THE NONPHOTOCHEMICAL REDUCTION OF PLASTOQUINONE IN LEAVES

Although it is generally assumed that the plastoquinone pool of thylak oid membranes in leaves of higher plants is rapidly oxidized upon dark ening, this is often not the case. A multiflash kinetic fluorimeter wa s used to monitor the redox state of the plastoquinone pool in leaves. It was found that in many species of plants, particularly those using the NAD-malic enzyme C4 system of photosynthesis, the pool actually b ecame more reduced following a light to dark transition. In some Amara nthus species, plastoquinone remained reduced in the dark for several hours. Far red light, which preferentially drives Photosystem I turnov er, could effectively oxidize the plastoquinone pool. Plastoquinone wa s re-reduced in the dark within a few seconds when far red illuminatio n was removed. The underlying mechanism of the dark reduction of the p lastoquinone pool is still uncertain but may involve chlororespiratory activity.