| Various techniques such as photosynthetic gas exchange, chlorophyll fluorescence kinetics as well as inhibitor treatment were used to investigate the effect of inactivation of photosystem II reaction centers on 'high energy' quenching after streptomycin sulphite (SM) treatment; the changes of photosynthetic activity as well as the distribution of excited energy between Photosystem I and II in the presence of 1,4-dithiothreitol (DTT), which inhibits the violaxanthin de-epoxidase (VDE). In addition, the excited energy capture, energy distribution as well as photoprotective mechanisms were also explored in iron- and manganese-deficient plants.The main results are as follows:1. In the presence of SM (streptomycin sulphite), the maximum PS II quantum yield (Fv/Fm) was suppressed markedly when exposure to high irradiance. The activity of PSII reaction centers (1/Fo-1/Fm) as well as electron transport rate (ETR) of PSII in SM treatment maize leaves decreased more significantly under high irradiance compared to that in control. Non-photochemical quenching (NPQ) and de-epoxidation level of xanthophyll cycle components in maize leaves were enhanced by SM treatment. However, 'high energy' quenching (qE), a major components of NPQ declined gradually in SM poisoned maize leaves subjected to strong irradiance; photoinhibitory component (qI), another components of NPQ,was enhanced obviously. It was suggested that the decline of 'high energy' quenching through xanthophyll cycle in maize leaves is mainly due to decrease in electron transport rate by SM-treatment.2. In DTT poisoned soybean leaves, the maximum PS II quantum yield (Fv/Fm) was hardly affected, however, the chlorophyll fluorescence decrease ratio (Rfd) decreased drastically under 1400μmol m-2 s-1 irradiance. Efficiency of excited energy captured by open PSII centers (Fv'/Fm') was enhanced about 30-40% by DTT treatment, whereas, photochemical quenching (qP) was depressed about 40% compared with that in control under high irradiance. When leaves exposed to strong light, DTT treatment caused by 30% decreasing of the excited energy distributed to PSI , whenas, that distributed to PSII increasing about 20%, implying that serious imbalance of excited energy distribution was observed in DTT treated leaves. Though high excitation pressure (1- qP) was resulted from DTT treatment, non-photochemical quenching (NPQ) was suppressed in the presence of DTT. Further experiments revealed that DTT completely inhibited the formation of zeaxanthin (Z), and the state transition was also significantly depressed by DTT. It is suggested that the imbalance of excitation distribution between the two photosystems induced by DTT mainly due to the enhancement of excitation capture in PSII antenna.3. Photosynthesis in iron-deficient soybean and maize leaves decreased drastically. The quantum yield of PSII electron transport (ФPSII), the efficiency of excitation energy capture by open PSII reaction centers (Fv'/Fm'), and photochemical quenching coefficient (qp) under high irradiance were decreased significantly as a result of iron deficiency, but non-photochemical quenching (NPQ) increased markedly. The analysis of the polyphasic rise of fluorescence transient showed that iron depletion induced a pronounced K step both in soybean and maize leaves. The maximal quantum yield of PSII photochemistry (φpo) decreased only slightly, however, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than QA (ψo) and the quantum yield of electron transport beyond QA (φEo) in iron deficient leaves decreased more significantly compared with that in control. Based on these data, it was proposed that not only the donor side but also the acceptor of PSII was damaged in iron deficient soybean and maize leaves.The decrease in photosynthesis might not due to the severely lowered pigment content. It is evident from the polyphasic rise of fluorescence transient that iron deficiency induced an increase of inactivated PSII reaction...
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