Garden pansy (Viola × wittrockiana Gams.) - a good candidate for the revitalisation of polluted areas

Abstract

In the current studies, heavy metal tolerance level, accumulation efficiency and sexual reproduction were determined in Viola × wittrockiana, a non-metallophytic ornamental cultivar in comparison to V. tricolor, a metallophyte, after zinc (Zn) or lead (Pb) treatment (0, 10, 100 and 1 000 ppm) in pot experiments. The seed germination frequency that was not reduced in comparison to the control, the effective Zn absorption from the soil and exclusion strategy for Pb, as well as the regular sexual reproduction of V. × wittrockiana treated with heavy metals all indicate the tolerance of this plant to heavy metals. The lack of a seed set under experimental conditions of V. × wittrockiana was due to the absence of pollinators, rather than the negative impact of heavy metals, as pollen viability and ovule development were normal under the treatments. The results indicate that V. × wittrockiana represents similar tolerance to Viola metallophytes and could be considered as a good material for the reclamation of polluted areas. The exceptional tolerance to heavy metals, the ability to initiate new generations in heavy-metal-burdened soil, which are additionally coupled with the unique beauty, make the garden pansy a good candidate to be potentially used in the future for phytoremediation purposes.

Innate, High Tolerance to Zinc and Lead in Violets Confirmed at the Suspended Cell Level

“…High tolerance of the cells of Viola · wittrockiana , a common bedding plant, is not an unexpected result. Sychta et al, 2020 [ 9 ] based on pot experiments measuring seed germination, heavy metal accumulation and sexual reproduction under the Zn and Pb treatments, confirmed that this non-metallicolous ornamental species could be treated as another metal-tolerant violet of similar tolerance to the metallicolous genotype of V. tricolor . Comparing our results with two accumulators Jatropha curcas and Prosopis laevigata [ 46 , 47 ], whose response to heavy metals varied depending on the metal applied, the dose and duration, is unauthorized neither due to the different methodology (biomass weight vs. cell viability) nor due to the fact that, in the latter biomass of all, not only living cells were measured.…”

“…Despite the lower mobility and limited use (for plants) of lead compared to zinc, cells treated with the same amount of either Zn or Pb accumulated at least several times more Pb than Zn. Although this trend is not observed in the field, where violets accumulate more zinc than lead in roots and in the aboveground parts [ 25 ], high BF (above 4) was confirmed in V. tricolor treated (to the soil) with 10 and 100 ppm of Pb [ 9 ]. Similarly, high BF was also observed in V .…”

“…Similarly, high BF was also observed in V . · wittrockiana at low concentrations (10 ppm added to the soil) [ 9 ]. On the other hand, in the cells of violets treated with high concentrations of Zn or Pb (2000 μM), the trend was reversed, cells accumulated significantly higher (10×) amounts of lead than zinc and revealed a very high BF (>50) in respect to the former element [ 11 ].…”

“…Five species of different metallophyte status were chosen: Arabidopsis thaliana —non-metallophyte (species intolerant to heavy metals), Viola philippica —Cd-hyperaccumulating metallophyte [ 21 ], V. x wittrockiana —metal-tolerant non-metallophyte which demonstrates tolerance to Zn and Pb in experimental conditions [ 9 ]; V. tricolor —facultative metallophyte, species occurring on both metalliferous and non-metalliferous soils, showing high tolerance to Zn and Pb [ 25 ], and Silene vulgaris subsp. humilis —facultative metallophyte [ 13 ].…”

“…Both could be the results of heavy metal action (for a revision, see Sychta et al, 2021 [ 8 ]). Suspended cells (derived from callus tissue) grown in in vitro culture reflecting many physiological and biological characteristics of the whole plant are a useful tool to study heavy metal toxicity and tolerance, including processes involved in their death [ 8 , 9 , 10 , 11 ].…”