Enlightening the Pathway of Phytoremediation: Ecophysiology and X-ray Fluorescence Visualization of Two Chilean Hardwoods Exposed to Excess Copper

Abstract

In the present climate emergency due to global warming, we are urged to move away from fossil fuels and pursue a speedy conversion to renewable energy systems. Consequently, copper (Cu) will remain in high demand because it is a highly efficient conductor used in clean energy systems to generate power from solar, hydro, thermal and wind energy across the world. Chile is the global leader in copper production, but this position has resulted in Chile having several hundred tailing deposits. We grew two Chilean native hardwood species, quillay (Quillaja saponaria Molina) and espino (Vachellia caven (Molina) Seigler & Ebinger, under three increasing Cu levels (0, 50, and 100 µM) for 6 months in a greenhouse setting. We measured growth, photosynthetic performance and elemental contents of leaves and roots to further evaluate their potential for phytoremediation. Growth of quillay was unaffected by Cu treatment but growth of espino was enhanced, as was its photosynthetic performance, indicating that espino may have an unusually high requirement for copper. Excess Cu was mostly restricted to the roots of both species, where X-ray fluorescence (XRF) mapping indicated some tendency for Cu to accumulate in tissues outside the periderm. Calcium oxalate crystals were prominently visible in XRF images of both species. Nickel (but not Cu) showed a concurrent distribution pattern with these crystals.

Keywords: Chile; native species; photosynthesis; synchrotron radiation; tailings.

Figure 1

Characteristics of soil mix from pots containing espino or quillay after 6 months growth; (a) calcium, (b) copper, (c) iron, (d) potassium, (e) manganese, (f) molybdenum, (g) organic matter, (h) pH, (i) zinc, (j) citric acid, (k) oxalic acid. Whiskers in boxplots show the data range excluding outliers.

Figure 2

Growth parameters of espino and quillay at time of harvest; (a) biomass, (b) shoot length, (c) root length, and (d) CCI.

Figure 3

Effects of Cu treatments on photosynthetic traits in espino and quillay; (a) maximum photosynthetic rate, (b) instantaneous water use efficiency, (c) stomatal conductance, (d) quantum yield.

Figure 4

Multi-element spectra in plant tissues; (a) espino leaf, (b) quillay leaf, (c) espino root, (d) quillay root. In every panel, the spectrum for Cu-treated tissue (100 μM treatment) is normalized relative to the spectrum for the control (0 μM treatment) by adjusting the total counts upwards or downwards, as appropriate to the sample. This normalization accounts for the lower signal-to-noise ratio for the Cu-treated quillay in panel b.

Figure 5

X-ray fluorescence images of leaflets of espino at 100 μm Cu. Scale bar of 100 μm; arrow in the lower left panel showcases the presence of calcium oxalate crystals in the secondary rachis.

Figure 6

X-ray fluorescence images of a leaf of quillay at 100 μm Cu. Scale bar of 100 μm; arrow in lower left panel points to calcium oxalate crystals at the mid-rib.

Figure 7

X-ray fluorescence images of an espino root at 100 μm Cu. Scale bar of 100 μm.

Figure 8

X-ray fluorescence images of a quillay root at 100 μm Cu. Scale bar of 100 μm.

Figure 9

X-ray fluorescence images of Ca and Ni, and their combined distributions, in leaflets of espino (first row) and roots of quillay root (second row), both at 100 μm Cu. Scale bar of 100 μm.