Plant photosystems, mechanism

In all covalent electron donor-acceptor systems produced earlier, triplet states observed by EPR were formed via a spin-orbit intersystem crossing (SO-ISC) mechanism. Another possible mechanism of triplet formation is RP-ISC, mentioned above, which results from radical ion pair recombination, and which had been observed previously by time-resolved electron paramagnetic resonance spectroscopy (TREPR) only in bacterial reaction centers and in the green plant Photosystem I and II reaction centers. These two mechanisms can be differentiated by the polarization pattern of the six EPR transitions at the canonical orientations. In SO-ISC,... 

Fig. 15. Plot ofthe extent of absorbance change due to P7007P430" recombination measured at 695 nm vs. the redox potential of 14 quinones and 7 non-quinone carbonyl compounds, the fluorenones (individual compounds are identified below the plot). The solid curve is the theoretical, one-electron Nernst curve centered near the redox potential of FeS-X in vivo. Data adapted from Itoh and Iwaki (1992) Exchange ofthe acceptor phylloquinone by artificial quinones and fluorenones in green plant photosystem I photosynthetic reaction center. In N Malaga, T Okada and H Masuhara (eds) Dynamics and Mechanism of Photoinduced Transfer and Related Phenomena. p.533. Elsevier,...

The present paper is a discussion of the photosystem II herbicides and their mechanisms of action. Among the topics covered are the green plant photosystems, photochemistry and electron transfers within photosystem II, requirements for herbicidal activity, mechanisms of action, herbicide selectivity and resistance, herbicide-binding proteins, and theoretical studies of herbicidebinding site interactions. 

This is why the plant had to develop the much more complex mechanism of two photosystems operating in series (see Figure 2). 

Higher plants and algae have developped an adaptation mechanism to cope with unbalanced excitation of the two photosystems. This mechanism, first described as producing State transitions (Bonaventura and Myers, 1969), corresponds to a change in antenna size of each photosystem the antenna size of the less excited photosystem increases at the expense of that of the overexcited photosystem. 

Several factors affect the salt sensitivity of plants. We first evaluated the effect of light intensity on the salt-stress. Figure 2 clearly showed that tobacco seedlings were more severely affected by salt stress under high light intensity both on the basis of chlorophyll content and fresh wei t increase. This result indicated that light, probably photosynthetic process, would be involved in the salt-stress. To characterize the salt-stress mechanism, we examine the effect of salt on the photosynthetic activities of isolated thylakoid membranes. Previously, we reported that the presence of salt in assay inhibited the photosystem II activity of tobacco thylakoid membranes but not photosystem I activities (Murota et al., 1994). Then, we further examined the effect of salt on the irreversible photodamage of thylakoid membrane activity. 

AG Voikov (1986) The molecular mechanism of functioning of photosystem II in higher plants A hypothesis. Photobiochem Photobiophys 11 1-7... 

AG Voikov (1987) Thylakoid membrane electrochemical mechanism of photosynthesis. The mechanism of oxygen evolution in the reaction center of photosystem II of green plants. Bioi Membr 4 984-993... 

Cyclic nitramine compounds such as RDX and HMX appear to have different modes of action in plants compared to the nitroaromatic compounds. Their basic structure and behavior in plants are similar to those of the triazine herbicides such as atrazine and simazine, which inhibit photosystem II in photosynthesis [32], The inhibition of photosynthesis may also be one of the modes of toxicity of cyclic nitramines based on symptoms such as chlorosis, necrosis, yellow leaf spots, and anthocyanin expression [27], However, the mechanism(s) of phytotoxicity of cyclic nitramines have not been investigated thoroughly. 

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