Aluminum (Al) is the most abundant element in the Earth's crust. In acidic soils (pH < 5.5) it is found as Al3+, which is toxic to most plants. In Al-sensitive plants, the first and most conspicuous symptom is the inhibition of the growth of the root system, where Al is covalently retained. Aluminum also causes indirect symptoms, such as reduced shoot growth. In Al-sensitive plants, Al-reduced CO2 assimilation rate (A) is observed, and there is evidence suggesting the impairment of photochemical reactions, as well as 30 to 80% reduction in stomatal conductance (gs). This suggests that when retained in the root system, the Al could affect the mesophyll hydration, like in a water deficit. On the other hand, plants from the Cerrado vegetation avoid Al uptake (Al non-accumulating species) and some species absorb Al but it does not cause any structural damage to their metabolism (Al-accumulating species). Based on these facts we will test the hypothesis that some of the symptoms of Al toxicity in sensitive plants, especially those related to the low growth and development of shoots, occur in response to the activation of mechanisms closely associated with the perception of water deficit, especially the biosynthesis of abscisic acid (ABA). We also expect that these associations do not occur in tolerant species, such as Styrax camporum. We will use S. camporum plants, which are considered moderate Al-accumulating species and 'Rangpur' lime plants (Citrus limonia), which are sensitive to Al. The two species will be studied separately, in nutrient solutions with and without Al. In these two groups of plants, in each experiment (S. camporum and C. limonia), we will measure the main root length, the shoot height, chlorophyll a fluorescence and gas exchange (including mainly gs). These non-destructive measurements will be made one day before the intervals of 1, 7, 15, 30, 45 and 60 days after planting (DAP). On these days, we will measure predawn and midday leaf water potentials, collect samples of roots and leaves for measuring the expression of the NCED gene, and the concentrations of ABA and its metabolites. After each collection, we will separate the organs (roots, stems + petioles, and leaves) to determine their respective dry masses. After that, these dried samples will be sent to a routine plant nutrition lab for quantification of the total Al contents in each of these organs.
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