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 Uptake
With the physiological transport of water and mineral ions throughout plant tissues in mind, we can talk more specifically about heavy metal uptake, which refers to the movement of ions from the soil into the plant. This discussion will focus on arsenic, lead, cadmium, and chromium because they have been well researched from a plant-physiology perspective. Plant heavy metal uptake is dependent on several factors that affect metal availability and mobility, such as the chemical form and speciation of the metal, soil pH, and soil composition. Furthermore, these variables directly impact each other and therefore cannot be viewed independently.
Take arsenic for example. The speciation of arsenic directly determines its bioavailability; as As(III) it is much less mobile in the soil, and thus less bioavailable than As(V) which has greater soil mobility (2). Soil pH also impacts the chemical form of arsenic in the soil and thus its bioavailability. Low soil pH (pH <4) tends to increase plant uptake of arsenic by enhancing its ability to complex with iron (2). Soil composition plays a role as well, since the presence of either manganese or iron oxides will increase arsenic uptake (2). So, a combination of low soil pH and high iron content together will increase the bioavailability of arsenic. Once a heavy metal makes it close enough to a root cell for plant uptake it enters via the symplastic pathway. Heavy metals often enter plant tissues via the same mechanisms through which desirable, essential minerals such as nitrogen or phosphorous are acquired. Arsenic generally enters plant tissues as As(V), through phosphate transport channels (2). This means that it competes with phosphate for root uptake because of their chemical similarities.
The speciation of chromium, pH of the soil, and soil composition all impact its bioavailability. Chromium enters the roots symplastically, as either Cr(III) or Cr(VI) (2). A pH greater than 5 can decrease Cr(III) solubility by increasing the formation of chromium oxide Cr(OH)3 (3). On the other hand a pH below 5 can decrease the mobility, and thus plant uptake, of Cr(III) because it results in greater adsorption of Cr(III) to clays and oxide minerals in the soil (3). This effect is highly dependent on soil composition; specifically, it is seen in soils with high clay or metal oxide content. So here again we can see that pH in conjunction with soil composition has a combined effect on chromium bioavailability. Chromium enters plant tissues by undergoing reduction, complexation with root exudates, or a combination of the two (2).
For cadmium, uptake is primarily impacted by the presence and concentration of specific minerals in the soil. Manganese enrichment of the soil can increase cadmium uptake while high iron levels will decrease it (2). Furthermore, lowering the pH of the soil appears to increase cadmium uptake by causing the chelation of cadmium with iron (2). This tends to occur when there is an iron deficiency that causes the plant to secrete organic acids into the soil in an attempt to increase iron uptake (2). This suggests that cadmium enters plants through the cellular mechanism that facilitates iron uptake. We do know with certainty that cadmium enters plant cells by flowing down the electrochemical gradient used to drive the cations of essential minerals into the plant cell (2).
Overall, lead has a low solubility and is not readily bioavailable to plants (2). It tends to form complexes with phosphates and sulfates, which are commonly found in the root zone since they are essential to plant nutrition (2). It seems that soil composition is the most important factor for the uptake of lead and that nutrient depleted soils are likely to have greater instance of lead uptake in plants. How lead enters plant cells is still unknown; there are no natural channels for lead uptake, but lead can and does bind to the carboxylic groups of the uronic acid mucilage that coats the surface of the root, protecting it from soil abrasion (2). For this reason, lead is more commonly found as a contaminant in root crops, potentially in greater quantities on the surface of the root rather than within plant tissues.
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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).