Taxon-Specific Physiological and Rhizosphere Responses of Deciduous Ornamental Shrubs to Humic- and Fulvic Acid-Based Biostimulant Treatment

In containerized finished plant production, the effects of biostimulants in nursery practice are often judged primarily on the basis of visual condition, while a more precise interpretation of treatment response requires leaf-level physiological and rhizosphere-level indicators. The aim of our study was to determine how the humic- and fulvic acid-based BiStep biostimulant influences the physiological functioning and, in part, the rhizosphere enzyme activity of three deciduous ornamental shrub taxa widely used both in nursery finished plant production and in urban green space plantings, namely, Forsythia × intermedia ‘Beatrix Farrand’, Weigela florida ‘Eva Rathke’, and Viburnum opulus ‘Roseum’, under commercial container conditions. In the experiment, control and biostimulant treatments were compared. Treatment effects were evaluated on the basis of net photosynthesis (Pn); transpiration (E); chlorophyll content; stomatal density; stomatal length; and acid phosphatase (ACP), alkaline phosphatase (ALP), and β-glucosidase (GLUC) activities. For Pn, a significant taxon × treatment interaction was observed (p = 0.002). Pn showed taxon-dependent numerical changes under BiStep: values were 22.212 µmol CO2 m−2 s−1 in F. intermedia, 4.182 µmol CO2 m−2 s−1 in W. florida, and 3.370 µmol CO2 m−2 s−1 in V. opulus, but pairwise differences from the control were not statistically significant. Transpiration also showed a significant taxon × treatment interaction (p < 0.001), although BiStep–control differences were not significant within taxa. Stomatal density increased significantly in F. intermedia and W. florida, while the BiStep–control difference was not significant in V. opulus. Chlorophyll content increased only in W. florida (from 699.6 to 924.4 µg g−1 fresh weight), but this change was not statistically significant. ACP activity showed significant treatment and interaction effects (p = 0.0107; p = 0.00546), whereas ALP and GLUC did not show a consistent treatment response. Based on the results, the effect of BiStep was clearly taxon-dependent and functionally selective. Therefore, in nursery finished plant production and subsequent urban plant use, it should not be considered a universally effective input, but rather a biostimulant whose relevance depends on the specific physiological and rhizosphere-level response of the taxon.