Effects of spectrum and duration of supplemental illumination on growth, yield and fruit quality of greenhouse sweet pepper

Abstract: The objective of this paper is to experimentally study the optimal spectrum and duration of illumination in regulating yield and quality of sweet pepper (Capsium annuum L.) grown in greenhouse. The cultivar of 'Aodaili' was used as the model plant and LED was used as the light source. We compared nine combinations of three light durations (2h, 18:00-20:00; 4h, 18:00- 22:00 and 8h, 18:00-02:00) and three light spectra measured with red (R) : blue (B) ratio at 2:1 (2R1B); 4:1, (4R1B), and 8:1 (8R1B); without supplemental light served as the control (CK). In each treatment, we measured the growth, yield and fruit quality of the sweet pepper. Results showed that the diameter of the canopy, soluble proteins, soluble sugar, and Vitamin C under 2R1B for 2h were the highest, increasing by 24.6%, 33.8%, 15.2% and 29.9%, respectively, compared to those in the CK. Nitrate content and acid-sugar ratio under 2R1B for 8h were the lowest, reduced by 19.4% and 27.0% respectively, compared to that in the CK. The dry biomass of the shoot and whole plant in 4R1B for 2h were the highest, increasing by 58.4% and 62.4% respectively, compared to that in the CK. In contrast, the fresh weight of the shoot and whole plant under 8R1B for 8h were the highest, increasing 55.1% and 48.9%, respectively, compared to that under the CK. Compared to those under the CK, the leaf area, fruit number and single plant yield under 4R1B for 8h increased by 32.4%, 37.9% and 47.2%, respectively. The SPAD, acid-sugar ratio and output-input ratio of the pepper under 8R1B for 2h increased significantly compared with that in CK, while no significant differences were found in the soluble proteins, soluble sugar, Vitamin C and nitrate content between them. These results revealed that the duration and spectrum of the supplemental light had a combined impact on growth, yield and fruit quality of sweet pepper at significant level. Supplemental lighting increased fruit number, single plant yield, soluble proteins, soluble sugar, titratable acid, Vitamin C, nitrate content, acid-sugar ratio. We used electricity consumption to analyze the effect of spectrum and duration of light on plant growth, based on the fuzzy membership function. The best three treatments calculated by the method are in the order of 8R1B for 2h > 4R1B for 8h > 8R1B for 8h. In summary, the optimal supplemental lighting for the greenhouse sweet pepper is 8R1B for 2h. The results presented in this paper has implications for using supplemental lighting to improve green peppers production in greenhouse.