An attempt is made to reveal the relation between the induction curves of delayed fluorescence (DF) registered at 0.35-5.5 ms and the prompt chlorophyll fluorescence (PF). A simple formulation was proposed to link the ratio of the transient values of delayed and variable fluorescence with the redox state of the primary electron acceptor of Photosystem II-Q_A, and the thylakoid membrane energization. The term luminescence potential (U_L) was introduced, defined as the sum of the redox potential of Q_A and the transmembrane proton gradient. It was shown that U_L is proportional to the ratio of DF to the variable part of PF. The theoretical model was verified and demonstrated by analysing induction courses of PF and millisecond DF, simultaneously registered from leaves of barley-wild-type and the chlorophyll b-less mutant chlorina f2. A definitive correlation between PF and DF was established. If the luminescence changes are strictly due to U_L, the courses of DF and PF are reciprocal and the millisecond DF curve resembles the first derivative of the PF(t) function.