Morphological and Physiological Responses of Weigela florida ‘Eva Rathke’ to Biostimulants and Growth Promoters

Ornamental horticulture and breeding, as well as urban landscape architecture, are facing increasing challenges driven by an intensely changing climate and urbanisation. The expansion of cities should be combined with an overall growth of green spaces, where ornamental plant species and cultivars will have to withstand a diverse range of environmental conditions, whereby they are often exposed to multiple stress factors. One of the most widely used ornamental shrub species Weigela florida ‘Eva Rathke’ was treated with the growth promoters Bistep with humic and fulvic acid, Kelpak® seaweed extract, and Yeald Plus with a high zinc content to test their applicability in a plant nursery. Bistep decreased the physiological parameters (the transpiration rate by 60%, the evapotranspiration rate by 56.5%, and the proline stress enzyme content level by 82.2%), indicating the stress level of the treated plants. The activity of β-glucosidase decreased with all growth-promoting treatments (11.5% for Kelpak and 9.5% for Yeald Plus), as did β-glucosaminidase (22.1% for Kelpak and 9.8% for Yeald Plus), but Bistep treatment reduced the activity of the enzymes less (9.9% for β-glucosidase and 3.3% for β-glucosaminidase). The measured alkaline phosphatase enzyme activity increased with treatment (by 10.7% for Kelpak, 11.7% for Yeald Plus, and 12.63% for Bistep). Based on the results, it was concluded that Bistep and Yeald Plus may be suitable for use in the studied variety, whereas Kelpak® may not be suggested in plant nurseries for growing W. florida ‘Eva Rathke’ plants.