We review the literature on the control of photosynthetic electron transport in intact plants, and the instrumentation currently available for exploring these reactions.
Under conditions of full ambient sunlight, the rate of photosynthesis is determined by three main parameters: the availability of substrate, the flux of excitation to the photosystems, and the sensitivity of the electron transfer reactions to low lumenal pH. Control of these three is finely tuned so that delivery of excitation is matched to substrate availability, and the lumenal pH is maintained above inhibitory levels. We discuss the reactions of the electron transfer chain which might be effected by the proton gradient, and suggest that an important function of the diversion of excitation away from photochemistry is to prevent the lumenal pH from dropping into an inhibitory range. As substrate (usually CO2) is depleted, the metabolite pools back up, and the proton gradient builds up. The main mechanism for control of photosynthetic electron transport is through a change of function of the antenna apparatus under these conditions from light-harvesting to exciton dumping. The switch results in a lowering of fluorescence (nonphotochemical quenching), as one or more dissipative pathways are activated which compete with both photochemistry and fluorescence. The switching signal is the lowering of the lumenal pH on generation of the proton gradient, and the evidence suggests that the mechanism reflects changes in the minor light-harvesting complexes (CP29, CP26 and CP24), possibly effecting ligation of chromophores. The amplitude of the fluorescence lowering is determined by the extent of de-epoxidation of violaxanthin in these complexes.
In order to explore these processes we must be able to assay the flux and poise of the partial reactions, including those of excitation delivery, electron transfer, the proton gradient and the metabolic acceptor pools, under the steady state conditions pertaining at maximal photosynthetic rates. We discuss the instrumentation available, and the limitations of methods based on measurement of parameters (fluorescence, delayed light emission, thermal radiometry) which reflect the interplay of many processes. These limitations can to some extent be overcome by selection of conditions in which only one or a few processes dominate. In the case of steady state fluorescence measurements, the saturation pulse technique has been used to distinguish photochemical and non-photochemical quenching. Since under physiological conditions the latter is dominated by fluorescence lowering, this has proved a useful practical approach, but at the expense of a simplification which precludes any detailed exploration of partial reactions. Deconvolution of fluorescence yield changes in the sub-ms time domain has proved a useful tool for exploring partial reactions close to PS II. Over the last five years, several groups using different technical approaches have developed spectrophotometers for measurement of absorbance changes in intact leaves, which have brought a new dimension to the analysis of photosynthesis in whole plants. Many of the reactions of the electron transfer chain, the xanthophyll cycle and the proton circuit can be measured directly. With the further development of a database of reliable spectra to aid deconvolution of overlapping changes, absorbance spectrophotometry can provide the specificity and sensitivity needed for analysis of partial reactions. We suggest that protocols based on perturbation of the steady state, and the simultaneous measurement of several parameters will be most rewarding. We note that future development in this field is hampered by the restricted availability of suitable instruments.