This work focused on the effects of aluminum (Al) stress on photosynthetic apparatus using Al-tolerant and sensitive cultivars of sorghum (Sorghum bicolor (L.) Moench). Plants were grown in Clark's nutrient solution aerated continuously and adjusted daily to pH 4.0 and containing two Al concentrations (0 and 185 mmol m À 3 ) supplied as Al 2 (SO 4 ) 3 Á 18H 2 O. Measurements were performed on recently expanded leaves from the eighth to sixteenth day after imposing Al treatments. In general, chloroplastidic pigment concentration, chlorophyll fluorescence parameters, and photosynthetic rates were initially affected by Al stress to a greater extent in the Al-tolerant cultivar than in the susceptible cultivar. However, the Al-tolerant genotype JOURNAL OF PLANT NUTRITION, 25(4), 821-832 (2002) 821 seemed to cope with high Al via progressive, sustained decreases in initial chlorophyll fluorescence and increases in chlorophyll a=b ratio, photosystem II photochemical efficiency, net carbon assimilation rate, and photosynthetic oxygen evolution. In contrast, the Al-sensitive cultivar did not show any apparent acclimation response to high Al application. The results suggest that the harmful effects of Al on photosynthesis might be partially alleviated through time-dependent mechanisms, particularly in the Al-tolerant sorghum cultivar.