Growth of <i>Paulownia</i> ssp. Interspecific Hybrid ‘Oxytree’ Micropropagated Nursery Plants under the Influence of Plant-Growth Regulators

The aim of the study was to examine the effects of application of plant-growth regulators (PGR) on the growth of paulownia plants and evaluate their usefulness in paulownia nurseries. The experiment was carried out on the paulownia interspecific hybrid (<i>Paulownia elongata</i> × <i>Paulownia fortunei</i>) ‘Oxytree’. Micropropagated plants grown in pots were sprayed with PGR solutions. Gibberellins (GA₃, GA₄+GA₇), 6-benzyladenine (BA), triclopyr (TPA), ethephon (ETH), daminozide (DA), prohexadione-Ca (PH), and trinexapac-ethyl (TE) were used. They were included in commercial products (Florgib 245 mg dm⁻³, Gibb plus 11 SL 5 cm³ dm⁻³, Globaryll 100 SL 1.5 cm³ dm⁻³, Topper 10 ST 200 mg dm⁻³, Agrostym 480 SL 2 cm³ dm⁻³, B-Nine 85 SG 2 g dm⁻³, Regalis plus 10 WG 2.5 g dm⁻³, Moddus 250 EC 1 cm³ dm⁻³, respectively). None of the studied preparations modified positive correlations between the longitudinal vs. transverse growth of stems and growth of leaves. Not only GA₃, GA₄+GA₇, but also BA (cytokinin) and unexpectedly TE (retardant) stimulated stem elongation and thickening. Furthermore, the effect of TE lasted longer than influence of GA_x and BA. Leaves of TE-treated plants were distinguished by a higher chlorophyll a/b ratio, and a lower relative chlorophyll content and efficiency of photosystem II (F_v/F_m, F_v/F₀). TPA strongly deformed the stems and leaves of liners. Other regulators (ETH, DA, PH) retarded the diameter growth of stems and leaf expansion. Some results obtained by GA_x, BA, and especially by TE treatment, might be beneficial for paulownia grown as ornamental and/or timber tree.