Evaluation of leaf morphology, structure and biochemical substance of balloon flower (Platycodon grandiflorum (Jacq.) A. DC.) plantlets in vitro under different light spectra

• Leaf morphology, structure and biochemical substance of plantlets in vitro were investigated to identify the effects of different LED-generated spectra on leaves under the same light intensity.
• The effects of spectral quality on the endogenous hormone content and leaf anatomical features of illuminated in vitro leaves were studied.
• Monochromic blue LEDs, monochromic red LEDs and three mixtures of blue plus red LEDs on the growth and morphogenesis of leaves on P. grandiflorum plantlets in vitro under the same light intensity were studied to select the best light source for this cultivation system.

In many parts of Asia, balloon flower (Platycodon grandiflorum (Jacq.) A. DC., P. grandiflorum) is much valued because of its ornamental and medicinal attributes. Although previous studies have described techniques of plant regeneration and culture, the influence of different spectra has not been documented. We investigated the effect of light spectra on leaf morphology, anatomy and chemical composition using cultured nodal explants grown under light-emitting diodes (LEDs) at a photosynthetic photon flux density of 50 μmol m−2 s−1. Treatments were: monochromatic blue light (B), 75% blue + 25% red light (BR31), 50% blue + 50% red light (BR11), 25% blue + 75% red light (BR13) and monochromatic red light (R). Fluorescent white lamps (FL) were used as a control. Treatment B induced larger leaf area, leaf thickness and dry mass of whole plant and higher leaf number than treatment R or FL. Plantlets from treatment R had the highest specific leaf mass, soluble sugar and sucrose content. Chlorophyll and L-ascorbic acid content was higher in leaves grown under LED variants than in those grown under FL. The indoleacetic acid content in B- and BR31-treated leaves was higher than that in the R-treated leaves. In addition, B and BR31 induced large and nearly rectangular palisade parenchyma cells, while the palisade parenchyma cells were elliptical in shape and small in the FL, BR11 and BR13 treatments, or irregular in the R treatment. Leaves from the R and BR11 treatments had the highest stomata frequency, moreover, FL-, B- and BR31-treated leaves possessed normal elliptical stomata. Treatment B induced the highest ETR, ΦPSII, and qp, significantly higher than treatment R or FL. Our data revealed that light spectrum strongly affected the morphology and growth index of P. grandiflorum plantlets cultured in vitro.