Influence of Different Light Spectra on Morphological Traits and Hypericin Content in St. Johnʼs Wort (Hypericum perforatum L.)

St. John’s wort (Hypericum perforatum L.) is a medicinal plant which used mainly in treatment of mild depression, neurological disorders and has been recently shown to have anticancer potential. The principle medicinal components of St. John’s wort are hypericin, pseudohypericin, and hyperforin. Light is one of the most important environmental factors affecting plant growth, survival, reproduction and distribution. The light quality, light intensity, duration and photoperiod directly affect plant growth. Light quality refers to the color or wavelength reaching the plant's surface. A prism (or raindrops) can divide sunlight into respective colors of red, orange, yellow, green, blue, indigo and violet. Red and blue have the greatest impact on plant growth. Green light is least effective (the reflection of green light gives the green color to plants). Blue light is primarily responsible for vegetative leaf growth. The principle objective of the current study was to evaluate the effects of different spectral quality including red, blue, green and white on the growth factors and production of hypericin.

This experiment was conducted on the basis of Completely Randomized Design with four treatments and 10 replications in the growth chamber in the Department of Horticulture, Ferdowsi University of Mashhad (FUM), Iran. In this study, seeds were obtained from the research greenhouse of FUM. Seeds after soaking in running water for 24 hours were planted in small pots (250g). After the seedlings have reached to height of 25cm, each 10 pots were put inside the boxes (20×30cm) made of colored filters. Experiment continued in a growth chamber with day and night temperature 25 and 21°C, respectively, relative humidity 45%, 16 hours of light with the intensity of 1000 lux and 8 hours of darkness for 50 days. Then morphological parameters including plant height, number of stems, number of leaves, number and length of internodes, fresh and dry weight of shoot and root were measured. To count the number of black nodules, the upper, middle and lower parts of seedling were evaluated individually. Hypericin content of the H. perforatum plantlets were measured according to the previous work of Azizi & Omidbaigi, 2002. Data were analyzed statistically by using SAS and Excel software. The significant differences between means were assessed by Tukeyʼs test at P < 0.05.

The results showed that morphological parameters including plant height, leaf number, internode length, root fresh weight, dry weight of stem and root were affected significantly by light treatments. Minimum and maximum of plant height was related to white and red lights, respectively. Increasing plant height under the influence of red light was due to the variation in levels of growth regulators. Red and blue light by changing of the GA hormone level in the plant and affected elongation of plant stem. Minimum and maximum of leaf number was related to blue and white light respectively. Also, other morphological traits including internode length, fresh and dry weights of stem and root showed significant differences. Internode length for white light was less than red, green and blue light. Maximum of the fresh and dry weight of shoot and root of seedlings was observed under white light. Results suggest that photosynthetic compounds move in plant under the influence of light quality. Also the number of black nodules in three different parts of plant and hypericin content were compared in plantlets under the effect of light quality. Results analysis also confirmed that different lights had the significant impact on the number of black nodules in upper and middle leaves of H. Perforatum seedlings. Seedlings treated with the red light had the highest number of black nodules in the middle section of H. perforatum. In the top third of St. John’s wort seedlings, red, blue and green light was inducted the highest number of black nodules formation than white light. The highest level of hypericin was related to red, white, blue and green lights, respectively. The number of black nodules in the plant and the hypericin content have positive correlation. It seems that in the seedlings treated with the red light, carbohydrates made from photosynthesis most used in biosynthesis of secondary metabolites than plant growth. In fact, carbon allelochemical compounds such as terpenes and phenolics have such metabolism direction to explain the increasing in secondary metabolites.

Different light spectra affects plant hormones levels and with alteration in the primary and secondary metabolites lead to a change in the morphological and biochemical traits of plant. In general, it can commented that the quantity and quality of light is able to affect the growth and the active metabolites of medicinal plants and using red and white mixture of light during seedling growth period have an effective role on generation of more strong seedlings with higher potential production of active ingredients.