Biostimulants in the Formation of Spondias tuberosa Rootstocks

The umbu tree (Spondias tuberosa Arruda) is a fruit-bearing species native to the semiarid region of Brazil that possesses high ecological and socioeconomic value; however, its cultivation remains limited by a lack of technologies for producing high-quality seedlings. In this context, the use of biostimulants offers a strategy to optimize seedling growth and physiological efficiency. This study aimed to evaluate the effects of different biostimulant sources and dosages on the growth, physiology, and quality of S. tuberosa seedlings. The experiment followed a 2 × 6 factorial design, with two sources (Bio-I and Bio-II) and six concentrations (0.0, 0.5, 1.5, 2.5, 3.5, and 4.5 mL L−1) of biofertilizer, arranged in a randomized block design. The evaluations included biometric variables, dry biomass, the Dickson quality index, leaf chlorophyll indices, chlorophyll fluorescence, and gas exchange. Leaf chlorophyll indices and gas exchange parameters were not significantly affected, indicating physiological stability. Intermediate doses (1.7–1.9 mL L−1) tended to increase the Fv/Fo and Fv/Fm values; however, these responses should be interpreted as descriptive trends since no significant treatment effect was detected. Height growth, shoot biomass accumulation, and the Dickson quality index increased with dosage, peaking at 4.5 mL L−1, whereas leaf production was greater with Bio II. Multivariate analysis indicated that leaf temperature played a major role in the growth characteristics and physiological processes evaluated. Lower leaf temperatures were associated with greater stomatal limitation of CO2 exchange and internal carbon accumulation. Foliar application of Bio I and Bio II improved the growth and quality of S. tuberosa seedlings, with the highest responses recorded for plant height and specific biometric traits at 4.5 mL L−1 within the evaluated concentration range.