Control of photosynthesis in leaves as revealed by rapid gas exchange and measurements of the assimilatory force FA

Abstract

The rapid transients of CO2 gas exchange have been measured in leaves ofHelianthus annuus L. In parallel experiments the assimilatory force FA, which is the product of the phosphorylation potential and the redox ratio NADPH/NADP, has been calculated from measured ratios of dihydroxyacetone phosphate to phosphoglycerate in the chloroplast stroma and in leaves. The following results were obtained: (i) When the light-dependent stroma alkalization was measured under steady-state conditions for photosynthesis in air containing 2000 μl · l(-1) CO2, alkalization increased with photosynthesis as the quantum flux density (irradiance) was increased. This contrasts to the light-dependent stroma alkalisation measured in dark-adapted leaves during the dark-light transient (Laisk et al. 1989, Planta177, 350-358) which reached a maximum at a quantum flux density far below that necessary to saturate photosynthesis. This maximum was about three times higher than the maximum stroma alkalization at light- and CO2-saturated photosynthesis. (ii) Accurate calculations of the assimilatory force FA require a consideration of the stromal pH. However, under many conditions, changes in the stromal pH resulting from changes in photosynthetic flux can be neglected because they are small. (iii) Stromal ratios of dihydroxyacetone phosphate to phosphoglycerate are generally lower than ratios measured in leaf extracts. The value of FA calculated from stromal metabolites was about 30% lower than FA calculated from cellular metabolites. Still, it appears sufficient for many purposes to calculate FA from metabolite measurements in leaf extracts. (iv) In the light, the catalytic capacity of the photosynthetic apparatus is adjusted to the level of irradiance. The response of carbon assimilation to large increases in irradiance is slow because it requires enzyme activation. Deactivation of the Calvin cycle induced by decreases in irradiance is slower than activation. (v) Changes in catalytic capacity and in the availability or level of substrates such as CO2 alter the flux resistance of the Calvin cycle. A decrease in flux resistance explains why FA often does not increase by much and may actually decrease when carbon flux is increased. Adjustments of flux resistances in the Calvin cycle and of photosystem-II activity in the electron-transport chain permit varying rates of photosynthesis at low levels of ATP and NADPH. As NADP remains available, the danger of over-reduction which leads to photoinactivation of electron transport is minimized.

In vivo modulation of nonphotochemical exciton quenching (NPQ) by regulation of the chloroplast ATP synthase

“…22 and 23). Under most conditions, high intensity illumination does not lead to complete reduction of Q A in PS II, indicating that light input is decreased as electron transfer nears saturation (23 25), implying that output from the light reactions and its consumption by the Calvin cycle are tightly coregulated. Furthermore, the pH of the lumen appears to be tightly regulated to a narrow range, where the stabilities of luminal components and the effective rate constants for electron transfer are near optimal (1).…”

Dynamic photosynthesis under a fluctuating environment: a modelling-based analysis

“…In the simulations used for Figure 5 . These values are in agreement with some estimations of free Pi in the stroma (Hampp et al, 1982;Sharkey and Vanderveer, 1989;Siebke et al, 1990), although other estimations result in values an order of magnitude higher (Dietz and Heber, 1984;Prinsley et al, 1986a). …”

“…Reported values of ADP/ATP in chloroplasts, across different irradiance and [CO2] levels are always higher than 0.35 (Hampp et al, 1982;Dietz and Heber, 1984;Prinsley et al, 1986a;Siebke et al, 1990;Heineke et al, 1991), whereas ATP/ADP/Pi remained lower than 1000 M −1 (Giersch et al, 1980;Siebke et al, 1990), so regulation of ATPase activity by low ADP concentrations may not occur in vivo (Kanazawa and Kramer, 2002). The underestimation of ADP/ATP by the model may be the result of the parameter values chosen for regulation of Rubisco activase (Section 5.2.7.1) or consumption by the Calvin cycle in the regeneration of RuBP (Section 5.2.7.2).…”

In Silico Analysis of the Regulation of the Photosynthetic Electron Transport Chain in C3 Plants

“…Estimations from experimental data indicated that Pi is expected to remain , 10 mM (Hampp et al, 1982;Sharkey and Vanderveer, 1989;Siebke et al, 1990), though the uncertainty on these estimations was not reported by the authors, even though assumptions on chloroplast volume or total chloroplast phosphate content were often required. This means that Pi could also contribute to the regulation of ATPase conductance, pmf, and NPQ at a wide range of light intensities and CO 2 levels, even when Pi does limit CO 2 assimilation.…”