Y(NA) (acceptor side limitation) represents the fraction of the overall P700 that cannot be oxidized by SP in a given state because of a lack of acceptors, and is enhanced by dark adaptation (deactivation of key enzymes of the Calvin–Benson cycle) and damage at the site of CO2 fixation.
Y(NA) is determined based on the equation Y(NA) = (Pm−Pm)/Pm.