Alumínio e Manganês: efeitos no desenvolvimento inicial do Guaranazeiro

Abstract

In acidic soils, the availability of aluminum (Al) and manganese (Mn) increases, intensifying their phytotoxic effects on plant metabolism. Although several studies have described the toxicity mechanisms of these elements in agronomically important species, their isolated effects on the initial growth of guarana remain poorly understood. To address this knowledge gap, we conducted two greenhouse experiments to evaluate the effects of increasing Al and Mn concentrations on physiological, biochemical, nutritional, and anatomical responses. In the first study, we evaluated the effects of five soil Al levels (0, 1.0, 2.0, 3.0, and 4.0 cmolc dm-3) on the initial development of the guarana cultivar BRS-Noçoquém. In the second study, we evaluated the factorial combination of six Mn levels (0.0, 2.5, 5.0, 10.0, 20.0, and 40.0 mg dm-3) and two guarana cultivars, BRS-Amazonas (CMU-300) and BRS-Maués (CMU-871). We grew the plants in pots under a completely randomized design, with one plant per experimental unit, using five replicates in the first study (25 plants) and four replicates in the second study (48 plants). In the first study, Al toxicity in the cultivar BRS-Noçoquém significantly reduced biomass, particularly root biomass, and caused anatomical changes, including xylem thickening and phloem reduction. The stress also impaired the nutritional status of the plants by reducing the concentration and accumulation of P and Cu in leaves and roots, Ca, Mg, and Fe in leaves, and Mn in roots, while restricting Ca translocation to the leaves and increasing proline accumulation and P use efficiency. In the second study, excess Mn reduced growth and altered the nutritional balance of the cultivars CMU-300 and CMU-871, mainly from 10 mg dm-3 Mn in the soil onward, with genotype-dependent responses. The cultivar CMU-871 showed greater tolerance, maintaining higher photochemical performance (Fv/Fm, PIabs, and PItotal), greater nutrient-use efficiency, and higher proline accumulation, particularly in the roots, whereas cultivar CMU-300 accumulated more Mn in the leaves and exhibited lower physiological performance. Overall, the results demonstrate that Al and Mn phytotoxicity act through distinct mechanisms: Al more strongly impairs root growth and architecture, whereas Mn predominantly affects leaf physiology. The evaluated cultivars showed sensitivity to excess levels of these elements, although they exhibited distinct responses and adopted metabolic maintenance strategies through biochemical and nutritional adjustments.