Lars-Göran Sundblad

S-state distribution and redox state of QA in barley in relation to luminescence decay kinetics

The decay of luminescence from barley showed a relative maximum that appeared when far-red preillumination was carried out in the presence of oxygen, but not if white light was used for pre-illumination or when oxygen was omitted. Far-red illumination kept the S-states of the water-splitting system randomized and the primary Photosystem II acceptor, Q A , oxidized. It was furthermore found that state S 3 was longer lived in the dark after far-red excitation than after white-light excitation. The presence of O 2 during and after far-red excitation also increased the lifetime of S 3 in the dark. Q A exhibited the most oxidized statein the presence of O 2 and immediately after far-red excitation after which it was partially reduced back to a steady-state level, in the dark, by reverse electron flow. It is concluded that the relative maximum during the decay of far-red excited luminescence was a result of preservation of S 2 , together with partial reduction of Q A by reverse electron flow,in the dark. The relative luminescence maximum was inhibited by the energy-transfer inhibitor tentoxin and its kinetics were altered by the phosphate translocator inhibitor 4,4-diisocyano-2,2′-disulfonic acid stilbene. It is suggested that the reverse electron flow was a result of reverse coupling.

A correlation between changes in luminescence decay kinetics and the appearance of a CO2-accumulating mechanism in Scenedesmus obliquus.

In experiments with the unicellular green algae Scenedesmus obliquus a correlation was found between the presence of the CO2-accumulating mechanism and the appearance of polyphasic luminescence decay kinetics. A potentiometric titration method was used to measure and calculate photosynthetic carbon uptake.Polyphasic luminescence decay kinetics was found when the algae showed photosynthetic characteristics typical of algae adapted to low-CO2 conditions. When high-CO2 grown algae were transferred to low-CO2 conditions they gradually developed polyphasic decay kinetics during the first 25–30 minutes. When low-CO2 grown algae were transferred to high-CO2 conditions the polyphasic decay kinetics disappeared. To account for these results a working hypothesis is presented on the basis of the energy requirement for a CO2-accumulating mechanism.

FAR-RED STIMULATED LONG-LIVED LUMINESCENCE FROM BARLEY PROTOPLASTS

Abstract The far-red stimulated luminescence decay kinetics, which in intact leaves from higher plants shows a relative maximum 30–90 s after excitation was studied on barley (Hordeum vulgare) protoplasts. When protoplasts were preilluminated with white light prior to far-red excitation, the luminescence maximum was consistently observed. The effect of the inhibitors 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), hydroxylamine, antimycin A and glycoladehyde and the uncoupler carbonylcyanide -p- trifluoromethoxyphenylhydrazone (FCCP) showed that: (1) The luminsecence expressed as a maximum is emitted from PS II, although it is stimulated through PS I. (2) The maximum is dependent on: reverse electron flow utilizing parts of both the cyclic, and the linear electron transport pathways; a transthylakoid ΔpH. (3) Operation of a complete Calvin cycle is not necessary for the maximum to appear. We conclude that protoplasts are a suitable experimental system for further work on the underlying mechanism behind the far-red stimulated relative maximum in the minute range decay of luminescence.

Luminescence decay kinetics in relation to the relaxation of the transthylakoid ΔpH from high and low CO2 adapted cells of Scenedesmus obliquus

Luminescence decay kinetics was shown to be polyphasic with two relative maxima observed, when high CO2 adapted cells of Scenedesmus obliquus were stressed by CO2 deficiency. Low CO2 adapted cells exhibited a single maximum. Luminescence from high CO2 adapted cells under high CO2 conditions decayed asymptotically without maxima. The magnitude of the transthylakoid ΔpH was shown to be related to the intensity of luminescence in an antiparallel way. The relaxation kinetics of the ‐ΔpH was shown to be complex, with an interval of increasing or maintained ΔpH. A working hypothesis is presented in order to explain polyphasic luminescence decay kinetics in terms of luminescence quenching by the transthylakoid ΔpH.

Dark reduction of QA in intact barley leaves as an effect of lowered CO2 concentration monitored by chlorophyll a luminescence and chlorophyll a F0 dark fluorescence

When the CO2 concentration in the atmosphere above an intact barley leaf was lowered in the dark after illumination, chlorophyll a luminescence and chlorophyll a dark fluorescence were stimulated. The stimulation was induced by lowered levels of CO2 in a wide concentration range including concentrations well above that saturating photosynthesis. The stimulation of luminescence by lowered CO2 concentrations was more pronounced after far-red excitation than after white light excitation. The difference in response to lowered CO2 concentrations after white/far-red excitation was less pronounced for fluorescence than for luminescence. Stimulation of luminescence was more pronounced when the CO2 concentration was lowered in an O2-containing atmosphere than under anaerobic conditions. It is concluded that lowering of the CO2 concentration in the dark after illumination causes a partial reduction of the primary Photosystem II acceptor QA.

Luminescence decay kinetics in relation to quenching and stimulation of dark fluorescence from high and low CO2 adapted cells of Scenedesmus obliquus and Chlamydomonas reinhardtii

Two green algal species, Chlamydomonas reinhardtii and Scenedesmus obliquus, exhibited a relative maximum during the decay of luminescence, when adapted to low CO2 conditions that was not observed in high CO2 adapted cells.From the kinetics of transient changes in the level of dark fluorescence, after illumination and parallel to the luminescence maxima, it was concluded that the maximum in Scenedesmus was mainly related to a decrease in nonphotochemical quenching, whereas in Chlamydomonas the maximum was mainly related to a dark reduction of the primary PS II acceptor QA.ATP/ADP ratios from low CO2 adapted Scenedesmus showed transient high levels after a dark/light transition that was not observed in high CO2 adapted cells. After 30 s of illumination the ATP/ADP ratios however stabilized at the same steady state level as in high CO2 adapted cells.Dark addition of HCO3- to low CO2 adapted cells of Chlamydomonas resulted in a rapid transient quenching of luminescence that was not observed in low CO2 adapted cells of neither species.It is concluded that the luminescence maxima present in both low CO2 adapted Scenedesmus and Chlamydomonas reflect adaptation of the cells to low CO2 conditions. It is further suggested that the difference in mechanistic origin of luminescence maxima in the two species reflects differences in adaptation.

Photosynthetic Acclimation to Low Carbon Concentrations in Chlamydomonas reinhardtii

Microalgae, have developed a CO2 concentrating mechanism which enables them to accumulate inorganic DIC inside the cells to concentrations far above those outside [1]. This increases the ratio of CO2/O2 inside the chloroplasts and the apparent affinity for CO2 of intact cells.