Effects of 24-Epibrassinolide Application on the Development of Dipteryx odorata: Between Growth Stimulation and Inhibitory Effects

Abstract Dipteryx odorata , the Tonka Bean tree, is an Amazonian tree species of high ecological and economic importance, widely used in reforestation and restoration programs. This study evaluated the effects of exogenous application of 24-epibrassinolide (24-EBL) on the morphological and metabolic development of young seedlings of D. odorata cultivated under semi-controlled nursery conditions with 50% shading at the Federal University of Pará, Brazil. A total of 115 seedlings were distributed among five treatments corresponding to 0, 10, 20, 30, and 40 nM of 24-EBL. Morphological parameters (height, basal diameter, number of leaves, and height/diameter ratio) and leaf physicochemical parameters (chlorophyll, nitrogen, humidity, and leaf temperature) were monitored over five to six months. To explore treatment-related differences among individuals and choose the variables for statistical modeling, we performed a principal component analysis. The effects of the treatments on these variables were evaluated using linear mixed-effects models with maximum likelihood estimation. The responses were strongly dose-dependent. The 30 nM treatment promoted the best overall performance, significantly increasing height, basal diameter, leaf production, and chlorophyll content, while 40 nM produced the best height/diameter ratio, indicating improved seedling quality and structural stability. In contrast, the 20 nM treatment induced inhibitory effects characterized by reduced growth, decreased chlorophyll production, and signs of physiological stress, suggesting a possible sensitivity window associated with increased ethylene production. These findings indicate the potential of 24-EBL, particularly at 30–40 nM, as a promising biotechnological tool to improve seedling quality and optimize large-scale production of D. odorata for Amazonian reforestation and ecosystem restoration programs.