Internal pH, pH, and the kinetics of electron transport in chloroplasts.

Abstract

The relation between the rate of electron transport, ΔpH, and internal pH was studied in lettuce chloroplasts, by following, in parallel, the fluorescence quenching of 9‐aminoacridine and the rate of electron transport. At low external pH (< 8.0) there was an initial burst in the rate of electron transport (R1) followed by a slower steady‐state rate (R2). However, at high external pH (> 8.5) R1 was slower than R2. Nevertheless, when analyzed as a function of internal pH, both R1 and R2 showed the same dependence with a maximal rate observed around internal pH 5.0. Thus, at low external pH, the initial pH is initially around 5.0 and, therefore, R1 is high, but as ΔpH is increased, the internal pH falls below 5.0 and R2 is lower. At high external pH, the internal pH is initially above 5.0 and, therefore, R1 is relatively low. As ΔpH increases the internal pH is lowered toward 5.0 and R2 becomes faster. A similar conversion of the ratio R1/R2 from R1/R2 > 1 to R1/R2 < 1 was also observed at constant external pH by varying the light intensity, or concentration of the uncoupler nigericin.In the presence of nigericin or at low light intensities, the maximal rate was shifted to higher internal pH values. At internal pH values above 5.0 in the presence of nigericin, the rate of electron transport was inversely proportional to ΔpH. These results indicate that the internal pH is not the only parameter that controls the rate of electron transport. It is shown that when all these diverse results are plotted as a function of an average pH (between the internal and external pH) they show a single optimum. Thus, it is suggested that the rate controlling site is embedded in the thylakoid membrane, and is a function of both the internal and external pH. This membrane pH and the size of ΔpH seem to be the major factors controlling the rate of electron transport in chloroplasts.