ELECTRON FLOW TO THE INTERSYSTEM CHAIN FROM STROMAL COMPONENTS AND CYCLIC ELECTRON FLOW IN MAIZE CHLOROPLASTS, AS DETECTED IN INTACT LEAVES BY MONITORING REDOX CHANGE OF P700 AND CHLOROPHYLL FLUORESCENCE

The functional pool size of electrons in the intersystem chain of the chloroplasts of maize was estimated to be about 25 per P700 by the redox change in P700 with single- and multiple-turnover lights under far-red light in intact leaves. This is about twice the pool size observed in C3 plants. Furthermore, the stromal pool size of electrons that can be donated to P700+ after actinic illumination was larger in maize leaves than in leaves of C3 plants, giving a maximum value of 225 electrons per P700. Maize leaves showed an increase in the yield of modulated Chl fluorescence after turning off of actinic light, which confirms the donation of electrons in the dark to the inter-system chain from the stromal donors that accumulated during actinic illumination. We propose that the mesophyll chloroplasts are responsible for a high level of electron-donating activity to the intersystem chain from stromal donors such as triose phosphates and malate with NADPH as an intermediate. The level of P700+ under strong far-red light was decreased after actinic illumination, suggesting the operation of an actinic light-triggered cyclic electron flow in chloroplasts of the bundle sheath cells.