The conductivity of the chloroplast ATP synthase is usually estimated from the time constant of a first-order exponential fit to the post-illumination relaxation kinetics of the electrochromic shift signal (ECS), and referred to with the gH+ parameter. However, as the chloroplast NADH dehydrogenase-like complex (NDH) that mediates cyclic electron flow / chlororespiration in darkness is a proton pump with 2 H+ / e- stoichiometry, doesn't the H+ "back pressure" from NDH activity also affect gH+? In other words, should we expect to see increased gH+ values in low CEF mutant plants and decreased gH+ in high CEF mutants? This definitely seems to be the case for example with hcef1 (high CEF, low gH+, Livingston et al 2010) as well as with pgr5 (low CEF, high gH+, Avenson et al. 2005)... Or is there something crucial I am missing here? For example, is the level of proton influx to the lumen generated by NDH so small relative to efflux via the ATP synthase that its effect on gH+ can be ignored?

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