Field controlled experiments on the physiological responses of maize (Zea mays L.) leaves to low-level air and soil mercury exposures

Abstract

Thousands of tons of mercury (Hg) are released from anthropogenic and natural sources to the atmosphere in a gaseous elemental form per year, yet little is known regarding the influence of airborne Hg on the physiological activities of plant leaves. In the present study, the effects of low-level air and soil Hg exposures on the gas exchange parameters of maize (Zea mays L.) leaves and their accumulation of Hg, proline, and malondialdehyde (MDA) were examined via field open-top chamber and Hg-enriched soil experiments, respectively. Low-level air Hg exposures (<50 ng m −3 ) had little effects on the gas exchange parameters of maize leaves during most of the daytime (p>0.05). However, both the net photosynthesis rate and carboxylation efficiency of maize leaves exposed to 50 ng m −3 air Hg were significantly lower than those exposed to 2 ng m −3 air Hg in late morning (p<0.05). Additionally, the Hg, proline, and MDA concentrations in maize leaves exposed to 20 and 50 ng m −3 air Hg were significantly higher than those exposed to 2 ng m −3 air Hg (p<0.05). These results indicated that the increase in airborne Hg potentially damaged functional photosynthetic apparatus in plant leaves, inducing free proline accumulation and membrane lipid peroxidation. Due to minor translocation of soil Hg to the leaves, low-level soil Hg exposures (<1,000 ng g −1 ) had no significant influences on the gas exchange parameters, or the Hg, proline, and MDA concentrations in maize leaves (p>0.05). Compared to soil Hg, airborne Hg easily caused physiological stress to plant leaves. The effects of increasing atmospheric Hg concentration on plant physiology should be of concern.

Quantitative Trait Loci for Mercury Accumulation in Maize (Zea mays L.) Identified Using a RIL Population

“…Hg pollution has aroused global concern because of its toxicity to organisms, persistence in the environment and long-range transport [25] . In some contaminated areas, food chain transfer is very common [31] – [34] .…”

“…In maize, there have been some studies on the physiological and biochemical responses to Hg tolerance. The net photosynthesis rate and carboxylation efficiency of maize leaves exposed to 50 ng m −3 Hg (in the air) were significantly reduced [25] . In vivo , HgCl 2 inhibited 5-aminolevulinic acid dehydratase activity in excised greening leaf segments of maize, and the inhibition could be alleviated by the addition of KNO 3 [26] .…”

Seasonal variations in metallic mercury (Hg&lt;sup&gt;0&lt;/sup&gt;) vapor exchange over biannual wheat–corn rotation cropland in the North China Plain

“…The study of Niu et al (2011) focusing on wheat and corn indicated a significant correlation between foliage Hg content and the exposure level of airborne Hg 0 for their principal growing stages. In a further study, Niu et al (2014) showed that only a moderate level of Hg 0 pollution in air (∼ 20 ng m −3 ) was required to induce measurable physiological stress on corn tissue.…”

Dynamic change of wheat eco-physiology and implications for establishing high-efficient stable agro-ecosystems under Hg stress

“…In this research, the absorption and accumulation of heavy metal ion Hg 2+ led to the drop in Pn of wheat seedlings to some extent at different times. Meanwhile, the accumulation of Hg 2+ also brought about relatively large magnitudes of Tr fluctuation, because the root systems of plants exerted certain inhibitive effects on the uptake of heavy metal ions (Niu et al, 2013), which partly resulted in the enhancement of Pn and Tr values in wheat seedlings (Figs. 2 and 3).…”