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Imbalance in Excited Energy Distribution Between Two Photosystems Induced by Feeding Dithiothreitol to Soybean Leaves

Chlorophyll fluorescence technique was used to investigate the effect of 1,4-dithiothreitol(DTT) on the distribution of excited energy between photosystem I(PS I) and photosystem II (PS II) in soybean leaves under high irradiance. The maximum PS II quantum yield(F v/F m) was hardly affected by the presence of DTT (Table 1,Fig.1), however, the chlorophyll fluorescence decrease ratio (R fd ) depressed drastically under 1 400 μmol m -2 s -1 irradiance(Fig.2). Efficiency of excitation captured by open PS II centers (F v′/F m′) was enhanced about 30%-40% by DTT treatment(Fig.3a), whereas photochemical quenching (q P) was depressed by about 40% (Fig.3b) by high irradiance. DTT treatment caused a 30% decrease in allocation of excited energy to PS I under strong light(Fig.4a) and a 20% increase to PS II(Fig.4b). Serious imbalance of excited energy distribution was observed in DTT-treated leaves(Fig.5). Though high excitation pressure(1-q P) resulted from DTT treatment(Fig.6a) , non-photochemical quenching(q N) was lowered(Fig.6b). Other experiments revealed that DTT completely inhibited the formation of zeaxanthin (Z)(Table 2), and also depressed the state transition(q T) significantly(Fig.7). It is suggested that the imbalance of excitation distribution between the two photosystems induced by DTT was mainly due to the enhancement of excitation energy capture by PS II antenna, and the imbalance induced the depression of R fd and activity of photosynthesis.