Morphology and carbohydrate accumulation in Pelargonium cuttings as affected by blue ratio of the supplemental lighting on stock plants

This study evaluated the influence of supplemental top LED lighting on stock plants of Pelargonium × hortorum ‘Ice Crystal’, with 3 R:B rates (H = 73% R + 23% B + 4% FR; M = 63% R + 33% B + 4% FR and L = 53% R + 43% B + 4% FR), on morphology, soluble carbohydrate and starch content of cuttings, in comparison with the natural lighting (NL). All LED lamps supplied the same PPFD at bench level (100±20 μmol m-2 s-1) for 14 h day-1. The growing period (November-January) was characterized by 9.5-10 natural light hours per day and a DLI ranging from 1.1 to 2.5 mol m-2 d-1 at bench level (NL). Supplemental lighting, regardless of the blue rate, reduced petiole length (-22.9%) and canopy diameter (- 16.0%) of stock plants but did not affected significantly their branching (3.7 stems plant-1) and leaf area (2352.4 cm2), compared to NL. Meanwhile, plants under lighting exhibited a double dry weight (+ 107.4%) mainly due to stem dry weight increase (+ 99.4%). Cuttings severed from plants under lighting were characterized by a lower leaf dry weight (-34.3%) and a smaller leaf area (-31.7% under M and L) than those grown under NL, despite of the higher leaf number (8.7 vs. 6.7). Moreover, supplemental lighting, increased carbohydrates concentrations in leaves, from 4.0 up to 10.7 mg g-1 dw and from 4.4 up to 110.4 mg g-1 dw with low blue ratio, for glucose and starch, respectively. At the same time lighting enhanced starch accumulation in stem, from 29.7 in NL up to 230 mg g-1 dw in H light, but halved glucose, indeed its concentration decreased from 6.7 to 3.0 mg g-1 DW under NL and lighting, respectively, without differences among spectra. Blue ratio in LED lamps should not exceed 23% for avoiding leaf damages (e.g., yellow and necrotic lamina) and promoting the production of cuttings with the highest quality.