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

Abstract

The photooxidation induced by high light (HL) during the grain-filling period usually causes great yield loss of wheat ( Triticum aestivum L.) in northern China. To understand the mechanism of wheat plant in response to HL, the photosynthetic characteristics of Xiaoyan 54, a winter wheat cultivar with high tolerance to HL, were investigated under HL condition. At third-leaf stage, seedlings of Xiaoyan 54 were transferred from low light to HL in a growth chamber and treated for 0, 1, 3, 8, 24, and 48 h. The net photosynthetic rate ( P n), chlorophyll content, and fluorescence parameters were measured with the second leaf. The activities of antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR), were also determined. In addition, the expression patterns of genes encoding pigment-binding proteins were also evaluated. The results showed that the P n increased gradually during the photosynthetic induction phase within 8 h of HL treatment, but decreased continuously during the photoinhibition phase longer than 8 h of HL treatment. The maximum P n value of 18 μmol CO 2 m −2 s −1 was observed at the 8 h timepoint. The G s, C i, and T r varied similarly to the P n and reached their peaks at the 8 h timepoint. The contents of total chlorophyll and chlorophyll a were changed slightly during 48 h of HL treatment. In contrast, chlorophyll b reduced significantly at the 24 and 48 h timepoints, but the ratio of chlorophyll a to b continuously increased from the 8 h timepoint. One hour of HL treatment significantly reduced the maximum quantum efficiency of PSII ( F v/ F m), the maximum fluorescence ( F m), and the variable fluorescence ( F v), but enhanced the heat dissipation process. The activities of SOD, CAT, APX, and GR were induced by HL stress and reached the highest values at 48 h timepoint. At the photoinhibition stage, the RNA transcripts of Talhcb genes that encode LHCII subunits declined. TaELIP1 and TaELIP3 encoding early light-induced protein were induced by short-term HL (within 3 h) and repressed by long-term HL. As the key enzymes in xanthophyll cycle, the transcripts of TaVDE and TaZEP responded differently to the HL stress. The transcripts of TaVDE decreased remarkably after 8 h timepoint and remained low level. However, the transcripts of TaZEP showed an increase trend from 3 to 24 h of HL, and decreased at 48 h timpoints. In conclusion, when wheat seedlings were exposed to continuous HL for 48 h, photooxidative stress occurred, which resulted in a series of variations in physiology and biochemistry, such as reductions of P n, F v/ F m, and Chl b, the down-regulated expressions of pigment-binding protein genes, and the activation of antioxidant enzymes.