Airborne foliar transfer of particular metals in Lactuca sativa L.: translocation, phytotoxicity, and bioaccessibility

Abstract

The uptake, translocation, and human bioaccessibility of metals originating from atmospheric fine particulate matters (PM) after foliar exposure is not well understood. Lettuce (Lactuca sativa L.) plants were exposed to micronic PbO, CuO, and CdO particulate matters (PMs) by the foliar pathway and mature plants (6 weeks old) were analyzed in terms of: (1) metal accumulation and localization on plant leaf surface, and metal translocation factor (TF) and global enrichment factor (GEF) in the plants; (2) shoot growth, plant dry weight (DW), net photosynthesis (Pn), stomatal conductance (Gs), and fatty acid ratio; (3) metal bioaccessibility in the plants and soil; and (4) the hazard quotient (HQ) associated with consumption of contaminated plants. Substantial levels of metals were observed in the directly exposed edible leaves and newly formed leaves of lettuce, highlighting both the possible metal transfers throughout the plant and the potential for human exposure after plant ingestion. No significant changes were observed in plant biomass after exposure to PbO, CuO, and CdO-PMs. The Gs and fatty acid ratio were increased in leaves after metal exposure. A dilution effect after foliar uptake was suggested which could alleviate metal phytotoxicity to some degree. However, plant shoot growth and Pn were inhibited when the plants are exposed to PbO, and necrosis enriched with Cd was observed on the leaf surface. Gastric bioaccessibility of plant leaves is ranked: Cd > Cu > Pb. Our results highlight a serious health risk of PbO, CuO, and CdO-PMs associated with consumption of vegetables exposed to these metals, even in newly formed leaves in the case of PbO and CdO exposure. Finally, the study highlights the fate and toxicity of metal rich-PMs, especially in the highly populated urban areas which are increasingly cultivated to promote local food.

Dual Role of Metallic Trace Elements in Stress Biology—From Negative to Beneficial Impact on Plants

“…According to Sgrigna et al [35], the enhanced air turbulence around the leaves that resulted from a higher leaf area index causes the increase in the dry deposition of contaminated particulate matters. In turn, Xiong et al [10] showed that the metal-enriched particles firstly adhere to the leaf surface as aggregates, and they are then partly trapped by the epicuticular waxes and partly penetrate in the leaf tissue through stomata. Subsequently, Pb, Cd, and Cu ions from directly exposed leaves are transferred to the newly formed leaves and roots.…”

“…The reduced root-to-shoot transport results from: (1) the presence of Casparian strips in the endodermis, which play a role of physical barrier regulating water and ions movement to the stele, (2) precipitation in the intercellular space as insoluble metal-salts, or sequestration in the vacuoles of cortex cells [40,41,42]. Similarly, after foliar deposition, a majority of the TEs remain in the organ by which they are absorbed and only less than 1% of ions is transported to root tissues and other organs [10,43]. However, the mechanisms preventing the translocation of leaf-absorbed TEs within the organism still remain unknown.…”

“…In the soils of temperate climate, the half-life time of chosen TEs varies from 75 to 380 years for Cd and from 1000 to 3000 years for Cu, Ni, Pb, Zn, and Se [4]. For this reason, all of these elements are considered to be non-biodegradable and persistent; however, they can be partially removed from the place where they are accumulated by the natural ability of plant species to uptake HMs via root or foliar penetration [7,10,11].…”

Genome-Wide Analysis of the ATP-Binding Cassette (ABC) Transporter Family in Zea mays L. and Its Response to Heavy Metal Stresses

“…The effects of heavy metals on plants are multifaceted, mainly including plant growth, plant dry weight, net photosynthesis, stomatal conductance, etc. [27]. Such studies are important for analyzing plant mechanisms of physiological and ecological adaptation after short-term exposure to Pb and Cd.…”

Effects of urban atmospheric particulate matter on higher plants using Lycopersicon esculentum as model species

“…During stress, plants suppress some life functions because their stress-signalling networks need energy for survive; one of these functions is the photosynthetic capacity. The increase in biomass depends on the quantity of bound carbon dioxide during photosynthesis [42,43]. Molecular biological studies demonstrated downregulation of photosynthetic gene expression by abiotic stress, and consequently, this regulation also decreases plant growth [44] We demonstrated that PAHs could reduce the biomass and the decrease in biomass was significantly higher with increasing the number of treatments; after the fourth treatment, the biomass compared to the control was 63.63%.…”