The effect of elevated temperature on electron flow to plastoquinone pool and to PSI from sources alternative to PSII was studied in barley (Hordeum vulgare L.) and maize (Zea mays L.) leaves. Alternative electron flow was characterized by measuring variable fluorescence of chlorophyll and absorption changes at 830 nm that reflect redox changes of P700, the primary electron donor of PSI. The treatment of leaves with elevated temperature resulted in a transient increase in variable fluorescence after cessation of actinic light. This increase was absent in leaves treated with methyl viologen (MV). The kinetics of P700+ reduction in barley and maize leaves treated with DCMU and MV exhibited two exponential components. The rate of both components markedly increased with temperature of the heat pretreatment of leaves when the reduction of P700+ was measured after short (1 s) illumination of leaves. The acceleration of both kinetic components of P700+ reduction by high-temperature treatment was much less pronounced when P700+ reduction rate was measured after illumination of leaves for 1 min. Since the treatment of leaves with DCMU and MV inhibited both the electron flow to PSI from PSII and ferredoxin-dependent cycling of electrons around PSI, the accelerated reduction of P700+ indicated that high temperature treatment activated electron flow to PSII from reductants localized in the chloroplast stroma. We conclude that the lesser extent of activation of this process by elevated temperature after prolonged illumination of heat-inhibited leaves is caused by depletion of the pool stromal reductants in light due to photoinduced electron transfer from these reductants to oxygen.
Russian Journal of Plant Physiology – Springer Journals
Published: Oct 11, 2004
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