Changes of photosynthetic characteristics, stomatal microstructure and proline metabolism in wheat seedlings under different combined treatments of zinc, iron and copper.

Heavy metals including iron (Fe) and copper (Cu) pollution in soil is threatening the ecological environment and crop yield. In this experiment, photosynthetic characteristics, stomatal microstructure and proline (Pro) metabolism in wheat variety 'Xihan 3' seedlings were assessed to explore the mechanism of phytotoxicity induced by various combinations of zinc (Zn), Fe and Cu. Compared with the control, Zn + Fe, Zn + Cu, Fe + Cu and Zn + Fe + Cu treatments obviously lowered the shoot and root lengths, reduced leaf area and root number, along with the increases of malondialdehyde, hydrogen peroxide and superoxide anion radicals levels and relative conductivity in wheat leaves. The net photosynthetic rate also lessened with the decreases of chlorophyll (Chl) level, Chl fluorescence parameters, stomatal conductance and stomatal length and width under these combined treatments. Additionally, different combinations of Zn, Fe and Cu increased the amount of osmotic regulators including Pro in wheat roots, up-regulated Δ1-pyrroline-5-carboxylic acid synthetase (P5CS), glutamate kinase, Δ1-pyrroline-5-carboxylate reductase (P5CR) and ornithine aminotransferase (OAT) activities and TaP5CS, TaP5CR and TaOAT expression, but down-regulated Pro dehydrogenase (ProDH) activity and TaProDH expression. Taken together, different combined treatments of Zn, Fe and Cu severely restricted root and leaf development through enhancing oxidative damage and weakening photosynthetic efficiency in wheat leaves, and the combined treatment-caused Pro accumulation in wheat roots could achieved through both glutamate and ornithine synthesis pathway and attenuated Pro degradation.