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.
Welcome to part I of the “Back to the Root” series of articles, where we will be delving into the botany of cannabis. Because all cannabis products begin with a plant, it behooves us to investigate the botanical nature of our matrix. Cannabis has proven to be a highly complex matrix and this complexity is directly tied to its botanical nature. A plant is a stationary organism; it cannot run away from threats, locate new resources when an area is depleted, or search out a mate. Instead plants use the language of chemistry to attract pollinators or herbivore predators, respond to threats with defense compounds, or direct the growth of new biomass. The goal of this article series is to help illuminate how understanding the botanical nature of cannabis is intrinsic to effective testing.
Our focus today is on heavy metal ions, particularly how they enter and move in plant tissues. When considering the movement of heavy metal ions into and throughout plant tissues, a basic understanding of plant anatomy is a prerequisite. Plants can be divided into two basic anatomical regions—the roots, consisting of everything below ground and the shoots, which is everything above ground such as the stems, leaves, and flowers.
To conduct water, essential nutrients, and the products of photosynthesis between the roots and shoots, plants have a two-part vascular system. The phloem transports photosynthates from “source to sink,” that is, it moves these compounds from where they are produced to where they are consumed. The xylem transports water and dissolved minerals from roots to shoots. While the phloem can move in both directions, the xylem is unidirectional. Another key difference between these systems is that the phloem consists of living cells that can assist in active transport, while the xylem tissue is dead at maturity and can function only via passive transport. This distinction is important because, as we will discuss in more detail later, it means that ions must be actively transported into the xylem by surrounding cells.
To maintain enough water to prevent wilting or death, plants have two primary means of facilitating the movement of water into the plant so that it can be distributed by the xylem. The most dramatic and effective means of drawing water out of the soil, which is effectively a concentrated solution, up to the top of a very tall tree is through transpiration. Transpiration is the evaporative loss of water from the shoots, which is controlled by the opening and closing of stomata, specialized pores embedded in the surface of the leaves that facilitate gas exchange. When the stomata are open, the pressure potential of the plant becomes very negative, creating a vacuum effect that draws water into the plant, moving it from roots to shoots. This flow of water into the plant and through the xylem as a result of transpiration is referred to as the transpiration stream.
The other secondary method for drawing water into the plant is by increasing the solute concentration of the roots, where most water uptake occurs. Concentration is increased by active transport of ions into the xylem from surrounding living cells, causing water to flow into and up the xylem. This phenomenon, referred to as root pressure, is most likely to occur at times when transpiration has stopped or slowed, such as at night or during times of stress, and water concentration in the soil is high.
Now that we have some understanding of the forces that move water from soil to plant we can discuss how minerals make their way into the vascular system. Most water and mineral absorption occurs in the youngest parts of the roots. The transport of minerals into the root is thought to be an active process that is energy dependent (1). We know that the concentration of essential minerals in root tissue is significantly higher than the surrounding soil, making the concentration gradient unfavorable for passive transport. By pumping protons out of the root cells, the positive charge of the soil increases creating a gradient that drives desirable cations into the cell by means of specialized transport channels. As we will see later, these transport channels don’t exclude chemically similar ions, making them a possible entry point for heavy metals.
Once a substance enters the root, it must make its way through the root tissue to the phloem and xylem systems located in the core. There are three possible pathways that substances can utilize to travel through the root cells on their way to xylem and phloem. The apoplastic pathway travels passively along the cell wall, without entering the cell itself. Minerals cannot be transported using this pathway, although water can travel apoplastically through some of the root. Once it reaches a strip of specialized cells that serve as the gatekeepers to the xylem and phloem, it must use another pathway called the symplastic pathway. The symplastic pathway travels actively from cell to cell along a highway of cytoplasmic threads (plasmodesmata). Minerals must use this pathway because they require active transport. Once the minerals reach the inner root tissue, they are actively secreted by surrounding cells into the mature, nonliving cells of the xylem. The transmembrane pathway travels through the plasma and vacuole membranes of cells. Vacuoles are special organelles found in plant cells that are used for storage and to support the cell wall; wilting is caused by vacuoles shrinking away from the plant cell wall because there is not enough water to keep them turgid.
To recap, the movement of minerals requires two active, energy dependent steps. The first occurs during mineral uptake at the root surface, and the second when minerals are secreted into the xylem by nearby living cells. Once mineral ions have entered the xylem, they are swept up by the transpiration stream and distributed throughout the plant. Some of the ions will move laterally throughout the roots and stems, while others will continue on to the leaves. Some ions remain in the transpiration stream and are delivered to the leaves where they escape into the atmosphere through the stomata. However, most ions will be actively transferred to the phloem in the leaves, entering the assimilation stream. From the assimilation stream they can be transferred to the youngest parts of the plant or to reproductive organs, such as the flower of a cannabis plant. Interestingly, there is no direct exchange between flowers or young shoots and the xylem; the phloem always acts as an intermediary.
- R.F. Evert and S.E. Eichhorn, Raven Biology of Plants, 8th Edition (W. H. Freeman, Macmillan, 2013) pp. 708–721.
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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).