Photoinhibitory damage

These two hypotheses for the nature of the phenotype of the mutant may yet prove to be compatible. The recently proposed scheme for the participation of cyt b559 in cyclic electron flow around PSn and protection of the reaction center from photoinhibitory damage may explain both the redox chemistry and protein assembly role of this cytochrome in PS II (8). Absence of cyt b559, and its associated protective redox reactions, may lead to increased photoinhibitory damage that would cause increased reaction center protein turnover. This increased turnover would then be observed as lack of protein assembly. It follows from this hypothesis that there should be environmental conditions that preclude photoinhibition and allow PS II assembly and function. Research in our laboratory is currently directed towards assessing the possible role of photoinhibition in the cyt b559 mutants. 

As expected the sensitivity of thylakoids to photoinhibitory damage is dependent on chlorophyll concentration. Low chlorophyll concentrations lead to more rapid inhibition of electron transport probably as a consequence of a higher exposition of membranes to light. 

Fig 2 shows that the decrease in the fluorescence ratio reflects decreased photosynthetic rates. This is consistent with photoinhibitory damage to the photosynthetic pathway. 

The kinetics of fluorescence induction in DCMU-poisoned thylakolds isolated from control, HL-chill and LL-chill treated leaves were measured and values of Fo, Fm, Fv and Fv/Fm determined (Table 1>. HL-chill treatment produced a large decrease in Fv/Fm, which when considered in conjunction with the decrease in PS2 electron transport (Fig. 2) and the previously observed decrease in atrazlne-bindlng capacity (3) is indicative of photoinhibitory damage to PS2. 

In this work we study the violaxanthin cycle and chlorophyll-a fluorescence in iron-deficient maize plants highly susceptible to photoinhibitory damage due to the low content of photosynthetic pigments. 

Alia, Pardha Saradhi P, Mohanty P. Proline enhances primary photochemical activities in isolated thylakoid membranes of Brassica juncea by arresting photoinhibitory damage. Biochem Biophys Res Commun 1991 181 1238-1244. 

Gilmore AM, Hazlett TL, Debrunner PG and Govindjee (1996b) Comparative time-resolved Photosystem II chlorophyll a fluorescence analyses reveal distinctive differences between photoinhibitory reaction center damage and xanthophyll cycle dependent energy dissipation. Photoschem Photobiol 64 552-563... 

The biochemistry of the violaxanthin cycle is well-characterised (1) but no clear function of this cycle, in relation to photosynthesis, has been demonstrated. Demmig et al, (2, 3) have suggested that zeaxanthin formation is related to a special function of this carotenoid under photoinhibitory conditions that serves to prevent damage. The xanthophyll cycle thus may play a crucial role in the protection of the photosynthetic apparatus... 

Common to all conditions in which there is an extrachloroplastic source of oxidising species, are both the production of xanthophyll acyl esters and the relative stability of 6-carotene particularly in the latter stages of treatments. This is a consequence of conditions prevalent in the cell once the pigments have been released into the cytosol. These are not seen in photoinhibitory or other conditions that lead to pigment destruction in which the source of damage is closely associated with photosynthesis itself. Similarly the production of a photooxidative product of 6-carotene, 6-carotene-5,6-epoxide, is associated with the latter and has not been seen under the former conditions. 

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