Effect of Different Light Spectral on Photosynthetic Performance, Growth Indicators and Essential Oil Content of Salvia officinalis L.

Background and objectives  Light is the main environmental factor for plant growth and development. Different attributes of light such as intensity, quality and duration affect plant growth and productivity. Light spectrum of growing environment is a determinant factor for plant growth and photosynthesis. The photosynthetic reactions are directly affected by various light parameters including its spectrum and intensity. Photosystem I and II (PSI and PSII) in the electron transport chain of photosynthetic apparatus are involved in converting solar energy to chemical compounds in plants. It has been found that the PSII is sensitive to light quality. Using The OJIP test, we can investigate the efficiency of various biological phases of the electron transport system. Light sources such as metal-halide, fluorescent, high-pressure sodium, neon lamps and light-emitting diode (LED) can be used for production of plants in closed environments instead of sunlight. Manipulation of the light spectrum of the lamps could trigger potential benefits by enhancing plant growth. Nowadays, by using the LED technology, it is possible to study the physiological effect of different light spectra for optimization of growth conditions and for increase the production of plants in controlled environments. This research was conducted to investigate photosynthetic apparatus, growth parameters, stomatal characteristics, transpiration rate and essential oil content of Salvia officinalis under different light spectra.   Materials and Methods  In this study, the effects of different light spectra were implemented and performed as a pot experiment using soilless media in the plant growth chamber based on a completely randomized design with 6 lighting spectra including White, Blue, Red and three combinations of R and B lights (R30:B70, R50:B50 and R70:B30) with three replications. The light intensity in all growth chambers was adjusted to photosynthetic photon flux density (PPFD) of 250 ±10 μmol m-2s-1 and light spectrum were monitored using a sekonic light meter (Sekonic C-7000, Japan). Growth condition was set at 14/10 h day/night cycles, 25/22oC day/night temperatures and 40% relative humidity. Three month following plant growth under different light spectra, the plants were evaluated for their growth parameters, stomatal characteristics (stomatal length, stomatal width, pore length or aperture) transpiration rate (E), relative water content (RWC), photosynthetic apparatus (evaluation of OJIP) and essential oil content. Data analysis of variance (ANOVA) was performed using IBM SAS software (Version 9.1) and the differences between means were assessed using Duncan’s multiple range tests at p≤ 0.05.   Results  The results showed that the stomata characteristics, photosynthetic performance, growth characteristics and essential oil content of Salvia officinalis were affected by different light spectra. Increasing the ratio of red light especially combined Red and Blue lights (R70:B30) led to the improvement of growth characteristics. Transient induction of chlorophyll fluorescence showed that the highest fluorescence intensities at all OJIP steps were detected in Red light. The lowest Fv/F0 and Fv/Fm were obtained in plants grown under Red light. Occurrence of leaf epinasty and decrease in Fv/Fm indicative of phenomenon of red light syndrome in the plants under Red treatment. Red light caused a reduction in performance index per absorbed light efficiency of (PIABS) and increase in quantum energy dissipation (ΦD0), light absorption (ABS/RC) and electron trapping (TR0/RC) per reaction center. The highest Fv/F0, Fv/Fm and PIABS were obtained under combination of Red and Rlue light. The highest ΦE0 was also detected in combination of Red and Blue light. The narrow and large stomatal apertures were detected under Red and Blue light, respectively. The highest transpiration rate was achieved in plants grown under Blue light LED. Increasing the ratio of Red light resulted in reduction in transpiration rate and improvement of leaf capacity to control water loss via reducing the opening of stomata. The highest amount of essential oil (1/75% v/w) was achieved in plants exposed to combination of Red:Blue light spectra (R70:B30). Conclusion  light spectrum during plant growth can change plant metabolism, LED can be used in favor of producing good-quality food in controlled environment agriculture due to their ease of application, waveband manipulation and limited heat production. Our result showed that photosynthetic apparatus, growth parameters, stomatal characteristics, transpiration rate, relative water content and essential oil content of plants were considerably influenced by light spectra. Using OJIP test confirmed that plants grown under monochromatic Red and Blue lights were less efficient to successfully transfer the excitons and most of the absorbed energy by the photosystems was dissipated as heat. In conclusion, combined Red and Blue lights (especially R70:B30) caused favorable growth, photosynthetic functionality and maximum essential oil content of Salvia officinalis. Therefore, combination of R and B lights (R70:B30) should be considered for production of Salvia officinalis under artificial light systems during commercial controlled environment production of plants.