Abstract
Light is one of most important factors to plants because it is necessary for photosynthesis. In this study, physiological and gene expression analyses under different light intensities were performed in the seedlings of rubber tree (Hevea brasiliensis) clone GT1. When light intensity increased from 20 to 1000 µmol m−2 s−1, there was no effect on the maximal quantum yield of photosystem II (PSII) photochemistry (Fv/Fm), indicating that high light intensity did not damage the structure and function of PSII reaction center. However, the effective photochemical quantum yield of PSII (Y(II)), photochemical quenching coefficient (qP), electron transfer rate (ETR), and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae (qL) were increased significantly as the light intensity increased, reached a maximum at 200 µmol m−2 s−1, but decreased from 400 µmol m−2 s−1. These results suggested that the PSII photochemistry showed an optimum performance at 200 µmol m−2 s−1 light intensity. The chlorophyll content was increased along with the increase of light intensity when it was no more than 400 µmol m−2 s−1. Since increasing light intensity caused significant increase in H2O2 content and decreases in the per unit activity of antioxidant enzymes SOD and POD, but the malondialdehyde (MDA) content was preserved at a low level even under high light intensity of 1000 µmol m−2 s−1, suggesting that high light irradiation did not induce membrane lipid peroxidation in rubber tree. Moreover, expressions of antioxidant-related genes were significantly up-regulated with the increase of light intensity. They reached the maximum expression at 400 µmol m−2 s−1, but decreased at 1000 µmol m−2 s−1. In conclusion, rubber tree could endure strong light irradiation via a specific mechanism. Adaptation to high light intensity is a complex process by regulating antioxidant enzymes activities, chloroplast formation, and related genes expressions in rubber tree.
Highlights
Light is an important source of photosynthetic apparatus for photosynthesis and carbohydrate assimilation in plants, alga, etc
The Fv/Fm has been widely used as an indicator of photoinhibition [15]. These results suggested that no photoinhibition was occurred even at high light intensity of 1000 mmol m22 s21 in rubber tree seedlings
Since low light intensity was not strong enough to induce the formations of chlorophyll and antioxidant enzymes in rubber tree, MDA content was increased at 20 to 100 mmol m22 s21. These results indicated that high light intensity increased reactive oxygen species (ROS) contents, more antioxidant enzymes were formed and the amounts of enzymes were increased at the meantime, resulting in per unit activity of enzymes was decreased
Summary
Light is an important source of photosynthetic apparatus for photosynthesis and carbohydrate assimilation in plants, alga, etc. Chloroplasts capture light energy and causes accumulation of reactive oxygen species (ROS). Ascorbate (vitamin C)-glutathione cycle, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) enzyme system, tocopherol (vitamin E), and carotenoids (xanthophyll cycle pigments) are commonly known as antioxidants [2,3]. All photosynthetic organisms regulate the synthesis of their photosystem in response to changes of environment. Plants regulate both chloroplast and nuclear gene expression in response to photosynthesis mediated changes in cellular redox [4,5]. Redox regulation of photosystem genes allows plants to ‘fine-tune’ synthesis of the photosystem in response to the light intensity
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