A Comparison of Indoor and Greenhouse Acclimation of Lavender Cuttings: Effects of Monochromatic Blue Light and Daminozide Sprays

Abstract Monochromatic blue (MB) light has shown potential to stimulate rooting of cuttings acclimated in indoor controlled environments but often leads to excessive shoot elongation. Our objective was to compare shoot and root growth and morphology of lavender ( Lavandula ) cuttings acclimated indoors under MB or broadband white (W) light-emitting diode (LED) fixtures, and to compare results with cuttings acclimated in a greenhouse under mist. A secondary objective was to compare responses of cuttings treated with or without daminozide (Dz), applied at 1000 mg∙L –1 to half of the cuttings as a height-control plant-growth regulator (PGR). Stem-tip cuttings of ‘Phenomenal’ ( Lavandula × intermedia Emeric ex Loisel) and ‘La Diva Vintage Violet’ ( Lavandula angustifolia Mill.) (‘Violet’) were acclimated for 14 and 21 d, respectively, under a photosynthetic photon flux density of 100 µmol·m⁻ 2 ·s⁻ 1 provided for 24 h·d⁻ 1 [daily light integral (DLI) of 8.6 mol·m⁻ 2 ·d⁻ 1 ] indoors, or in a greenhouse with a DLI of 11.4 mol·m⁻ 2 ·d⁻ 1 provided by sunlight. A greenhouse finishing phase lasting 14 d was then conducted to evaluate carryover effects of all treatments applied during the acclimation phase. In general, cuttings rooted better in the greenhouse than indoors, suggesting that when environmental conditions are moderate in the greenhouse, indoor acclimation may not be warranted for lavender propagation. For cuttings acclimated indoors, MB light stimulated shoot elongation of ‘Phenomenal’, but not of ‘Violet’, showing a species-specific response. However, W light tended to increase leaf number, leaf area, and shoot dry mass (DM) of both species, suggesting a more favorable light environment for shoot growth and development than MB light. Although slight increases were measured under MB compared with W light for root length, root DM, and root mass fraction (RMF), these effects generally were limited to cuttings that did not receive a Dz spray. Furthermore, although spraying Dz effectively helped maintain compact cuttings, it reduced leaf number and shoot DM by 8% to 12% and 9% to 12%, respectively. Nonetheless, cuttings treated with Dz had a higher RMF during finishing, indicating that that a greater share of accumulated biomass was allocated to root production. Our findings indicate that more research is needed to demonstrate practical advantages of acclimating cuttings indoors, and to understand the potential of applying height-control PGRs during early stages of propagation.