Seedlings of Citrus grandis (L.) Osbeck cv. Tuyou were irrigated daily for 5 months with nutrient solution containing 0 (control), 0.2, 0.6 or 1.6 mM aluminum (Al) from AlCl(3).6H(2)O. Shoot growth was more sensitive to Al toxicity than root growth, gas exchange, chlorophyll (Chl) concentration, polyphasic Chl a fluorescence (OJIP) induction and related parameters. Leaves of Al-treated plants showed decreased CO(2) assimilation and Chl concentration, yet intercellular CO(2) concentration increased and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was unchanged. Chlorophyll a fluorescence induction analysis of Al-stressed leaves showed a large rise at the O-step and a large depression at the P-step, accompanied by two new bands at 300 micros (K-band) and at about 150 micros (L-band). Maximum fluorescence, maximum quantum yield of primary photochemistry, oxygen-evolving complex (OEC), quantum yield of electron transport, quantum yield of electron transport from Q(A) (-) to the Photosystem I end electron acceptors, IP phase and total performance index were decreased in leaves of Al-treated plants, whereas minimum fluorescence, relative variable fluorescence at the J-step and I-step, and dissipated energy were increased. We propose that impaired electron transport capacity accompanied by lack of reducing equivalents were the main factors contributing to decreased CO(2) assimilation in Al-treated plants. Aluminum-induced photoinhibition occurring at both the donor (i.e., the OEC) and the acceptor sides of Photosystem II may be associated with growth inhibition. Besides decreased light absorption due to reduced Chl concentration, enhanced energy dissipation protected the leaves of Al-treated plants from photo-oxidative damage in high light.