For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757

Physical mechanisms of generation and deactivation of singlet oxygen.

Crop leaves in full sunlight dissipate damaging excess absorbed light energy as heat. When sunlit leaves are shaded by clouds or other leaves, this protective dissipation continues for many minutes and reduces photosynthesis. Calculations have shown that this could cost field crops up to 20% of their potential yield. Here, we describe the bioengineering of an accelerated response to natural shading events in Nicotiana (tobacco), resulting in increased leaf carbon dioxide uptake and plant dry matter productivity by about 15% in fluctuating light. Because the photoprotective mechanism that has been altered is common to all flowering plants and crops, the findings provide proof of concept for a route to obtaining a sustainable increase in productivity for food crops and a much-needed yield jump.

/pdf/improving-photosynthesis-and-crop-productivity-by-3bkz3nefnt.pdf

Improving photosynthesis and crop productivity by accelerating recovery from photoprotection.

The role of the xanthophyll cycle and of lutein in photoprotection of photosystem II

Antioxidant functions of carotenoids.

https://science.sciencemag.org/content/sci/162/3857/963.full.pdf

Mechanisms of Photosensitized Oxidation

Studies on the light and dark interconversions of leaf xanthophylls.

— Eleven strains of microorganisms were examined for their susceptibility to death by irradiation with 21–30 mW of light at 6328 A from a continuous-wave gas laser. Washed cells of seven species were rapidly killed when irradiated aerobically in aqueous solutions of toluidine blue: Sarcina lutea (two strains), Escherichia coli, Chromobacterium violaceum, Arthrobacter atrocyanus, Pseudomonas aeruginosa, Saccharomyces cerevisiae, and Rhodotorula glutinis. Cells remained viable when irradiated anaerobically in toluidine blue. This is not proof that oxygen is required for death since toluidine blue is subject to reduction to the leuco form under anaerobic conditions. None of the microbes tested, including cells of Bacillus cereus, Rhodospirillum rubrum, and Euglena gracilis, were killed in aerated suspensions without toluidine blue. Thus, it is unlikely that any strain has an endogenous photosensitizer capable of acting at 6328 A. However, more than 95 per cent of the cells of R. rubrum lost their motility. A colorless mutant of S. lutea began dying 10 min faster than the wild strain, which is known to be protected from photosensitized death by carotenoids for at least 2 hr when irradiated in toluidine blue with 1000 ft candles of tungsten light. Since toluidine blue is bleached in irradiated cell suspensions, logarithmic death ceases within an hour with 2·5 × 10=6 M toluidine blue. Yeasts are less susceptible than bacteria, exhibiting longer lags before death begins. The size of colonies produced by irradiated yeast cells decreases with irradiation time. This suggests that multiple hits are required for death.

Lethal photosensitization of microorganisms with light from a continuous-wave gas laser.

Biosynthetic origin of oxygen in the leaf xanthophylls

Summary

The incorporation of oxygen from water enriched in H218O into the leaf xanthophylls during the Hill reaction was investigated. No enrichment was detectable in violaxanthin and neoxanthin. A low incorporation into lutein, which was not dependent on the Hill reaction, was observed.



The results indicate that the epoxy pigments do not function as an oxygen transport system in photosynthesis under the conditions of these experiments.

Xanthophylls and the hill reaction

— The sensitivity of older cells of Rhodotorula glutinis containing a high concentration of carotenoids was compared with young cells containing little visible pigmentation. The younger cells died faster, suggesting that the carotenoids protected. Further, cells grown in the presence of diphenylamine, an inhibitor of carotenoid syntheses, were more susceptible than were normally grown cells. A carotenoidless mutant, isolated after exposure of the wild strain to u.v. light, was more susceptible to lethal photosensitization than the wild strain. The difference in sensitivity, however, was not as great as between old and young cultures of the wild strain. Young cultures of the white mutant were more susceptible to death than were older cultures, therefore, sensitivity is related to age as well as to carotenoid content. The difference in sensitivity was increased between the white mutant and the wild strain when lower light intensities were used. At 10 mW the difference in the lag period was 60 min, while at 30 mW the difference was only 10 min. The lag period was also dependent on the temperature at which cell suspensions were irradiated. At 3°C, no death occurred in 1 hr at 27 mW with either wild or mutant strains. The lag was progressively shorter with increased temperature. The rate of death, however, was apparently not affected by differences in temperature.



An analysis of the carotenoids from the irradiated cells did not show any detectible changes even though it was demonstrated that β-carotene could be bleached when irradiated in vitro in the presence of toluidine blue.

Function of carotenoids in protection of rhodotorula glutinis against irradiation from a gas laser

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