Short campanula (Campanula portenschlagiana ‘PGM Get MEE’®) stock plants present a difficulty in machine-harvesting of cuttings. Light adjustment may be an effective approach to mediate plant elongation. Two experiments were performed to 1) investigate whether short-term (five weeks) daily 24-h dynamic lighting (DL) with red and blue light-emitting diodes (LEDs) can promote elongation without inducing flowering, and 2) explore whether DL can be used to modify stock plant morphology to improve the cutting quality and rooting success in a controlled environment. Two lighting treatments were used: concurrent lighting (CL) with red (85%) and blue (15%) LEDs (RB) at 100 µmol·m−2·s−1 and DL with red (170 µmol·m−2·s−1), blue (30 µmol·m−2·s−1), and RB (100 µmol·m−2·s−1) LEDs sequentially at three different lighting stages, respectively, in both experiments. In Expt. 1, at final harvest of stock plants, the side branches were longer under DL compared with CL, but the five (= 2 + 2 + 1) weeks of 24-h daily lighting resulted in visible flower buds under both treatments. Based on the results of Expt. 1, a second experiment (Expt. 2) was conducted with the same cultivar and experimental conditions, but with a shorter photoperiod (10 h·d−1) for 11 (= 8 + 2 + 1) weeks. In Expt. 2, at final harvest, DL compared with CL caused more upright side branches, and reduced the dry biomass of side branches with one branching order and leaf chlorophyll content. However, the harvested cutting quality and rooting success were similar between both treatments. In both experiments, side branch number under DL was greater compared with CL at the end of the first lighting stage. Stock plants under DL were taller from the second lighting stage on to final harvest compared with CL, and the final heights of stock plants under DL met the target for machine-harvest in both experiments. Therefore, if the lighting strategy is further optimized, DL can potentially benefit controlled-environment production of campanula cuttings.
Dynamic versus Concurrent Lighting with Red and Blue Light-emitting Diodes as the Sole Light Source Can Potentially Improve Campanula Stock Plant Morphology for Cutting Production