Effects Of Light Intensity On Anthocyanins Contents And Photosynthetic Characteristics In Ocimum Basilicum L. Leaves

Ocimum basilicum L. is an upmarket healthy vegetable which combines edible medicinal and ornamental functions. It has been the study focus in medical health foods and chemical industry area around the world. Anthocyanins contents, anatomical structure, gas exchange, chlorophyll a fluorescence kinetics and reflectance spectroscopy of leaves under different light intensity were investigated in ocimum basilicum leaves. The main results are as follows:1. Effects of light intensity on anatomical structure and photosynthetic characteristics in ocimum basilicum leavesCompared with the plants under the high light (ambient condition), the leaves became thinner under the weak light. Palisade tissue became loose, and its proportion in leaf decreased. Specific leaf area, the chlorophyll and carotenoid content per leaf area, stomatal density and stomatal index on adaxial and abaxial surfaces were all significant reduced. Anthocyanins content in purple basil leaves also declined, while leaf area had no significant changes. It was found that purple basil leaves turned green from purple in the end. There was no anthocyanins generated in green basil leaves, but its leaf area became larger.The light compensation point (LCP) and light saturation point (LSP) were significantly reduced; the net photosynthetic rate (Pn), maximal photochemical efficiency (Fv/Fm) and photosystemⅡcapability index (PI) sharply decreased; the efficiency of open centers of photosystemⅡ(Fv'/Fm'), photochemical quenching (qP) and actual photosystemⅡefficiency (ΦPSⅡ) gradually decreased, while non-photochemical quenching (NPQ) enhanced under the weak light. The change of photochemical reflectance index (APRI) of leaves were slightly increased in purple basil leaves, but slightly declined in green basil leaves under the weak light.2. Changes of photoprotect mechanism in ocimum basilicum L. leaves after transferred from low to high photosynthetic photoflux densityThe chlorophyll and carotenoid content per leaf area and the chlorophyll a/b and anthocyanins content in purple basil leaves increased by time after transferred from low illumination environment to high photosynthetic photoflux density.Photoinhibition in photosynthesis occurs easily in the first day after transferred from low to high light, when the net photosynthetic rate (Pn) and maximal photochemical efficiency (Fv/Fm) were sigificiant decreased. But with the time expand the net photosynthetic rate (Pn) and maximal photochemical efficiency (Fv/Fm) gradually raise again. Compared with the green basil, the purple basil has the lighter photoinhibition.The efficiency of open centers of photosystemⅡ(Fv'/Fm'), photochemical quenching (qP) and actual photosystemⅡefficiency (ΦPSII) were gradually enhanced, while non-photochemical quenching (NPQ) decreased in the new functional leaf; thermal dissipation relying on xanthophyll cycle reinforced in green basil leaves, but thermal dissipation relying on xanthophyll cycle reductioned in purple basil leaves with the time expand after transferred from low to high photosynthetic photoflux density.On these bases, we concluded that light intensity could regulate photosynthetic capacity by influencing leaves thickness, stomatal density, specific leaf area, pigment content and photosystem II activity. The plants relieve photoinhibition through improving photosynthetic capacity under strong light. We also discovered that light intensity played an important role in regulating anthocyanins synthesis, and high light promoted anthocyanins synthesis. Also, anthocyanins synthesis was beneficial to relieving strong light stress in Purple Basil leaves; the changes of anthocyanins content could contribute to adapting different light intensity environment. Compared with the green basil, the purple basil had the lighter photoinhibition and the higher light saturation point (LSP).Thermal dissipation relying on xanthophyll cycle may play an important role in dissipating excessive excitated energy under strong light in green basil leaves.