Towards greenhouse cultivation of Artemisia annua: The application of LEDs in regulating plant growth and secondary metabolism

Abstract

Artemisinin is a sesquiterpene lactone produced in glandular trichomes of Artemisia annua, and is extensively used in the treatment of malaria. Growth and secondary metabolism of A. annua are strongly regulated by environmental conditions, causing unstable supply and quality of raw materials from field grown plants. This study aimed to bring A. annua into greenhouse cultivation and to increase artemisinin production by manipulating greenhouse light environment using LEDs. A. annua plants were grown in a greenhouse compartment for five weeks in vegetative stage with either supplemental photosynthetically active radiation (PAR) (blue, green, red or white) or supplemental radiation outside PAR wavelength (far-red, UV-B or both). The colour of supplemental PAR hardly affected plant morphology and biomass, except that supplemental green decreased plant biomass by 15% (both fresh and dry mass) compared to supplemental white. Supplemental far-red increased final plant height by 23% whereas it decreased leaf area, plant fresh and dry weight by 30%, 17% and 7%, respectively, compared to the treatment without supplemental radiation. Supplemental UV-B decreased plant leaf area and dry weight (both by 7%). Interestingly, supplemental green and UV-B increased leaf glandular trichome density by 11% and 9%, respectively. However, concentrations of artemisinin, arteannuin B, dihydroartemisinic acid and artemisinic acid only exhibited marginal differences between the light treatments. There were no interactive effects of far-red and UV-B on plant biomass, morphology, trichome density and secondary metabolite concentrations. Our results illustrate the potential of applying light treatments in greenhouse production of A. annua to increase trichome density in vegetative stage. However, the trade-off between light effects on plant growth and trichome initiation needs to be considered. Moreover, the underlying mechanisms of light spectrum regulation on artemisinin biosynthesis need further clarification to enhance artemisinin yield in greenhouse production of A. annua.

Keywords: Artemisia annua; artemisinin; biomass; glandular trichome; light spectrum; plant morphology.

Figure 1

Leaf number (A, B), leaf area on the main stem (C, D) and plant height (E, F) during the experiment in treatments of supplemental radiation within (A, C, E; Exp. 1) and outside (B, D, F; Exp 2) the range of photosynthetically active radiation (PAR) (values are mean ± s.e.; n = 4, with five plants in each statistical replicate). The supplemental radiation of different colours in panels A, C and E had an intensity of 23 mmol m^-2 s^-1. The control in panels B, D and F is treatment without supplemental radiation; supplemental far-red radiation resulted in a red to far-red ratio of 0.3 at the plant level, and supplemental UV-B had an intensity of 0.53 W m^-2. “F” and “U” respectively indicate a significant effect of far-red and UV-B on a specific day after transplanting (p < 0.05). “NS” indicates non-significant effect was found.

Figure 2

Total leaf area (from both main stem and side shoots) per plant (A, B) and specific leaf area (calculated as leaf area divided by leaf dry weight; average value from all leaves) (C, D) measured at final harvest in treatments of supplemental radiation within (A, C; Exp 1) and outside (B, D; Exp 2) the range of PAR (values are mean ± s.e.; n = 4, with 15 plants in each statistical replicate). Details on the treatment abbreviations can be found in Figure 1. In panel A and C, letters indicate significant differences (p < 0.05) and “NS” indicates non-significant difference. In panel (B and D, “F” and “U” respectively indicate a significant effect of far-red and UV-B (p < 0.05).

Figure 3

Internode length (A, B), leaf length (C, D) and leaf elevation angle compared with horizontal (E, F) for each phytomer rank on the main stem at final harvest in treatments of supplemental radiation within (A, C, E; Exp. 1) and outside (B, D, F; Exp. 2) the range of PAR (values are mean ± s.e.; n = 4, with 15 plants in each statistical replicate). Details on the treatment abbreviations can be found in Figure 1. “F” and “U” respectively indicate a significant effect of far-red and UV-B on the traits measured at a specific rank (p < 0.05). “NS” indicates non-significant effects for all ranks.

Figure 4

Plant dry weight at final harvest in treatments of supplemental radiation within (A, Exp. 1) and outside (B, Exp. 2) the range of PAR (values are mean ± s.e.; n = 4, with 15 plants in each statistical replicate). Solid bars are stem dry weight. Dashed bars are leaf dry weight. Details on the treatment abbreviations can be found in Figure 1. In panel (A), letters indicate significant differences (p < 0.05). In panel (B), “F” and “U” respectively indicate a significant effect of far-red and UV-B (p < 0.05).

Figure 5

Trichome density measured at three weeks (seedling stage; (A, B) and five weeks (branching state; (C, D) after transplanting in treatments with supplemental radiation within (A, C; Exp. 1) and outside (B, D; Exp. 2) PAR wavelength range (values are mean ± s.e.; n = 4, with three plants in each statistical replicate). Details on the treatment abbreviations can be found in Figure 1. “NS” in panel A and B indicates non-significant treatment effects. Different letters in panel C indicate significant differences between treatments (p < 0.05; ANOVA test was done using log transformed data). “U” in panel D indicates significant effects from supplemental UV-B (p < 0.05).