Why does cyclic electron transport occur

Chlorophyll fluorescence in the leaves was evaluated simultaneously with the gas exchange measurements using a leaf fluorometer chamber ; Li-Cor Inc. Because leaf absorbance L abs in tobacco differs little between sun and shade leaves Miyake et al. The constant of 0. On assumption that the water—water cycle in leaves is not a major alternative electron sink for dissipating excess excitation energy when CO 2 assimilation is restricted Driever and Baker, , we allocated the electron flow through PSII to the carboxylation J C and oxygenation J O of RuBP.

These were estimated according to the method of Valentini et al. Where A net is the measured rate of CO 2 assimilation, R d is the day respiration rate measured after 30 min dark adaptation. The total amount of electron transport required for Rubisco carboxylation and oxygenation J g was computed as:. The total amount of ATP demand from Rubisco carboxylation and oxygenation was obtained with the following formula:.

The following equations were used:. This useful tool for assessing the P redox state has been used in numerous studies Gao et al. The photochemical quantum yield of PSI, Y I , is defined by the fraction of overall P that, in a given state, is reduced and not limited by the acceptor side.

All results were displayed as mean values from four to six independent experiments. The combination of a higher Rubisco content as indicated by V cmax ; see Yamori et al. Values of R d , J max , and V cmax for the sun and shade leaves. Thus, rates were approximately twice as high for sun leaves at both light intensities. Values for ETRI did not differ between the sun and shade leaves under low light intensities, i. Asterisks indicate significant differences in the sun leaves compared to the shade leaves.

Non-photochemical quenching can be subdivided into three components: energy-dependent thermal dissipation qE , photoinhibition qI , and state transitions qT. As the light became more intense, Y ND also gradually increased. Values were much higher in the shade leaves than in the sun leaves, especially under high light. These results indicated that the shade leaves up-regulated NPQ and the P oxidation ratio under high light to prevent over-reduction of the electron transfer chain.

Furthermore, their different capacities to utilize light energy mean that sun and shade leaves do not require the same level of photoprotection at a given light. Although CEF activation is involved in ATP synthesis and photoprotective functions, the specific role of CEF under different light intensities in each leaf type is unclear.

This work was to test whether the role of CEF can be regulated flexibly in response to incident light in the sun and shade leaves. Under stronger light, however, the rise in CEF was very large for sun leaves but only slight for shade leaves.

Furthermore, these results support an earlier conclusion by Miyake et al. Therefore, our estimates of CEF in sun and shade leaves are reliable. Such a change in energy demand necessitates a flexible mechanism to add ATP synthesis and then balance that ratio.

This hypothesis is supported by research with mutants that lack key enzymes for the CEF pathway. For tobacco plants, light saturating point LSP of CO 2 assimilation is lower in the shade leaves than the sun leaves Huang et al. Our results strongly suggest that the main function of CEF is flexibly changed according to lighting conditions in sun and shade leaves of tobacco.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

National Center for Biotechnology Information , U. Journal List Front Plant Sci v. This is not what is seen in higher plants. Furthermore, given the very high concentration of protein in the granal regions, diffusion of plastoquinol in the membrane is expected to be highly restricted Joliot et al.

Thus, it is difficult to envisage that a significant rate of electron transfer could occur from PSII to cytochrome b 6 f complexes in the stromal lamellae. Such considerations have led a number of authors to suggest that PSI and cytochrome b 6 f in the stromal lamellae might be primarily involved in CET, with PSI at the margins and cytochrome b 6 f in the granal stacks being involved in linear electron transport Albertsson, ; Joliot et al. This effect was measured by determining the signal change in nm absorbance when a flash of saturating light was given to the leaf, followed by immediate transfer to total darkness.

This allowed the three types of reaction centre to be defined. Those that were reduced but could be oxidized by a flash; those that were oxidized and were reduced rapidly following a transition to darkness; and those whose redox state did not change during the measurement. Given the coincidence of the proportion of centres in the stromal lamellae and those that were inactive under optimal conditions, it is suggested that the latter was the case and that this difference in behaviour reflected their location in the membrane Golding and Johnson, If the above model is correct, it is necessary to consider what might be the activation step involved, what is the nature of the CET switch.

A recent paper by Rajagopal et al. The fact that the size of this population is similar to the pool of stromal lamellae PSI and of stress-activated PSI leads to the hypothesis that these three pools are identical and that CET is activated in response to an overreduction of the chloroplast stroma under stress conditions Fig.

The NADP H sensitivity of a pool of PSI might be explained by their different environment possibly involving interaction with lipids in the membrane or may indicate a difference in peptide composition.

Many of the minor polypeptides associated with PSI have poorly defined function and might not be uniformly distributed Jensen et al. Furthermore, some of these exists in different isoforms, including PsaE which has been implicated in CET in cyanobacteria Thomas et al.

Model for the organization and regulation of cyclic electron transport. Under optimal conditions moderate light, high CO 2 linear electron transport proceeds using PSI centres at the margins of the granal stacks. PSI centres in the stromal lamellae are inactive. These are involved in cyclic electron transport. Coincident with the activation of CET is an inhibition of linear electron flow Golding and Johnson, This can be explained according to two alternative models.

Early studies of the effects of pH on electron transport showed that electron flow through the cytochrome b 6 f complex is sensitive to the presence of a low pH in the thylakoid lumen Haehnel, There is little evidence for this occurring under steady-state conditions in plants Krieger et al.

In an alternative model, evidence has recently been published that the cytochrome b 6 f complex might be regulated by the redox potential of the chloroplast stroma, via a thioredoxin-linked mechanism Johnson, This requires confirmation as a physiological process, however, if this were the case, the coincidence of down-regulation of linear flow and activation of CET would be explained by their both responding to a common signal: stromal redox poise.

Measurements of the decay of the electrical field across the thylakoid membrane following a light—dark transition might give some clues to this. Comparing a leaf at high or low CO 2 , it could be expected that there would be a substantially different flux through the ATPase, in line with the different demands for ATP in the chloroplast.

However, measurements of the decay of the potential gradient reveal that the rate of this varies only slightly between these conditions Sacksteder et al. This might suggest that the rate of ATP consumption is also only weakly dependent on the activity of the Benson—Calvin cycle, however, this seems unlikely.

Alternatively, it is necessary to suggest that the decay of the field is only poorly coupled to ATP consumption under low CO 2 conditions, i. Evidence from isolated thylakoids indicates that the ATPase can become uncoupled in the absence of adenosine nucleotides, but it has been argued that in vivo concentrations of these will be sufficient to prevent such a slip Groth and Junge, However, it has also been discussed that high light conditions or a strong potential gradient might also induce such a slip Berry and Rumberg, After a history of 50 years, it is only recently that research into cyclic electron transport is starting to make progress.

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