Comparison of sole-source and supplemental lighting on callus formation and initial rhizogenesis of gaura and salvia cuttings

Variability in outdoor daily temperatures and photosynthetic daily light integrals (DLIs) from early spring to late fall limits the ability of propagators to accurately control propagation environments to consistently callus, root, and yield compact herbaceous perennial rooted liners. We evaluated and compared the effects of sole-source lighting (SSL) delivered from red (R) and blue (B) light-emitting diodes (LEDs) to supplemental lighting (SL) provided by high-pressure sodium (HPS) lamps on herbaceous perennial cutting morphology, physiology, and growth during callusing and initial rhizogenesis. Cuttings of perennial sage (Salvia nemorosa L. ‘Lyrical Blues’) and wand flower (Gaura lindheimeri Engelm. and A. Gray ‘Siskiyou Pink’) were propagated in a walk-in growth chamber under multilayer SSL provided by LEDs with [R (660 nm)]: [B (460 nm)] light ratios (%) of 100:0 (R₁₀₀:B₀), 75:25 (R₇₅:B₂₅), 50:50 (R₅₀:B₅₀), or 0:100 (R₀:B₁₀₀) delivering 60 µmol·m^-2·s^-1 for 16 hours (total DLI of 3.4 mol·m^-2·d^-1). In a glass-glazed greenhouse (GH control), cuttings were propagated under ambient solar light and day-extension SL provided by HPS lamps delivering 40 µmol·m^-2·s^-1 to provide a 16-hour photoperiod (total DLI of 3.3 mol·m^-2·d^-1). At 10 days after sticking cuttings, callus diameter and rooting percentage were similar among all light-quality treatments. For instance, callus diameter, a measure of growth, of wand flower cuttings increased from an average 1.7 mm at stick (0 day) to a range of 2.7 to 2.9 mm at 10 days after sticking, regardless of lighting treatment. Relative leaf chlorophyll content was generally greater under SSL R₇₅:B₂₅ or R₅₀:B₅₀ than all other light-quality treatments. However, stem length of perennial sage and wand flower cuttings propagated under SSL R₅₀:B₅₀ at 10 days were 21% and 30% shorter and resulted in 50% and 8% greater root biomass, respectively, compared with those under SL. The herbaceous perennial cuttings propagated in this study under SSL R₅₀:B₅₀ were of similar quality or more compact compared with those under SL, indicating that callus induction and initial rooting can occur under LEDs in a multilayer SSL propagation system.