On the eastern plains of China, the overlapped area of farmland and coal resources (OAFCR) covers an area of 1.33 × 105 km2 and accounts for 31.93% of the cultivated land area (Hu et al., 2014). Coal is the primary energy source in China and is widely used for power generation. According to the Statistical Bulletin of 2022 National Economic and Social Development of the People’s Republic of China, the installed thermal power capacity was 1332.39 million kW in 2022, an increase of 2.7% from the previous year. To reduce the process of transporting coal, many coal-fired power plants have been built near coal resources. The combustion of coal releases particulate matter, including fly ash, that can affect the areas surrounding power plants. Furthermore, coal combustion cannot completely destroy inorganic materials such as heavy metals (HMs), which are present in fly ash and are an environmental pollutant (Nguyen et al., 2022). The winter heating season has led to a five-fold increase in PM2.5 in the air (Xiao et al., 2015) and an increase in HMs attached to PM2.5 (Deng et al., 2020) compared with normal levels in many parts of China. Particulates and HMs produced by coal combustion can be deposited on the land surrounding coal-fired power plants, potentially influencing the normal growth of crops in OAFCR and threatening food security.
Stem elongation is considered a crucial stage in determining the final yield of wheat and is sensitive to environmental changes (Kronenberg et al., 2017). Stress can cause an increase in flag leaves and reduce the total chlorophyll content during stem elongation (Fan et al., 2019). Notably, the accumulation of HMs during stem elongation is higher than that in other stages under plant stress (Jin et al., 2010; Su et al., 2019). Moreover, stem elongation is vital for determining the number of fertile florets at anthesis and is directly related to wheat yield (Doghozlou & Emam, 2022).
Similar to the role of plant roots in absorbing HMs, aboveground plant organs, such as leaves, fruits, and flowers, also have a strong absorption capacity (Song et al., 2021). In the OAFCR, the HMs carried in fly ash are deposited onto plant parts, particularly leaves. Leaves can actively absorb HMs through stomata, surface cracks, aqueous pores, and ectodesmata (Fernandez & Brown, 2013; Kovár et al., 2023). Many factors affect the uptake of HMs on the leaf surface, such as the physical and chemical characteristics of the cuticle, plant physiology, the properties of the metal, and exposure time (Wang et al., 2020). After foliar penetration, HMs can be transported to other plant organs through the phloem vascular system in the same manner as substances are transported during photosynthesis (Shahid et al., 2017). Subsequently, heavy metals are transported between cells via active transport within the cell symplastic pathway (Carini & Bengtsson, 2001). Cells can mitigate the toxicity of absorbed HMs to organelles by blocking their entry with the cell wall and separating the HMs in vacuoles (Shahid et al., 2014). Foliar absorption of single metals has been widely studied (Cakmak et al., 2000; Gao et al., 2021a). However, fly ash contains a wide variety of HMs in complex forms and in different amounts (Altikulac et al., 2022). The interactions between different HMs are also complicated. Therefore, the mechanisms underlying HM uptake and transport pathways in plants under fly ash stress require further in-depth analysis.
Photosynthesis is an essential process for ensuring yield (Han et al., 2016). A high fly ash content with attached HMs (Jin et al., 2012; Sagardoy et al., 2010) substantially reduces stomatal conductance, albedo, and photosynthesis (Raja et al., 2014), which inhibits CO2 and O2 exchange. Additionally, HMs can change the chlorophyll structure by forming complexes, such as that formed when Cu replaces the Mg atom in the center of the chlorophyll (Petrovic et al., 2006). Many enzymes are involved in the process of photosynthesis, including ribulose-1,5-bisphosphate carboxylase (Rubisco), a key enzyme in CO2 assimilation that catalyzes the formation of ribulose biphosphate (RuBP), which regulates the Calvin cycle (Han et al., 2016) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and fructose-1,6-bisphosphate aldolase (FBA), which are involved in glycolysis and the Calvin cycle (Patipong et al., 2019; Yu et al., 2020). Excessive HM content inhibits the activity and function of Rubisco and carbonic anhydrases (Arena et al., 2017), whereas it increases GAPDH activity (Chen et al., 2005). Meanwhile, FBA is sensitive to various stressors, like water shortage or salt stress (Cai et al., 2022). Stress conditions induced by particulate matter and attached HMs lead to reactive oxygen species (ROS) generation in plants, which causes oxidative stress and damage to essential cell components (Matic et al., 2021). Specifically, plants increase the activity of superoxide dismutase (SOD) to eliminate O2•− (Zhang et al., 2021) and then increase the activities of peroxidase (POD) (Gajewska & Sklodowska, 2008) and catalase (CAT) to scavenge H2O2 to maintain homeostasis under oxidative stress induced by environmental stress.
Wheat grown in OAFCR is subjected to fly ash and its associated HMs, which causes combined stress with complex effects; however, the mechanisms of the impact of fly ash and HMs on wheat require further study. Therefore, in this study, we simulated different levels of fly ash stress on both the roots and shoots of wheat during the stem elongation period with the aim of determining: (1) the impact of HM absorption and transport on stem elongation in wheat, (2) the effects of particulate matter stress from coal-burning on wheat photosynthesis, and (3) the changes in the antioxidant mechanisms of wheat under particulate matter stress from coal-burning.