Photosynthetic control in broken spinach chloroplasts

Abstract

1. Introduction West and Wiskich [l] showed that the rate of photosynthetic electron transport in class 1 pea chloro- plasts was regulated by the presence or absence of the intermediates required for photophosphorylation. By analogy with respiratory control in mitochondria [2] they termed this regulation photosynthetic control. West and Wiskich [l] showed that in the presence of magnesium and phosphate the initial rate of elec- tron transport was stimulated by addition of ADP. After conversion of all of the ADP to ATP the rate of electron transport decreased to a rate lower than the initial rate (i.e. to the controlled rate). The ratio of the fast phosphorylating rate to the slow controlled rate was termed the photosynthetic control (PC) ratio. Kraayenhof [3] obtained similar results with class 1 chloroplasts. West and Wiskich [l] showed that the PC ratio was decreased by exposing their whole chloroplasts to increasing periods of sonic oscillation and therefore concluded that photosynthetic control is dependent on the structural integrity of the chloroplasts. However, the loss of control in their experiments was accom- panied by a decrease in the ADP/O ratio. Earlier workers using different measurement tech- niques had shown that the rate of electron transport was higher under phosphorylating that under non- phosphorylating conditions [4] and that the rate of electron transport decreased when all the ADP was phosphorylated [5] . We have reinvestigated photosynthetic control, in chloroplasts broken by osmotic shock, Broken chloro- plasts retain the ability to catalyse photophosphoryla-