Back to the Root—The Role of Botany and Plant Physiology in Cannabis Testing, Part I: Understanding Mechanisms of Heavy Metal Uptake in Plants: Page 3 of 4

March 6, 2020
Volume: 
3
Issue: 
2
Abstract / Synopsis: 

This article series will explore the effects of plant physiology on testing, including an examination of matrix effects, how specific types of analytes are transported through plant tissues, and synthesis pathways for compounds of interest such as terpenes and cannabinoids. This installment focuses on the physiology of heavy metal transport and translocation into and within plant tissues. It includes a brief explanation of the plant vascular system, how it functions, and how ions enter and move within this system. It finishes with a discussion of how nonessential, toxic substances (that is, heavy metals) enter plant systems, where they accumulate, and potential implications for testing.

Heavy Metal Testing and Research

Because there are so many variables that can impact heavy metal uptake, research that seeks to address issues of heavy metal contamination in cannabis will be necessarily complex. When attempting to troubleshoot contamination it will be important to have an understanding of the metal speciation and form in the contaminated soil. Depending on the metal contaminant, measurement of soil pH could prove useful for elucidating enhanced metal mobility and thus uptake by plants. Determination of soil composition, because it also can directly impact mobility and uptake, might prove useful as well. Unfortunately, to diagnose and correct metal contamination, it is likely that each of these variables would need to be explored. This process could be further complicated depending on the conditions of the farm itself. Indoor farms range from relatively to highly controlled environments, while outdoor farms are open systems that essentially equate to field research.

Once a heavy metal enters a plant, several things can happen: metals can be sequestered in root cell vacuoles or walls, biotransformed to less toxic molecules, or transported from the roots to the shoots (translocation) (2,4). Remember that in the case of minerals, translocation occurs primarily in the xylem for roots and stems, but minerals are transferred to the phloem to reach the flowers and youngest shoots. Which of these options occurs is plant species dependent. Because there is little research on heavy metal uptake in cannabis, what happens after metal uptake occurs in cannabis plants is not well understood.

The following information comes primarily from agricultural and phytoremediation research, which is extensive, with the goal of giving the reader some idea of how these metals might behave in cannabis plants. In general, there are three groups that plant species can fall into for each respective metal. Some species are able to sequester significant concentrations of toxic metals in root tissues, before increased soil concentration forces translocation to avoid poisoning (2). Other species are more likely to translocate metals to aerial tissues and still others can maintain equilibrium with external metal levels in their shoots and roots until toxicity occurs (2). Further, since each metal has unique chemical properties they often behave differently within plant tissues.

Once arsenic has entered plant tissues in the form of As(V), it is often reduced to As(III) or biotransformed into a less toxic compound (2). Some of these compounds include dimethylacetamide (DMA), methyl methacylate (MMA), or inorganic As(III) that has been complexed with thiol groups (2). We know that plants can synthesize enough arsenic reductase because of experiments that showed that both species of arsenic were present in plant roots, but only As(III) was found in the plant’s shoots (2). Research has also shown that plants grown in As(III) amended soil will still uptake a percentage of the arsenic after oxidizing it to As(V).

While it is possible for chromium to be translocated, most chromium is sequestered in the vacuole or cell wall of root cells (2). However, it has been shown that chromium mobility within the plant is dependent on its chemical form (2). As mentioned earlier, chromium uptake sometimes involves complexation with root exudates, many of which are also organic acids. Complexation with these organic acids increases chromium solubility, making it more mobile in the root xylem (2).

Cadmium can be stored in the vacuoles of root cells by phytochelatins that act as chelators to bind cadmium, restricting its movement through the root (2). If cadmium is not bound it can move unrestricted through the root in some plant species via a low-affinity calcium transporter. Translocation can occur in this case, but an unknown mechanism controls and regulates translocation to the aerial shoots (2). Certain plants that can accumulate high concentrations of cadmium are detoxifying it by complexing cadmium with low molecular weight thiols (2). 

Some plant species can detoxify lead by complexation or inactivation and the roots appear to be the primary place where lead is found (2). Lead can be bound to plant cell walls at ion exchange sites, but unbound lead will move towards the root core via calcium transport channels until it reaches a strip of specialized cells called the Casparian strip (2). The Casparian strip can act as a lead barrier to the vascular system at lower concentrations (2).

This hopefully provides a general appreciation for the many ways that specific heavy metals behave once they are in plant tissues. Some overarching patterns include root sequestration, detoxification through biotransformation, or translocation.

References: 
  1. R.F. Evert and S.E. Eichhorn, Raven Biology of Plants, 8th Edition (W. H. Freeman, Macmillan, 2013) pp. 708–721.
  2. J.R. Peralta-Videa, M.L. Lopez, M. Narayan, G. Saupe, and J. Gardea-Torresdey, Int. J. Biochem. Cell Biol. 41(8–9), 1665–1677 (2009). 
  3. R.A. Wuana and F.E. Okieimen, ISRN Ecol, A 402647, 1–20 (2011).
  4. S. Citterio, A. Santagostino, P. Fumagalli, N. Prato, P. Ranalli, and S. Sgorbati, Plant Soil, A 256, 243–252 (2003). 
  5. P. Linger, A. Ostwald, and J. Haensler, Biol. Plant. A 4, 567–576 (2005).

About the Author

Gwen Bode, B.S., is an aspiring doctoral candidate and botanist with a strong chemistry background. As an undergraduate at Eastern Washington University she investigated the vitamin content of a wild edible plant via HPLC. She has since worked at the front line of the cannabis testing industry, integrating her botanical knowledge with the practical aspects of analytical testing. Direct correspondance to: [email protected]

How to Cite this Article

G. Bode, Cannabis Science and Technology 3(2), 26–29, 45 (2020).