Responses of Micropropagated Rhubarb (Rheum rhaponticum) Plantlets to Different Growing Media and Light Conditions in the Greenhouse

Cultivating red-stalked rhubarb plants is an important source of raw materials for producing health-promoting foods. The quality and quantity of rhubarb crops are significantly dependent on planting material. To obtain high-quality planting material for the value selection of the rhubarb 'Raspberry', we evaluated the morphological and physiological responses of micropropagated plantlets to different growth substrates and light quality during early growth ex vitro in the greenhouse. The plantlets were grown in high-EC (GM1) and low-EC (GM2) peat substrates under four light-emitting diodes (LED) light treatments as supplementary lighting (SL) in the wintertime: 100% red (R), 100% blue (B), white light [44.4% green (G), 24.4% B, 28.9% R; 2.2% far red (FR)] and R+B+G+FR (49.4/16.3/10.3/23.8%) light. Compared to the control (natural sunlight), applied LED lighting significantly increased all growth parameters, but only in plantlets grown in GM1 substrate. Among LED treatments, R+B+G+FR light had the most stimulative effect on all growth parameters (length of leaf petioles, leaf area, biomass) and soluble sugar production. Still, it decreased the levels of phenolic compounds in the leaf petioles. Phenolic synthesis, mainly anthocyanins, was the highest under white light (622.8 mg center dot 100 g(-1) dry mass), followed by red (601.8 mg center dot 100 g(-1)), blue (464.4 mg center dot 100 g(-1)), and R+B+G+FR light (416.4 mg center dot 100 g(-1)). High anthocyanin accumulation under R-LED light was associated with high antioxidant activity and growth cessation. Hence, for optimal effects related to plant growth and anthocyanin biosynthesis, the use of W-LED lighting is recommended for the early growth ex vitro of micropropagated rhubarb plantlets.