STIMULATION OF PHOTOPHOSPHORYLATION BY ASCORBATE AS A FUNCTION OF LIGHT-INTENSITY

When isolated, stroma-free thylakoids are illuminated in the presence of ADP and orthophosphate in the absence of any electron acceptor exce pt O-2, the addition of ascorbate stimulates electron transport throug h the formation of the radical monodehydroascorbate and the coupled sy nthesis of ATP (G. Forti and G. Elli [1995] Plant Physiol 109: 1207-12 11). The stimulation is shown here to be higher at low light intensity . These observations are explained in terms of the increase of the ele ctron transport rate by ascorbate, which established a higher value of the steady-state pH gradient, causing activation of ATP synthase, whi ch is known to be dependent on the level of the H+-electrochemical pot ential difference, and a higher rate of proton flux across the membran es available for utilization by ATP synthesis.