High abscisic acid and low root hydraulic conductivity may explain low leaf hydration in 'Mandarin' lime exposed to aluminum

Abstract

The first symptom of aluminum (Al) toxicity is the inhibition of root growth, which has been associated with low leaf hydration, with negative consequences for leaf gas exchange including stomatal conductance (g(s)) observed in many plant species. Here we asked whether low leaf hydration occurs before or after the inhibition of root growth of Citrus x limonia Osbeck ('Mandarin' lime) cultivated for 60 days in nutrient solution with 0 and 1480 mu M Al. The length, diameter, surface area and biomass of roots of plants exposed to Al were lower than control plants only at 30 days after treatments (DAT). Until the end of the study, estimated g(s) (measured by sap flow techniques) was lower than in control plants from 3 DAT, total plant transpiration (E-plant) and root hydraulic conductivity (Lp(r)) at 7 DAT, and midday leaf water potential (psi md) and relative leaf water content at 15 DAT. Abscisic acid (ABA) in leaves was twofold higher in Al-exposed plants 1 DAT, and in roots a twofold higher peak was observed at 15 DAT. As ABA in leaves approached values of control plants after 15 DAT, we propose that low g(s) of plants exposed to Al is primarily caused by ABA, and the maintenance of low g(s) could be ascribed to the low Lp(r) from 7 DAT until the end of the study. Therefore, the low leaf hydration in 'Mandarin' lime exposed to Al does not seem to be caused by root growth inhibition or by a simple consequence of low water uptake due to a stunted root system.