Aluminum (Al) is toxic to most plants. Although inhibition of root elongation can occur even under 10 M Al, above 1000 M damage to photochemical performances has been reported, reducing the CO 2 assimilation rate (A). However, Al is retained in the roots of most plants, including Citrus limonia ('Rangpur' lime) with no direct explanation for the low A. In this study, 'Rangpur' lime plants were cultivated hydroponically in a nutrient solution with 1480 M Al, and we expected to determine the time range within which Al-induced decrease in A starts. To our surprise, low stomatal conductance (gs) rather than low photochemical performances was evident 45 days after exposing the roots to Al. Aluminum was mostly retained in the roots and histochemically observed in the stele, suggesting reduction in water uptake/transport in the xylem that became fibrous. We also found a 35% reduction in the midday leaf water potential in plants exposed to Al, indicating association between the fibrous xylem vessels and low gs, which could explain the Al-induced decrease in A in 'Rangpur' lime plants.

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In the Cerrado vegetation, generally known as 'Brazilian savanna', aluminum (Al) accumulating and non-accumulating plants coexist, growing on soils that are acidic, poor in nutrients and rich in Al. Differing from Al-sensitive species, these plants are not expected to experience Al injuries. Using Styrax camporum, a non-accumulating plant, we recorded biometric variations in leaves, shoots and roots of young plants exposed to 0 and 1480 lM Al in a nutrient solution. Photosynthetic responses were measured biweekly over 91 days. Plants exposed to Al drastically reduced flushing, indicating that Al interferes with the functioning of the shoot apex. Aluminum caused low CO 2 assimilation rate, largely explained by low stomatal conductance, while Al-induced decrease in photochemical performance occurred only on some dates during the experiment. In addition, the absorbed Al was mostly retained in the roots. Although counterintuitive, as this species grows on Al-rich soils, we noted that high Al availability impairs lateral root formation, causing an impact on water uptake and gas exchange rates of this species.

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