The present work aimed to study the physiological effects of mixed heavy metals, cadmium (Cd), copper (Cu), chromium (Cr), and zinc (Zn) in four poplar hybrids. Poplar hybrids were treated once with six different concentrations (0, 5, 50, 100, 200, and 500 ppm) of mixed heavy metals for a period of 3 months. The photosynthetic and transpiration rates were measured before and during treatment using an LCiÀSD portable photosynthesis system whereas the photosynthetic pigments were analytically determined. According to the results obtained, poplar hybrids exhibited a decrease in chlorophyll content and carotenoids except for hybrid 1 (Eco 28) which showed an increase at 200 and 500 ppm heavy metal concentrations. The photosynthetic rate decreased with increasing heavy metal concentrations; however, an increase was observed at the highest concentration of 500 ppm at 14.54 mmol m ¡2 s ¡1 . Transpiration rates were significantly affected by increased heavy metals. Hybrid 1 (Eco 28) was deduced as the hybrid that has the ability to tolerate high levels of mixed heavy metals. Among the four poplar hybrids tested, hybrid 1 (Eco 28) was the best candidate for phytoremediative experiments under greenhouse conditions. These results are valuable in understanding how various tree hybrids respond to mixed heavy metal stress, especially when using them as bio indicators for phytoremediation experiments.

The present work aimed to determine the phytoextraction potential of four poplar hybrids when treated with mixed heavy metals, cadmium (Cd), chromium, (Cr), copper (Cu), and zinc (Zn) under greenhouse conditions. Poplar hybrids were treated once with six different concentrations (0, 5, 50, 100, 200, and 500 mg L ¡1) of mixed heavy metals over a period of 3 months, after which the plants were harvested, dried, digested using the aqua regia method and the heavy metal concentrations in the roots, stem, and leaves were determined. Individual heavy metal analysis using ICP-AES of separate heavy metals in plant parts demonstrated higher Zn accumulation in leaves, compared to all other heavy metals. As for Cd, Cr, and Cu the concentration accumulation in the leaves, stem, and roots varied across hybrids. Based on the phytoextraction potential of the four hybrids, hybrid 1 (Eco 28) was determined as a suitable candidate for phytoextraction of multi-contaminated sites, with DN 034 also showing selective phytoextraction potential in the early stages of growth.

This study aimed to identify the effects of oxidized brown coal humic acid fertilizer on the relative growth rate of several tree species intended for reforestation. Field experiments were carried out during 2011-2014 at the Research and Experimental Center for Combating Desertification located at the Elsen Tasarkhai station in central Mongolia. The trees studied were Populus sibirica Tausch., Salix ledebouriana Trautv., and Acer tataricum L. The experiment was conducted with concentrations of 2000, 10,000, and 20,000 mg L −1 of humic acid fertilization treatment. Measurement of the relative height growth rate (RHGR) was undertaken for a period of four years. The results demonstrated significant differences between the humic fertilizer concentrations, which varied depending on the species. Compared to monthly RHGR over the study period, the treatment using fertilizers yielded significantly better tree growth. P. sibirica, when treated with 2000 mg L −1 and 10,000 mg L −1 humic acid fertilizers, had significant height growth rates. S. ledebouriana with 20,000 mg L −1 of humic acid fertilzers treatments showed the highest RHGR. In addition, when the humic acid treatments were compared to the control, results showed that oxidized brown coal humic acid fertilizers as an organic fertilizer can have a significant effect on the growth of A. tataricum. The results equally showed that the soil chemical properties EC, CO 2 , NO 3 , and K 2 O were significant among all the treatments compared to control. The effect on P 2 O 5 significantly increased in all the treatments; however, there was no significant effect on pH and Mg among all treatments. Combining the results obtained with reforestation and sustainable land-management practices can help to improve soil organics in degraded sandy soil regions.

The effects of mixed heavy metals differ not only in different plants but also on the hybrids exposed. In this chapter, we focus on phytoremediation and the physiological effects of mixed heavy metals on four poplar hybrids. According to the results obtained from greenhouse pot experiments with mixed heavy metals, the photosynthetic and transpiration rates were affected by increased heavy metal concentrations. The concentration of heavy metals copper, chromium, cadmium, and zinc in the plant roots, stem and leaves varied with the concentration of mixed heavy metal as well as individual heavy metals. Based on the phytoextraction potential; hybrid 1 (Eco 28) was deduced as the best candidate for phytoremediation in mixed heavy metal contamination treatment. The results obtained are valuable in understanding how specific hybrids respond to mixed heavy metal stress especially when using them as bioindicators for phytoremediation experiments in multi-metal contaminated sites. Selection of new plants along with field trials over extended periods will increase the possibility of further enhancing and establishing phytoremediation technology in the future.

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