The variable chlorophyll (Chl) a fluorescence yield is known to be related to the photochemical activity of photosystem I1 (PSII) of oxygen-evolving organisms. The kinetics of the fluorescence rise from the minimum yield, F,, to the maximum yield, F,, is a monitor of the accumulation of net reduced primary bound plastoquinone (QA) with time in all the PSII centers. Using a shutter-less system (Plant Efficiency Analyzer, Hansatech, UK), which allows data accumulation over several orders of magnitude of time (40 11s to 120 s), we have measured on a logarithmic time scale, for the first time, the complete polyphasic fluorescence rise for a variety of oxygenic plants and cyanobacteria at different light intensities. With increasing light intensity, the fluorescence rise is changed from a typical 0-I-P characteristic to curves with two intermediate levels J and I, both of which show saturation at high light intensity but different intensity dependence. Under physiological conditions, Chl a fluorescence transients of all the organisms examined follow the sequence of 0-J-I-P. The characteristics of the kinetics with respect to light intensity and temperature suggest that the 0-J phase is the photochemical phase, leading to the reduction of QA to QA-. The intermediate level I is suggested to be related to a heterogeneity in the filling up of the plastoquinone pool. The P is reached when all the plastoquinone (PQ) molecules are reduced to PQH2. The addition of 3-(3-4-dichIorophenyl)-1,l -dimethylurea leads to a transformation of the 0-J-I-P rise into an 0-J rise. The kinetics of 0-J-I-P observed here was found to be similar to that of 0-1,-12-P, reported by Neubauer and Schreiber (2. Naturforsch. 42c, 1246-1254, 1987). The biochemical significance of the fluorescence steps 0-J-I-P with respect to the filling up of the plastoquinone pool by PSII reactions is discussed.
The process of photosynthesis is initiated by the capture of sunlight by a network of light-absorbing molecules (chromophores), which are also responsible for the subsequent funneling of the excitation energy to the reaction centers. Through evolution, genetic drift, and speciation, photosynthetic organisms have discovered many solutions for light harvesting. In this review, we describe the underlying photophysical principles by which this energy is absorbed, as well as the mechanisms of electronic excitation energy transfer (EET). First, optical properties of the individual pigment chromophores present in light-harvesting antenna complexes are introduced, and then we examine the collective behavior of pigment-pigment and pigment-protein interactions. The description of energy transfer, in particular multichromophoric antenna structures, is shown to vary depending on the spatial and energetic landscape, which dictates the relative coupling strength between constituent pigment molecules. In the latter half of the article, we focus on the light-harvesting complexes of purple bacteria as a model to illustrate the present understanding of the synergetic effects leading to EET optimization of light-harvesting antenna systems while exploring the structure and function of the integral chromophores. We end this review with a brief overview of the energy-transfer dynamics and pathways in the light-harvesting antennas of various photosynthetic organisms.
Tocopherols (vitamin E) are lipid soluble antioxidants synthesized by plants and some cyanobacteria. We have earlier reported that overexpression of the gamma-tocopherol methyl transferase (gamma-TMT) gene from Arabidopsis thaliana in transgenic Brassica juncea plants resulted in an over six-fold increase in the level of alpha-tocopherol, the most active form of all the tocopherols. Tocopherol levels have been shown to increase in response to a variety of abiotic stresses. In the present study on Brassica juncea, we found that salt, heavy metal and osmotic stress induced an increase in the total tocopherol levels. Measurements of seed germination, shoot growth and leaf disc senescence showed that transgenic Brassica juncea plants overexpressing the gamma-TMT gene had enhanced tolerance to the induced stresses. Analysis of the chlorophyll a fluorescence rise kinetics, from the initial "O" level to the "P" (the peak) level, showed that there were differential effects of the applied stresses on different sites of the photosynthetic machinery; further, these effects were alleviated in the transgenic (line 16.1) Brassica juncea plants. We show that alpha-tocopherol plays an important role in the alleviation of stress induced by salt, heavy metal and osmoticum in Brassica juncea.
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