Balancing the central roles of the thylakoid proton gradient. 
The photosynthetic electron transfer chain generates proton motive force (pmf), composed of both electric field (Deltapsi) and concentration (DeltapH) gradients.  Both components can drive ATP synthesis, whereas the DeltapH component alone can trigger feedback regulation of the antenna.  It has often been suggested that a relatively large pmf is needed to sustain the energetic contributions of the ATP synthase reaction (DeltaG(ATP)), and that the Deltapsi component is dissipated during illumination, leading to an acidic lumen in the light.  We suggest that this is incompatible with the stabilities of lumenal components and the observed activation of downregulation.  Recent work on the chloroplast ATP synthase suggests that a more moderate pmf can sustain DeltaG(ATP).  In addition, in vivo probes suggest that a substantial fraction of pmf can be stored as Deltapsi.  Together, these factors should allow sufficient DeltaG(ATP) to maintain lumen pH in a range where lumenal enzyme activities are nearly optimal, and where the level of NPQ is regulated.