adjusts almost proportionally with the latter, whereas its Chl antenna correspondingly increases. Even under N deficiency, a likely condition in the natural plant community, a substantial part of N is stored in non-photosynthetic proteins under abundant irradiation, but much less under limiting irradiation. At least in trees the general sequence of down-regulation due to shade adjustment is the following: (1) non-protein cell structures and non-photosynthetic proteins; (2) carbon assimilation proteins; (3) light reaction centre proteins, first PSI; and (4) chlorophyll-binding proteins.

Notes: Two silver birch (Betula pendula Roth) clones K1659 and V5952 were grown in open-top chambers over 3 years (age 7–9 years). The treatments were increased CO2 concentration (+CO2, 72 Pa), increased O3 concentration (+O3, 2 × ambient O3 with seasonal AOT40 up to 28 p.p.m. h) and in combination (+CO2 + O3). Thirty-seven photosynthetic parameters were measured in the laboratory immediately after excising leaves using a computer-operated routine of gas exchange and optical measurements. In control leaves the photosynthetic parameters were close to the values widely used in a model (Farquhar, von Caemmerer and Berry, Planta 149, 78–90, 1980). The distribution of chlorophyll between photosystem II and photosystem I, intrinsic quantum yield of electron transport, uncoupled turnover rate of Cyt b6f, Rubisco specificity and Km (CO2) were not influenced by treatments. Net photosynthetic rate responded to +CO2 with a mean increase of 17% in both clones. Dry weight of leaves increased, whereas protein, especially Rubisco content and the related photosynthetic parameters decreased. Averaged over 3 years, eight and 17 mechanistically independent parameters were significantly influenced by the elevated CO2 in clones K1659 and V5952, respectively. The elevated O3 caused a significant decrease in the average photosynthetic rate of clone V5952, but not of clone K1659. The treatment caused changes in one parameter of clone K1659 and in 11 parameters of clone V5952. Results of the combined treatment indicated that +O3 had less effect in the presence of +CO2 than alone. Interestingly, changes in the same photosynthetic parameters were observed in chamberless grown trees of clone V5952 as under +O3 treatment in chambers, but this was not observed for clone K1659. These results suggest that during chronic fumigation, at concentrations below the threshold of visible leaf injuries, ozone influenced the photosynthetic parameters as a general stress factor, in a similar manner to weather conditions that were more stressful outside the chambers. According to this hypothesis, the sensitivity of a species or a clone to ozone is expected to depend on the growth conditions: the plant is less sensitive to ozone if the conditions are close to optimal and it is more sensitive to ozone under conditions of stress.

Notes: The present study was performed to investigate the adjustment of the rate parameters of the light and dark reactions of photosynthesis to the natural growth light in leaves of an overstorey species, Betula pendula Roth, a subcanopy species, Tilia cordata P. Mill., and a herb, Solidago virgaurea L., growing in a natural plant community in Järvselja, Estonia. Shoots were collected from the site and individual leaves were measured in a laboratory applying a standardized routine of kinetic gas exchange, Chl fluorescence and 820 nm transmittance measurements. These measurements enabled the calculations of the quantum yield of photosynthesis and rate constants of excitation capture by photochemical and non-photochemical quenchers, rate constant for P700+ reduction via the cytochrome b6f complex with and without photosynthetic control, actual maximum and potential (uncoupled) electron transport rate, stomatal and mesophyll resistances for CO2 transport, Km(CO2) and Vm of ribulose-bisphosphate carboxylase-oxygenase (Rubisco) in vivo. In parallel, N, Chl and Rubisco contents were measured from the same leaves. No adjustment toward higher quantum yield in shade compared with sun leaves was observed, although relatively more N was partitioned to the light-harvesting machinery in shade leaves (H. Eichelmann et al., 2004). The electron transport rate through the Cyt b6f complex was strongly down-regulated under saturating light compared with darkness, and this was observed under atmospheric, as well as saturating CO2 concentration. In vivo Vm measurements of Rubisco were lower than corresponding reported measurements in vitro, and the kcat per reaction site varied widely between leaves and growth sites. The correlation between Rubisco Vm and the photosystem I density was stronger than between Vm and the density of Rubisco active sites. The results showed that the capacity of the photosynthetic machinery decreases in shade-adjusted leaves, but it still remains in excess of the actual photosynthetic rate. The photosynthetic control systems that are targeted to adjust the photosynthetic rate to meet the plant's needs and to balance the partial reactions of photosynthesis, down-regulate partial processes of photosynthesis: excess harvested light is quenched non-photochemically; excess electron transport capacity of Cyt b6f is down-regulated by ΔpH-dependent photosynthetic control; Rubisco is synthesized in excess, and the number of activated Rubisco molecules is controlled by photosystem I-related processes. Consequently, the nitrogen contained in the components of the photosynthetic machinery is not used at full efficiency. The strong correlation between leaf nitrogen and photosynthetic performance is not due to the nitrogen requirements of the photosynthetic apparatus, but because a certain amount of energy must be captured through photosynthesis to maintain this nitrogen within a leaf.

Notes: 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 CO2 response curves of net CO2 assimilation at 21 and 2 kPa O2; transients from limiting to different saturating CO2 concentrations at 2 kPa O2; post-illumination CO2 fixation transient; dark–light induction of O2 evolution; O2 yield from one saturating single-turnover flash; chlorophyll fluorescence F0, Fs and Fm during the light and CO2 response curves; leaf transmission at 820 nm (P700+) during the light and CO2 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 CO2 fixation (YCO2), time constant of P700 re-reduction (τ′), relative optical cross-sections of PSII and PSI antennae (aII, aI), PSII and PSI density per leaf area unit, plastoquinone pool, total mesophyll resistance, mesophyll diffusion resistance, Vm, Km(CO2) and CO2/O2 specificity of Rubisco, RuBP pool at CO2 limitation (assimilatory charge). An example of the routine and calculations are shown for one leaf and data are presented for leaves of 8-year-old-trees of two birch clones growing in Suonenjoki Forest Research Station, Finland, during summer 2000. Parameters YCO2, basic τ′, aII, aI, Km(CO2) and Ks varied little in different leaves [relative standard deviation (RSD) 〈 7%], other parameters scattered widely (RSD typically 10–40%). It is concluded that the little scattered parameters are determined by basic physico-chemical properties of the photosynthetic machinery whereas the widely scattered parameters are adjusting to growth conditions. The proposed non-destructive routine is suitable for diagnosing the photosynthetic machinery of leaves and may be applied in plant ecophysiology and in genetic engineering of plants.