The extent to which the spatial separation between photosystems I and II associated with granal formation limits noncyclic electron flow in isolated lettuce chloroplasts.

Uncoupled noncyclic electron flow in stacked (granal) chloroplasts with a lateral heterogeneity in the distribution of the two photosystems has been compared with that in unstacked (agranal) chloroplasts with a near-uniform distribution. Chloroplasts were maintained in either structural state in the same assay medium so as to equalize effects of ionic composition which may influence reaction rates. The assay medium, an ion-deficient solution, was capable of supporting high rates of electron flow from water to methyl viologen. At high irradiance, unstacked chloroplasts exhibited an uncoupled rate which was 30% (in chloroplasts isolated from lettuce grown in low light) or 55% (in chloroplasts isolated from lettuce grown in high light) higher than that of stacked chloroplasts; the percentage remained relatively constant in the temperature range 7 to 22 degrees C for both high-light and low-light chloroplasts. At low irradiance, stacked low-light chloroplasts, despite the spatial separation of the two photosystems, gave higher rates of electron flow than did unstacked low-light chloroplasts. The addition of MgCl2 to stacked chloroplasts increased the uncoupled rate of noncyclic electron flow, but only at relatively high irradiances. The differences observed for stacked and unstacked chloroplasts, and for high-light and low-light chloroplasts are discussed. The approach taken in this work should be useful in other comparisons of stacked and unstacked chloroplasts.