Boro e potássio em mudas de açaizeiro: crescimento, trocas gasosas e diagnose visual

Abstract

In recent decades, the cultivation of the açaí palm (Euterpe oleracea Mart.) has gained significant economic importance in Northern Brazil; however, information regarding its nutritional management remains scarce. B stands out as one of the most common nutrient deficiencies in this crop, playing a key role in cell wall formation and stabilization, with direct impacts on plant growth. K, in turn, acts in osmotic control, stomatal regulation, and cellular metabolism. This study aimed to evaluate the physiological and morphological responses, biomass accumulation, and visual symptoms of deficiency and toxicity in açaí palm seedlings subjected to varying doses of B and K. The experiment was conducted in a greenhouse from November 2024 to May 2025 at the Federal Rural University of the Amazon (UFRA), Belém campus, Pará state, Brazil. Plants were grown individually in 8 L pots containing a medium textured, low-fertility Yellow Oxisol (Xanthic Ferralsol) substrate, with base saturation adjusted to 60%. A completely randomized design (CRD) was adopted in a 4 × 4 factorial scheme. The nutrient sources used were potassium chloride (KCl, 60% K2O) and disodium octaborate / pentahydrate tetraborate (Granubor, 15% B) at doses of 0.1 (K0), 0.2 (K1), 0.3 (K2), and 0.4 g dm−3 (K3) of K, and 0 (B0), 0.001 (B1), 0.002 (B2), and 0.003 g dm−3 (B3) of B. Analysis of variance revealed no significant interaction (𝑝 > 0.05) between B and K for any of the evaluated variables. K significantly influenced gas exchange, promoting linear reductions in net CO2 assimilation rate (𝐴), stomatal conductance (𝑔𝑠), and transpiration rate (𝐸). Conversely, B exerted a significant quadratic effect on the intercellular-to-ambient CO2 concentration ratio (𝐶𝑖/𝐶𝑎), collar circumference (CC), root dry mass (RDM), and root volume (RV). The technical optimal B dose for CC was 0.0012 g dm−3 , whereas for RDM and RV it was 0.0013 g dm−3—the same dose at which the 𝐶𝑖/𝐶𝑎 ratio reached its minimum point. The sharp decline observed in these variables at higher doses demonstrated that excess B is more detrimental to growth than B omission itself. Visual diagnosis allowed the characterization of distinct symptoms: B deficiency caused crinkled leaves, leaflets fused in a "fishbone" pattern, stipe cracking, as well as root apical thickening and necrosis. K deficiency manifested in older leaves as marginal chlorosis and necrosis, premature defoliation, and susceptibility to whitefly. Excess B induced marginal chlorosis and scorching on older leaves, whereas excess K caused chlorosis and necrotic lesions. Careful management of B and K fertilization is required during seedling production due to the high sensitivity of the species to B and the risk of osmotic stress induced by high doses of KCl. Further studies are recommended to understand the nutritional requirements of the açaí palm across different phenological stages.