Under elevated CO2 conditions the photosynthetic rate is enhanced, thus biomass production is positively influenced. According to Wang et al. (2012) [28], the increase in total Cd uptake by poplar (Populus sp.) and willow (Salix sp.) genotypes due to increased biomass production under elevated CO2 conditions suggests an alternative way of improving the efficiency of phytoremediation in heavy metal contaminated soils.

The use of fertilizers is another useful practice that should be adopted by the researchers in order to enhance biomass production under extreme heavy metal toxicity conditions. Some Brassica species, which are suitable to be used as phytoremediators, may suffer from Fe or Mn deficiency symptoms under Cu, or Zn toxicity conditions. In that case, leaf Fe and Mn fertilizations should be done in order to increase their biomass production [29], thus their ability to absorb and accumulate great amounts of heavy metals in contaminated soils, i. e. the efficiency of phytoremediation. According to Li et al. (2012) [25], in order to achieve large biomass crops, heavy fertilization has been practiced by farmers. Application of fertilizers not only provides plant nutrients, but may also change the speciation and mobility of heavy metals, thus enhances their uptake. According to Li et al. (2012) [25], NPK fertilization of Amaranthus hypochondriacus, a fast growing species grown under Cd toxicity conditions, greatly increased dry biomass by a factor of 2.7-3.8, resulting in a large increment of Cd accumulation. High biomass plants may be beneficed and overcome limitations concerning metal phytoextraction from the application of chemical amendments, including chelators, soil acidifiers, organic acids, ammonium e. t.c. [21]. Mihucz et al. (2012) [79] found that Poplar trees, grown hydroponically under Cd, Ni and Pb stress, increased their heavy metal accumulation by factor 1.6-3.3 when Fe (Ш) citrate was used.

Mycorrhizal associations may be another factor increasing resistance to heavy metal toxicity, thus reducing the depression of biomass due to toxic conditions. Castillo et al. (2011) [80] found that when Tagetes erecta L. colonized by Glomus intraradices displayed a higher resistance to Cu toxicity. According to the same authors, Glomus intraradices possibly accumulated excess Cu in its vesicles, thereby enhanced Cu tolerance of Tagetes erecta L. [80].

Finally, other factors, such as the influence of Bacillus sp. on plant growth, in contaminated heavy metal soils, indicate that biomass may be stimulated under so adverse conditions. According to Brunetti et al. (2012) [81], the effect of the amendment with compost and Bacillus licheniformis on the growth of three species of Brassicaceae family was positive, since it significantly increased their dry matter. Furthermore, the strain of Bacillus SLS18 was found to increase the biomass of the species sweet sorghum (Sorghum bicolor L.), Phytolacca acinosa Roxb., and Solanum nigrum L. when grown under Mn and Cd toxicity conditions [82].