The hypothesis that changes in the IP amplitude of the fluorescence transient OJIP reflect changes in leaf photosystem I (PSI) content was tested using mineral-deficient sugar beet plants. Young sugar beet plants (Beta vulgaris) were grown hydroponically on nutrient solutions containing either 1 mM or no Mg2+ and 2.1 mu M to 1.88 mM SO42- for 4 weeks. During this period two leaf pairs were followed: the already developed second leaf pair and the third leaf pair that was budding at the start of the treatment. The IP amplitude [FIP (fluorescence amplitude of the I-to-P-rise) and its relative contribution to the fluorescence rise: ?VIP (amplitude of the relative variable fluorescence of the I-to-P-rise = relative contribution of the I-to-P-rise to the OJIP-rise)] and the amplitude of the transmission change at 820 nm (difference between all plastocyanin and the primary electron donor of photosystems I oxidized and reduced, respectively) relative to the total transmission signal (?Imax/Itot) were determined as a function of the treatment time. Correlating the transmission and the two fluorescence parameters yielded approximately linear relationships in both cases. For the least severely affected leaves the parameter ?VIP correlated considerably better with ?Imax/Itot than ?FIP indicating that it is the ratio PSII:PSI that counts. To show that the relationship also holds for other plants and treatments, data from salt- and drought-stressed plants of barley, chickpea and pea are shown. The relationship between ?VIP and PSI content was confirmed by western blot analysis using an antibody against psaD. The good correlations between ?Imax/Itot and ?FIP and ?VIP, respectively, suggest that changes in the IP amplitude can be used as semi-quantitative indicators for (relative) changes in the PSI content of the leaf.