Biostimulant-Induced Physiological Changes in Forsythia × intermedia ‘Beatrix Farrand’

In container-grown ornamental shrub production, limited substrate volume can rapidly alter plant water status and affect leaf-level physiological performance. A randomized block experiment was conducted to evaluate the effects of three foliar-applied biostimulants, BiStep, Kelpak, and Yeald Plus, compared with an untreated control in container-grown Forsythia × intermedia ‘Beatrix Farrand’. Each treatment comprised 25 plants distributed across five blocks. Stomatal density, stomatal length, leaf transpiration, net photosynthesis, total chlorophyll content, and gravimetrically determined daily water loss were measured. Compared with the control, BiStep significantly increased stomatal density, whereas Yeald Plus significantly decreased it. Kelpak and Yeald Plus significantly increased stomatal length compared with the control. Leaf transpiration was significantly lower under Kelpak and Yeald Plus than in the control, while Kelpak and BiStep had significantly higher net photosynthetic rates than Yeald Plus, whereas the control did not differ significantly from any treatment. Total chlorophyll content did not differ significantly among treatments. Whole-plant daily water loss was higher in all biostimulant-treated plants than in the control, with the highest values recorded under BiStep. These results show that the tested biostimulants induced treatment-specific physiological changes rather than a uniform response. Kelpak provided the most favorable leaf-level balance by reducing instantaneous transpiration while maintaining high photosynthetic activity, whereas BiStep promoted a more water-demanding physiological profile. Based on these outcomes, Kelpak may be recommended where improved leaf-level physiological efficiency is required under container-grown nursery conditions, while BiStep should be applied only with adequate irrigation management.