This article is the second installment of “Back to the Root,” which seeks to explore the effects of plant physiology on topics related to cannabis testing. The term “pesticide” is a broad umbrella that encompasses any substance used to control unwanted or detrimental organisms including plants, insects, and microbes. In an attempt to elucidate the botanical side of plant-pesticide interactions, this article discusses the ways in which chemical pesticides interact with plant physiology, in particular native defense systems. It also draws comparisons between the chemical similarities and differences of synthetic and biologically sourced pesticides (biopesticides). Finally, it explores the unique botanical issues of pesticide interactions in cannabis, a plant with a plethora of novel, poorly understood phytochemicals.
The word pesticide is a broad term that refers to any substance used to control pests, pathogens, or weeds. Pesticides can be defined by the pest that they control (that is, herbicide, fungicide, insecticide, and so on), their chemical structure (organic versus inorganic), their method of action (systemic versus contact), or their source (synthetic versus biological). “Pesticide” can also refer to biological controls such as predatory insects or bacteria that attack a pest organism. For the purposes of this article, which seeks to explore the role that physiology plays in plant-pesticide interactions we will focus on systemic, chemical pesticides, which function by entering plant tissues rather than contact pesticides that interact with the target organism directly. As we will discuss in more detail later on, the fact that systemic pesticides enter plant tissues has important implications for pesticide residue analysis.
In the part I of the “Back to the Root” article series (1), we began with a brief overview of the plant vascular system so that we could discuss the mechanics of plant heavy metal uptake. As a quick reminder, plants have a two-part vascular system that consists of the xylem, which transports water and dissolved minerals from roots to shoots, and the phloem, which transports photosynthates from where they are produced to where they are consumed, commonly referred to as “source to sink” movement. The phloem is also the primary transport system for phloem-mobile nutrients and signaling molecules related to defense responses (2,3). Some important distinctions between these two systems are that the xylem is a one-way system consisting of cells that are dead at maturity, while the phloem has specialized living cells and can transport substances in both directions. We will talk in more detail about the phloem and how it functions shortly, as it relates to the movement of systemic pesticides within plant tissues.
When considering how pesticides might enter plant tissues, and which system will transport them, it is important to consider the water solubility of the compound in question. Generally speaking, compounds that are more water soluble will translocate (move through plant tissues) via the transpiration stream in the xylem. On the other hand, less water soluble compounds will enter the assimilation stream and translocate in the phloem. Because the transpiration stream requires the tiny pores in the leaf surface, called stomata, to be open, application is best done during the day when plants are photosynthesizing to maximize uptake. In the past, water-soluble systemic pesticides were applied to the soil or root zone in the form of liquids or granules, which makes logical sense because the xylem is a one-way system that moves from roots to shoots. The use of liquid and granular xylem-mobile systemic pesticides used to be more common, however environmental concerns and pest resistance have led to a transition to seed-coating as the primary mode of application (4).
Foliar application of systemic pesticides is usually reserved for compounds that translocate via the phloem. Outdoor cannabis grows might want to be particularly wary of neighbors that are applying systemic pesticides to foliage because there is potential for spray drift and contamination. Phloem-systemic pesticides can also be applied to the root zone because the phloem is an adaptive multidirectional system. This also means that soil contamination of phloem-mobile systemic pesticides can be a potential source of unintentional pesticide uptake. While root uptake of phloem-mobile systemic pesticides is a possible topic of discussion, for the sake of brevity and to avoid repetition of information, this article will focus on systemic pesticides that are applied to foliage.
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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 correspondence to: [email protected]
How to Cite this Article
G. Bode, Cannabis Science and Technology 3(4), 36–41 (2020).