Copper stress-induced phytotoxicity associated with photosynthetic characteristics and lignin metabolism in wheat seedlings

Copper (Cu) pollution is one of environmental problems that adversely affects the growth and development of plants. However, knowledge of lignin metabolism associated with Cu-induced phytotoxicity mechanism is insufficient. The objective of this study was to reveal the mechanisms underlying Cu-induced phytotoxicity by evaluating changes in the photosynthetic characteristics and lignin metabolism in the seedlings of wheat cultivar 'Longchun 30 '. Treatment with varying concentrations of Cu clearly retarded seedling growth, as demonstrated by a reduction in the growth parameters. Cu exposure reduced the photosynthetic pigment content, gas exchange parameters, and chlorophyll fluorescence parameters, including the maximum photosynthetic efficiency, po-tential efficiency of photosystem II (PS II), photochemical efficiency of PS II in light, photochemical quenching, actual photochemical efficiency, quantum yield of PS II electron transport, and electron transport rate, but notably increased the nonphotochemical quenching and quantum yield of regulatory energy dissipation. Addi-tionally, a significant increase was observed in the amount of cell wall lignin in wheat leaves and roots under Cu exposure. This increase was positively associated with the up-regulation of enzymes related to lignin synthesis, such as phenylalanine ammonia-lyase, 4-coumarate:CoA ligase, cinnamyl alcohol dehydrogenase, laccase, cell wall bound (CW-bound) guaiacol peroxidase, and CW-bound conifer alcohol peroxidase, and TaPAL, Ta4CL, TaCAD, and TaLAC expression. Correlation analysis revealed that lignin levels in the cell wall were negatively correlated with the growth of wheat leaves and roots. Taken together, Cu exposure inhibited photosynthesis in wheat seedlings, resulting from a reduction in photosynthetic pigment content, light energy conversion, and photosynthetic electron transport in the leaves of Cu-stressed seedlings, and the Cu-inhibitory effect on seedling growth was related to the inhibition of photosynthesis and an increase in cell wall lignification.