Wheat growth, photosynthesis and physiological characteristics under different soil Zn levels

In recent years, heavy metal hazards in the soil have seriously affected agricultural production. This study aims to examine the effects of different levels of heavy metal Zn on the growth, photosynthesis and physiological characteristics of wheat, and provide a theoretical basis for the diagnosis and control of heavy metal pollution in agricultural production. The field test method was used to explore the changes of wheat agronomic traits, photosynthetic capacity, chlorophyll fluorescence parameters, spectral characteristic curve, active oxygen metabolism system, cell ultrastructure, and yield, under different Zn levels (0, 250, 500, 750, and 1 000 mg kg–¹). The results show that, low-level Zn treatments can effectively promote the synthesis of wheat chlorophyll, improve photosynthetic capacity, and increase yield. The yield of ZnL1 (250 mg kg–¹) was the highest in the two-year test, which increased by 20.4% in 2018 and 13.9% in 2019 compared with CK (0 mg kg–¹). However, a high Zn level had a significant stress effect on the photosystem of wheat. PIₐbₛ (reaction center performance index) and Fᵥ/Fₘ (maximum photochemical efficiency) were significantly reduced, the active oxygen metabolism system was damaged, and the photosynthetic capacity was reduced, which in turn led to reduced yield. Among them, the yield of ZnL4 (1 000 mg kg–¹) was the lowest in the two-year test, which was 28.1 and 16.4% lower than CK in 2018 and 2019, respectively. The green peak position of ZnL3 and ZnL4 had “red shift” to the long wave direction, while the red valley position of ZnL4 had “blue shift” to the short wave direction. Under ZnL4, some wheat leaf organelles began to decompose, vacuoles increased, cytoplasm decreased, cell walls thickened, chloroplast basal lamellae were disordered, and mitochondrial membranes disintegrated. Stepwise regression and Path analysis showed that Pₙ (net photosynthetic rate) played a leading role in the formation of yield. Redundancy (RDA) analysis showed that the optimal Zn level for wheat growth was about 250 mg kg–¹, and wheat would be stressed when the soil Zn level exceeded 500 mg kg–¹ in the test condition of this study. Findings of this study provide a theoretical basis for the diagnosis and prevention of heavy metal (Zn) pollution in the soil.