Leaf factors affecting the relationship between chlorophyll fluorescence and the rate of photosynthetic electron transport as determined from CO2 uptake

CO2 uptake and chlorophyll fluorescence were measured under non-photorespiratory conditions in leaves from 14 plant species. The rate of CO2-dependent electron transport (JCO2) was calculated as four times rate of gross photosynthesis. The quantum yield of electron transport in photosystem II was estimated from the ratio delta F/Fm', where delta F is the difference between steady-state and maximal fluorescence in the light. As photon flux density (PFD) increased, JCO2 increased linearly first, and then reached saturation. The product (delta F/Fm')PFD, which is a function of electron transport rate, showed a similar response. Therefore, the relationship between (delta F/Fm') PFD versus JCO2 was proportional. However, under high light, a linear correlation was not always maintained. Factors affecting the linear correlation were analyzed by measuring CO2 uptake and chlorophyll fluorescence under illumination from either the upper (adaxial) or lower (abaxial) leaf surface, and by using plants with anatomically symmetric leaves having palisade tissues on both sides. Consequently, it was shown that the parameter delta F/Fm' is based on chlorophyll fluorescence emitted from chloroplasts present near the illuminated surface. Further, it was suggested that this restriction of the origin of fluorescence actually measured is significant in a leaf with high chlorophyll content, resulting in the deviation from linearity in the relationship between JCO2 and (delta F/Fm')PFD.

The relationship between photosystem II efficiency and quantum yield for CO(2) assimilation is not affected by nitrogen content in apple leaves

Bench-grafted Fuji/M.26 apple (Malus domestica Borkh.) trees were fertigated with different concentrations of nitrogen by using a modified Hoagland's solution for 45 d. CO(2) assimilation and photosystem II (PSII) quantum efficiency in response to incident photon flux density (PFD) were measured simultaneously in recent fully expanded leaves under low O(2) (2%) and saturated CO(2) (1300 micromol mol(-1)) conditions. A single curvilinear relationship was found between true quantum yield for CO(2) assimilation and PSII quantum efficiency for leaves with a wide range of leaf N content. The relationship was linear up to a quantum yield of approximately 0.05 mol CO(2) mol(-1) quanta. It then became curvilinear with a further rise in quantum yield in response to decreasing PFD. This relationship was subsequently used as a calibration curve to assess the rate of non-cyclic electron transport associated with Rubisco and the partitioning of electron flow between CO(2) assimilation and photorespiration in different N leaves in response to intercellular CO(2) concentration (C(i)) under normal O(2) conditions. Both the rate of non-cyclic electron flow and the rate of electron flow to CO(2) or O(2) increased with increasing leaf N at any given C(i). The percentage of non-cyclic electron flow to CO(2) assimilation, however, remained the same regardless of leaf N content. As C(i) increased, the percentage of non-cyclic electron flow to CO(2) assimilation increased. In conclusion, the relationship between PSII quantum efficiency and quantum yield for CO(2) assimilation and the partitioning of electron flow between CO(2) assimilation and photorespiration are not affected by N content in apple leaves.

Tansley Review No. 36 Excited leaves

Photosynthesis is largely to do with energy transduction; the conversion of light energy into electrical energy into chemical energy. Precisely how much light energy is needed to bring about the reduction of one molecule of carbon dioxide and the release of one molecule of oxygen (the quantum requirement) is a matter of fundamental importance and one which has attracted much past controversy. This article concludes that a minimum quantum requirement of eight, as demanded by the Z-scheme, is obviously consistent with much contemporary work which puts the measured value for C₃ leaves close to nine. Moreover, while values of less than eight (obtained in some circumstances with micro-organisms), are a reminder that nothing is beyond challenge they are not, in the absence of confirmation and extension, sufficiently compelling to demand rejection of either the Z-scheme or current measuring procedures. This article also shows why, even if the underlying minimum requirement was now accepted beyond all reasonable doubt, there would still be very good reasons for continuing, indefinitely, to measure actual photosynthetic efficiency in the natural environment. It discusses some of the implications of the fact that all plants, if not stressed, appear to photosynthesize at the same rate in low light. It explains the role of fluorescence in its relation to quantum yield, the possibility that the rate of photosynthesis might be determined from fluorescence measurements alone, and that a combination of fluorescence and gas exchange measurements could provide new information about the manner in which 'dark respiration' is affected by light. It indicates how contemporary interest in all of these matters has focused attention on the necessity for safe dissipation of excitation energy by leaves and on the manner by which this might be achieved. CONTENTS Summary 325 I. Excitation 325 II. Quantum requirement 326 III. Learning from fluorescence 335 IV. Safely dissipated 340 Acknowledgements 342 References 342.

On the relationship between the quantum yield of Photosystem II electron transport, as determined by chlorophyll fluorescence and the quantum yield of CO2-dependent O 2 evolution

We tested the two empirical models of the relationship between chlorophyll fluorescence and photosynthesis, previously published by Weis E and Berry JA 1987 (Biochim Biophys Acta 894: 198-208) and Genty B et al. 1989 (Biochim Biophys Acta 990: 87-92). These were applied to data from different species representing different states of light acclimation, to species with C3 or C4 photosynthesis, and to wild-type and a chlorophyll b-less chlorina mutant of barley. Photosynthesis measured as CO2-saturated O2 evolution and modulated fluorescence were simultaneously monitored over a range of photon flux densities. The quantum yields of O2 evolution (ØO2) were based on absorbed photons, and the fluorescence parameters for photochemical (qp) and non-photochemical (qN) quenching, as well as the ratio of variable fluorescence to maximum fluorescence during steady-state illumination (F'v/F'm), were determined. In accordance with the Weis and Berry model, most plants studied exhibited an approximately linear relationship between ØO2/qp (i.e., the yield of O2 evolution by open Photosystem II reaction centres) and qN, except for wild-type barley that showed a non-linear relationship. In contrast to the linear relationship reported by Genty et al. for qp×F'v/F'm (i.e., the quantum yield of Photosystem II electron transport) and ØCO2, we found a non-linear relationship between qp×F'v/F'm and ØO2 for all plants, except for the chlorina mutant of barley, which showed a largely linear relationship. The curvilinearity of wild-type barley deviated somewhat from that of other species tested. The non-linear part of the relationship was confined to low, limiting photon flux densities, whereas at higher light levels the relationship was linear. Photoinhibition did not change the overall shape of the relationship between qp×F'v/F'm and ØO2 except that the maximum values of the quantum yields of Photosystem II electron transport and photosynthetic O2 evolution decreased in proportion to the degree of photoinhibition. This implies that the quantum yield of Photosystem II electron transport under high light conditions may be similar for photoinhibited and non-inhibited plants. Based on our experimental results and theoretical analyses of photochemical and non-photochemical fluoresce quenching processes, we conclude that both models, although not universal for all plants, provide useful means for the prediction of photosynthesis from fluorescence parameters. However, we also discuss that conditions which alter one or more of the rate constants that determine the various fluorescence parameters, as well as differential light penetration in assays for oxygen evolution and fluorescence emission, may have direct effect on the relationships of the two models.