Impact of leaf absorption of atmospheric heavy metals on plants: a review

Fossil fuel combustion, vehicle exhaust emissions, metallurgical industry emissions, and mining activities discharge significant amounts of heavy metals into the atmosphere. This has intensified heavy metal pollution, posing serious threats to the environmental quality of soil, water, and plants. After dry and wet deposition from the atmosphere, these heavy metals participate in plant biochemical cycles. While root-based heavy metal uptake is well-documented, the mechanisms and impact of heavy metal absorption by leaves remain poorly studied. This paper summarizes previous work, offering a systematic review of the mechanisms governing plant leaf absorption and transport of atmospheric heavy metals. Plant leaves primarily absorb heavy metals through their stomata and cuticles, with subsequent transport to the phloem through the apoplast or symplast pathway, and final deposition in the leaves, fruits, roots, and other organs, similar to photosynthetic products. Morphological changes in heavy metals in the plant are associated with absorption and transport processes. Toxicity mechanisms primarily involve mechanical damage, inhibition of photosynthesis-related processes, and oxidative stress. Detoxification mechanisms in plants include retention within the cell walls, sequestration in the soluble parts of plants, and defense by the antioxidant system.