Pb residues were found in both industrial and human living soils, which will be further transferred to plants to accumulate (Doris, Jorge et al., Gao, Zhang et al. 2021). A study reported that Pb can cause chlorosis, oxidative stress and growth and development disorders in plant leaves (Kanwal, Farhan et al. 2020). This study evaluated the effects of different doses (300 mg/kg, 600 mg/kg, 900 mg/kg) of Pb stress on the leaf chlorosis, oxidative stress and growth and development of pakchoi. The results showed that Pb stress in pakchoi caused oxidative stress, abnormal mineral content, inhibition of AsA-GSH system and photosynthesis, abnormal chlorophyll content, and abnormal expression of chloroplast development genes in a dose-dependent manner.
Heavy metal stress can cause mineral imbalance in humans, animals and plants (Xu, Xiaojing et al. 2021). In addition, Cd and As are proven toxic mineral elements, which are easy to accumulate in plants and affect growth and development (Irshad, Noman et al. 2021). The ICP-MS method can gauge the content of all elements in plant and animal tissues, and principal component analysis and correlation analysis can simplify the complex relationship between the elements (Xu, Xiaojing et al. 2021), so that we can observe that Pb is exposed to small changes of element content in pakchoi leaves. This experiment found that as the dose of Pb in the soil increased, the content of growth important elements B and Se decreased, while the content of toxic mineral elements such as Cd, As and Cu increased. These results indicate that Pb stress decreases the absorption of beneficial elements in pakchoi leaves in a dose-dependent manner, while the deposition of other toxic metal elements increases. Boron is an vital element for plant reproduction and growth, which plays an important role in the physiological processes of crop plant leaf expansion and meristem development (Pinho, Monnerat et al. 2015). In PCA, Se, B, Hg, Tl and Ba belong to the first component. Li, K, Na, Mo, V, Co, Al, Mn, Cu, Zn and Fe belong to the second component. Se and B were positively correlated with component one; Pb was negatively correlated with component one. Therefore, Se and B are negatively correlated with Pb, which is similar to that obtained by correlation analysis. So, this also suggests to us that adding B or Se to Pb-stressed pakchoi may be used as an antagonist of Pb stress. Pb treatment can reduce Se content and Se supplementation also can reduce Pb content. There was a negative correlation between Pb and Se (Huang, Chen et al. 2021).
The AsA-GSH system is composed of the AsA (that is, vitamin C)-DHA and GSH-GSSG processes and the enzymes involved in these two processes and resists environmental stresses such as low light (Hu, Li et al. 2019). According to reports, Cd and Cu will accumulate in plants, causing the activities of MDHAR, APX and DHAR to decrease, the abnormal levels of AsA and DHA, and the decreases of GSH and GSSG content, leading to the oxidative stress and the imbalance of AsA-GSH cycle (Zhou, Huo et al. 2018, Jung, Lee et al. 2021). Salt-alkali mixed stress reduces the key enzymes of AsA synthesis pathway L-galactose dehydrogenase (GDH) and L-galactose-1,4-lactone dehydrogenase (GLDH) activities, and weakens the AsA-GSH cycle efficiency, thereby causing oxidative damage to naked oats (Liu, Liu et al. 2021). Additionally, ammonia gas stress decreased the activities of antioxidant systems (SOD, T-AOC, and GSH-Px), whereas increased the concentration of MDA in chickens (Han, Zhang et al. 2020). Boron (B) and chromium (Cr) stress increased MDA and caused oxidative stress in wheats (Ashraf, Rasheed et al. 2022). The results of this study are similar to the above-mentioned literature. It was found that under medium and high doses of Pb treatment, the contents of AsA and vitamin C synthesis key enzyme (GLDH) continued to decrease, indicating that Pb has a dose-dependent inhibitory effect on the vitamin C synthesis of pakchoi. With the increase of Pb dose, the oxidative stress marker MDA continued to increase, and the activities of antioxidant enzymes continued to decrease. This shows that the dose dependent Pb stress causes the decrease of the antioxidant capacity and the increase of oxidative stress level. From another angle, we found that the tolerance of pakchoi to adverse environment is reduced from another angle through the ratio of GSH/GSSG. With the help of APX and DHAR activity abnormalities, we can summarize the above results as: Pb stress dose-dependently causes AsA-GSH circulatory system imbalance, which reduces the tolerance of pakchoi to oxidative stress.
Chloroplast is the place where plants photosynthesize. The chlorophyll in the chloroplast absorbs light energy to participate in the normal progress of photosynthesis. The net photosynthetic rate is a key indicator for evaluating the photosynthesis efficiency of plants. Under high-dose metal accumulation stress (60 mg/kg Cd + 90 mg/kg Cu), the photosynthetic characteristics (chlorophyll a and b content, Pn, Tr, Gr and Ci) and nutrients of pea are reduced (Lei, Zhu et al. 2021). This experiment found that medium and high doses of Pb stress reduced the values of Pn, Tr and Gs, and high doses of Pb treatment significantly reduced the values of Ci, which directly explained the negative effects of Pb stress on the photosynthesis of pakchoi. In addition, PSII photoreaction is an important stage of photoreaction (Ci, Jiang et al. 2009), and Fv/Fm and Fv/Fo can be used to measure the original light energy conversion efficiency and maximum light energy conversion potential of PSII system in pakchoi. Research by Li et al. confirmed that under 100 µmol/L Cd hydroponic conditions, the photosynthetic parameters Fv/Fo and Fv/Fm of elsholtzia serrata were significantly reduced (Li, Yang et al. 2015). This study found that as the Pb dose increased, Fv/Fm continued to decrease, and showed a dose-dependent effect. This shows that the negative effect of Pb stress on the photosynthesis of pakchoi via the abnormality of PSII light response system. In addition, the effects of medium and high doses of Pb on the contents of chlorophyll a and chlorophyll b were obviously reduced in a dose-dependent manner. Studies have found that GLK expression leads to the increased levels of chlorophyll and LHC (Li, Wang et al. 2020), and genes such as PIF and HCF are also involved in chloroplast development and chlorophyll synthesis (Schmitz, Schöttler et al. 2012, Zhang, Xiong et al. 2021). The EGY1 (Ethylene-dependent gravitropism-deficient and yellow-green 1) gene encodes a thylakoid membrane-localized protease involved in chloroplast development in mesophyll cells (Sanjaya, Muramatsu et al. 2021). Our research also found that after Pb stress, the expression of chlorophyll synthesis genes (HCF and PIF) and chloroplast development-related genes (GLK, GLN2 and EGY1) of pakchoi were down-regulated to varying degrees, this further confirms that Pb stress may affect the photosynthesis of pakchoi through the development of chloroplast and the downregulation of chlorophyll synthesis. The above results indicate that Pb exposure affects the PSII photoresponse system of photosynthesis by affecting the development of chloroplasts and the synthesis of chlorophyll, which also presents a dose-dependent effect.
In conclusion, we found that Pb stress dose-dependently has an adverse effect on the mineral content of pakchoi, AsA-GSH and photosynthesis. In detail, Pb induces oxidative stress in pakchoi, and the AsA-GSH cycle and photosynthesis are weakened, which further leads to abnormal chlorophyll content and decreasing chloroplast development gene expression. Because heavy metals accumulate in plants through the food chain in the environment, they will eventually endanger humans and animals, and even the entire ecological environment. The results of this study supplement the toxicology of heavy metals and provide instructions for the planting of pakchoi and warnings of heavy metal hazards.