ALL-trans β-CAROTENE-5,6-EPOXIDE IN THYLAKOID MEMBRANES

All- trans β-carotene-5,6-epoxide has been found in the thylakoid membranes of spinach and of the cyanobacterium Synechococcus vulcanus Copeland. The epoxide was extracted from the thylakoid membranes with acetone, and was isolated by high-performance liquid chromatography (HPLC). The structure of the epoxide was identified by means of mass, Raman, and electronic absorption spectroscopy. Changes in the amount of the epoxide, as a result of epoxidation and (apparent) de-epoxidation reactions in the membranes, were traced by analysis of extracts on HPLC. In isolated thylakoid membranes, only the epoxidation reaction took place. The reaction was caused by irradiation or by the addition of ferricyanide, suggesting that electron transport reactions in the membranes are involved in the epoxidation. In intact spinach leaves, however, both epoxidation and de-epoxidation took place; the extent of epoxidation correlated with the intensity of light incident on the leaves. The epoxidation and de-epoxidation of all- trans β-carotene are contrasted with those of xanthophylls (in the violaxanthin cycle).     

ACTION SPECTRA FOR BIOSYNTHESIS OF CHLOROPHYLLS a AND b AND β-CAROTENE

Abstract— Action spectra for the formation of chlorophyll b and β-carotene were determined with etiolated wheat leaves and compared with the action spectrum for the formation of chlorophyll a determined for the same samples. The action spectra were measured with etiolated leaves which had been pre-illuminated for 10 min and incubated in the dark for 4 h to eliminate induction of pigments. The action spectra for chlorophyll b and for β-carotene accorded with the action spectrum for chlorophyll a and with the absorption spectrum of protochlorophyllide in intact etiolated leaves. It is postulated from this result with chlorophyll b that this pigment is formed from protochlorophyllide through chlorophyll a or some intermediates to chlorophyll a. Complexing between chlorophylls and β-carotene and proteins is postulated to interpret the action spectrum for β-carotene. It is assumed that the low concentration of chlorophylls formed photochemically limits the rate of complexing, and that consumption of β-carotene for the complexing induces formation of new β-carotene.     

DEVELOPMENT OF THERMOLUMINESCENCE BANDS DURING GREENING OF WHEAT LEAVES UNDER CONTINUOUS AND INTERMITTENT ILLUMINATION

Abstract— Mature wheat leaves excited by 1-min illumination at a low temperature of -60° C showed five thermoluminescence bands at -45, -10, +25, +40 and +55° C (denoted as Z_u, A, B₁ B₂ and C bands, respectively). The development of these bands during greening of etiolated wheat leaves under continuous and intermittent illumination was investigated, and the following results were obtained. (1) Etiolated leaves showed only the C band. When these leaves were greened under continuous light, the B₁ and B₂ bands appeared at 3 h and the Z_u band appeared at 10 h. The B₁ and B₂ bands were intensified during prolonged greening under continuous illumination to be the strong bands observed for mature leaves. The A band and the group of B₁ and B₂ bands were alternative: Similar experiments by excitation of thermoluminescence at -20° C showed the development of the A band instead of these B₁ and B₁ bands. (2) When the etiolated leaves were greened under intermittent illumination (1-ms light + 5-min dark), the Z_u band first appeared after 5 h of illumination (60 flashes) and was gradually intensified during further illumination with 340 flashes but, interestingly, neither the B₁ nor the B₂ band appeared even after 24–28 h of illumination with 280–340 flashes. (3) On exposure of such leaves greened under intermittent illumination to continuous light, the B₁ and B₂ bands were rapidly developed. The appearance of these bands was accompanied by the generation of the Hill activity (DCIP photoreduction with water as electron donor). (4) These results were discussed in relation to the previous study on photoactivation of the latent water-splitting system accumulated in the leaves greened under intermittent illumination. It was deduced that the structure responsible for the A band or the group of B₁ and B₂ bands is involved in the evolution of oxygen in chloroplasts, and probably involves cations of the Mn²⁺ catalyst generated by the action of light.     

INFLUENCE OF CHANGES IN THE PHOTON PROTECTIVE ENERGY DISSIPATION ON RED LIGHT-INDUCED DETRAPPING OF THE THERMOLUMINESCENCE Z-BAND

-Thermoluminescence emission at 110 K (Z-band) was markedly diminished when thylakoid membranes were exposed to red light during or after Z-band charging with blue light. Analysis of this phenomenon showed that deactivation of Z-band-emitting chlorophyll species occurred preferentially on the low temperature side of the glow curve, and red light of670–680 nm was most efficient in the deactivation. In order to test our hypothesis that this detrapping is related to local heating effects caused by dissipation of absorbed energy, we measured thermoluminescence glow curves and Z-band emission spectra from spinach leaf discs and thylakoid membranes during induction of nonphotochemical chlorophyll fluorescence quenching. Pretreatment of the plant material was designed to achieve different levels of (1) de-epoxidized xanthophylls in the photosynthetic apparatus and (2) the proton concentration in the thylakoid lumen. In comparison, measurements were performed in aggregated and trimeric light-harvesting pigment-protein complexes of photosystem II. We observed on all three levels of organization that a higher capacity of excitation energy dissipation was accompanied by a stronger red light-induced detrapping of Z-band thermoluminescence.