Pool Size of Electrons That Can Be Donated to P700+ As Determined in Intact Leaves: Donation to P700+ from Stromal Components Via the Intersystem Chain

A method for estimation of the functional pool size of electrons that can be donated to P700+ after the illumination of intact leaves with actinic light is described. The complementary areas between the stationary level of P700+ attained by irradiation with far-red light and the oxidation curves of P700 by far-red light after a 50-ms multiple-turnover light (MT-area), and after the illumination of actinic light (AL-area) were determined. Since the MT-area represents the pool size of electrons in the intersystem chain, the ratio of the AL-area to the MT-area allows us to estimate the pooi size of electrons stored during actinic illumination that can be donated to P700+ The ratio of the AL-area to the MT-area was determined for intact leaves of several C3 plants, and it was found to increase to a value of two or three with increases in the intensity of actinic light, and it was further increased by anaerobiosis. The pool size of electrons that can be donated to P700+ from the stroma was estimated to be in the range of 12 and 28 electrons per P700 under aerobic conditions. These results indicate that stromal components donate electrons to P700+ through the intersystem chain in chloroplasts of C3 plants.