Finite mesophyll diffusion conductance (gm) significantly constrains net assimilation rate (An), but gm variations and variation sources in response to environmental stresses during leaf development are imperfectly known. The combined effects of light and water limitations on gm and diffusion limitations of photosynthesis were studied in saplings of Populus tremula L. An one-dimensional diffusion model was used to gain insight into the importance of key anatomical traits in determining gm. Leaf development was associated with increases in dry mass per unit area, thickness, density, exposed mesophyll (Smes/S) and chloroplast (Sc/S) to leaf area ratio, internal air space (fias), cell wall thickness and chloroplast dimensions. Development of Smes/S and Sc/S was delayed under low light. Reduction in light availability was associated with lower Sc/S, but with larger fias and chloroplast thickness. Water stress reduced Sc/S and increased cell wall thickness under high light. In all treatments, gm and An increased and CO2 drawdown because of gm, Ci-Cc, decreased with increasing leaf age. Low light and drought resulted in reduced gm and An and increased Ci-Cc. These results emphasize the importance of gm and its components in determining An variations during leaf development and in response to stress.

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

The spectral global quantum yield (YII, electrons/photons absorbed) of photosystem II (PSII) was measured in sunflower leaves in State 1 using monochromatic light. The global quantum yield of PSI (YI) was measured using low-intensity monochromatic light flashes and the associated transmittance change at 810nm. The 810-nm signal change was calibrated based on the number of electrons generated by PSII during the flash (4·O2 evolution) which arrived at the PSI donor side after a delay of 2ms. The intrinsic quantum yield of PSI (yI, electrons per photon absorbed by PSI) was measured at 712nm, where photon absorption by PSII was small. The results were used to resolve the individual spectra of the excitation partitioning coefficients between PSI (aI) and PSII (aII) in leaves. For comparison, pigment-protein complexes for PSII and PSI were isolated, separated by sucrose density ultracentrifugation, and their optical density was measured. A good correlation was obtained for the spectral excitation partitioning coefficients measured by these different methods. The intrinsic yield of PSI was high (yI=0.88), but it absorbed only about 1/3 of quanta; consequently, about 2/3 of quanta were absorbed by PSII, but processed with the low intrinsic yield yII=0.63. In PSII, the quantum yield of charge separation was 0.89 as detected by variable fluorescence Fv/Fm, but 29% of separated charges recombined (Laisk A, Eichelmann H and Oja V, Photosynth. Res. 113, 145-155). At wavelengths less than 580nm about 30% of excitation is absorbed by pigments poorly connected to either photosystem, most likely carotenoids bound in pigment-protein complexes.

A computer model comprising light reactions, electron-proton transport, enzymatic reactions, and regulatory functions of C3 photosynthesis has been developed as a system of differential budget equations for intermediate compounds. The emphasis is on electron transport through PSII and PSI and on the modeling of Chl fluorescence and 810 nm absorptance signals. Non-photochemical quenching of PSII excitation is controlled by lumenal pH. Alternative electron transport is modeled as the Mehler type O2 reduction plus the malate-oxaloacetate shuttle based on the chloroplast malate dehydrogenase. Carbon reduction enzymes are redox-controlled by the ferredoxin-thioredoxin system, sucrose synthesis is controlled by the fructose 2,6-bisphosphate inhibition of cytosolic FBPase, and starch synthesis is controlled by ADP-glucose pyrophosphorylase. Photorespiratory glycolate pathway is included in an integrated way, sufficient to reproduce steady-state rates of photorespiration. Rate-equations are designed on principles of multisubstrate-multiproduct enzyme kinetics. The parameters of the model were adopted from literature or were estimated from fitting the photosynthetic rate and pool sizes to experimental data. The model provided good simulations for steady-state photosynthesis, Chl fluorescence, and 810 nm transmittance signals under varying light, CO2 and O2 concentrations, as well as for the transients of post-illumination CO2 uptake, Chl fluorescence induction and the 810 nm signal. The modeling shows that the present understanding of photosynthesis incorporated in the model is basically correct, but still insufficient to reproduce the dark-light induction of photosynthesis, the time kinetics of non-photochemical quenching, 'photosynthetic control' of plastoquinone oxidation, cyclic electron flow around PSI, oscillations in photosynthesis. The model may find application for predicting the results of gene transformations, the analysis of kinetic experimental data, the training of students.

Oscillations in the rate of photosynthesis of sunflower (Helianthus annuus L.) leaves were induced by subjecting leaves, whose photosynthetic apparatus had been activated, to a sudden transition from darkness or low light to high-intensity illumination, or by transfering them in the light from air to an atmosphere containing saturating CO2. It was found that at the first maximum, light-and CO2-saturated photosynthesis can be much faster than steady-state photosynthesis. Both QA in the reaction center of PS II and P700 in the reaction center of PS I of the chloroplast electron-transport chain were more oxidized during the maxima of photosynthesis than during the minima. Maxima of P700 oxidation slightly preceded maxima in photosynthesis. During a transition from low to high irradiance, the assimilatory force FA, which was calculated from ratios of dihydroxyacetone phosphate to phosphoglycerate under the assumption that the reactions catalyzed by NADP-dependent glyceraldehydephosphate dehydrogenase, phosphoglycerate kinase and triosephosphate isomerase are close to equilibrium, oscillated in parallel with photosynthesis. However, only one of its components, the calculated phosphorylation potential (ATP)/(ADP)(Pi), paralleled photosynthesis, whereas calculated NADPH/NADP ratios exhibited antiparallel behaviour. When photosynthetic oscillations were initiated by a transition from low to high CO2, the assimilatory force FA declined, was very low at the first minimum of photosynthesis and increased as photosynthesis rose to its second maximum. The observations indicate that the minima in photosynthesis are caused by lack of ATP. This leads to overreduction of the electron-transport chain which is indicated by the reduction of P700. During photosynthetic oscillations the chloroplast thylakoid system is unable to adjust the supply of ATP and NADPH rapidly to demand at the stoichiometric relationship required by the carbonreduction cycle.