How acidic is the lumen?

Proton motive force (pmf), established across the thylakoid membrane by pho tosynthetic electron transfer, functions both to drive the synthesis of ATP and initiate processes that down-regulate photosynthesis. At the same time , excessively low lumen pH can lead to the destruction of some lumenal comp onents and sensitization of the photosynthetic apparatus to photoinhibition . Therefore, in order to understand the energy budget of photosynthesis, it s regulation and responses to environmental stresses, it is essential to kn ow the magnitude of pmf, its distribution between Delta pH and the electric field (Delta phi) as well as the relationships between these parameters an d Delta G(ATP), and down-regulatory and inhibitory processes. We review pas t estimates of lumen pH and propose a model that can explain much of the di vergent data in the literature. In this model, in intact plants under permi ssive conditions, photosynthesis is regulated so that lumen pH remains mode rate (between 5.8 and 6.5), where it modulates the activity of the violaxan thin deepoxidase, does not significantly restrict the turnover of the cytoc hrome b(6)f complex, and does not destabilize the oxygen evolving complex. Only under stressed conditions, where light input exceeds the capacity of b oth photosynthesis and down-regulatory processes, does lumen pH decrease be low 5, possibly contributing to photoinhibition. A value of n = 4 for the s toichiometry of protons pumped through the ATP synthase per ATP synthesized , and a minor contribution of Delta phi to pmf, will allow moderate lumen p H to sustain the observed levels of Delta G(ATP).