Variation in Photosynthetic Traits and Antioxidant Enzyme Activities of Wheat Seedlings Transferred from Low to High Light Growth Condition

Abstract

When plants absorb excessive light energy, a large number of reactive oxygen species is generally produced resulting in the degradation of DNA, proteins, and pigments in plants. For wheat (Triticum aestivum L.) grown in North China, the photooxidation induced by high light (HL) during grain-filling period usually causes great losses in grain yield. It is important to understand the mechanism of wheat plant in response to HL for HL-tolerant breeding in wheat. Xiaoyan 54, a wheat cultivar with high resistance to HL, is an ideal material to disclose photosynthesis characteristics of wheat when exposed to HL. In this study, the third-leaf seedlings of Xiaoyan 54 were grown under the condition low light to HL in a growth chamber. The seedlings were sampled at 0, 1, 3, 8, 24, and 48 h after HL treatment. Simultaneously, the net photosynthetic rate (Pn), chlorophyll content (Chl), and fluorescence parameters were measured with the second leaf. The activities of antioxidant enzymes, such as superox-ide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR), were also determined. In addition, expression pattern of the pigment binding protein related genes were evaluated. The results showed that Pn increased in the photosynthetic induction stage that was from 0 h to 8 h of HL treatment, but decreased continuously during the photoinhibi-tion stage that was from 8 h to 48 h of HL treatment. The maximum Pn value of 18 μmol CO2 m?2 s?1 was observed at the 8 h timepoint of HL treatment. The parameters Gs, Ci, and Tr changed similarly to Pn and reached the peaks at the 8 h timepoint of treatment. The contents of total chlorophyll and chlorophyll a only changed slightly during the 48 h of HL treatment. In contrast, chlorophyll b reduced significantly from 24 h to 48 h of treatment, and the ratio of chlorophyll a/b increased from 8 h to 48 h of HL treatment. At the one hour timepoint of HL treatment, the maximum quantum efficiency of PSII (Fv/Fm), the maximum fluo-rescence (Fm), and variable fluorescence (Fv) were down-regulated significantly, when the heat dissipation was enhanced. The activities of SOD, CAT, APX, and GR were induced to higher levels with the highest value at the 24 h timepoint of HL treatment. From 8 h to 48 h of HL treatment, the Talhcb genes, encoding LHCII subunits, were down-regulated at the RNA levels. At early stage of HL treatment (0–3 h), TaELIP1 and TaELIP3 were induced, but repressed from 8 h to 48 h. As key enzymes in xantho-phyll cycle, the transcripts of TaVDE and TaZEP responded differently to HL treatment. The expression of TaVDE decreased remarkably at 8 h of HL treatment and maintained a rather low level till 48 h. However, the expression of TaZEP showed an increase trend from 3 h to 24 h, and decreased at 48 h. In conclusion, when wheat seedlings exposed to continuous HL for 48 h, photooxidative stress occurred resulting in reductions of Pn, Fv/Fm, Chl b, and the expressions of pigment binding protein genes, but activation of the antioxidant enzymes.