The phytotoxic aluminum species (Al3+) is considered as the primary factor limiting crop productivity in over 40 % of world's arable land that is acidic. We evaluated the responses of two wheat cultivars (Triticum aestivum L.) with differential Al resistance, cv. Yecora E (Al-resistant) and cv. Dio (Al-sensitive), exposed to 0, 37, 74 and 148 mu M Al for 14 days in hydroponic culture at pH 4.5. With increasing Al concentration, leaf Ca2+ and Mg2+ content decreased, as well as the effective quantum yield of photosystem II (PSII) photochemistry (I broken vertical bar (PSII) ), while a gradual increase in leaf membrane lipid peroxidation, Al accumulation, photoinhibition (estimated as F (v) /F (m) ), and PSII excitation pressure (1 - q (p) ) occurred. However, the Al-resistant cultivar with lower Al accumulation, retained larger concentrations of Ca2+ and Mg2+ in the leaves and kept a larger fraction of the PSII reaction centres (RCs) in an open configuration, i.e. a higher ratio of oxidized to reduced quinone A (Q(A)), than plants of the Al-sensitive cultivar. Four times higher Al concentration in the nutrient solution was required for Al-resistant plants (148 mu M Al) than for Al-sensitive (37 mu M Al), in order to establish the same closed RCs. Yet, the decline in photosynthetic efficiency in the cultivar Dio was not only due to closure of PSII RCs but also to a decrease in the quantum yield of the open RCs. We suggest that Al3+ toxicity may be mediated by nutrient deficiency and oxidative stress, and that Al-resistance of the wheat cultivar Yecora E, may be due at least partially, from the decreased Al accumulation that resulted to decreased reactive oxygen species (ROS) formation. However, under equal internal Al accumulation (exposure Al concentration: Dio 74 mu M, Yecora E 148 mu M) that resulted to the same oxidative stress, the reduced PSII excitation pressure and the better PSII functioning of the Al-resistant cultivar was probably due to the larger concentrations of Ca2+ and Mg2+ in the leaves. We propose that the different sensitivities of wheat cultivars to Al3+ toxicity can be correlated to differences in the redox state of Q(A). Thus, chlorophyll fluorescence measurements can be a promising tool for rapid screening of Al resistance in wheat cultivars.