Journal:Cadmium bioconcentration and translocation potential in day-neutral and photoperiod-sensitive hemp grown hydroponically for the medicinal market

Full article title	Cadmium bioconcentration and translocation potential in day-neutral and photoperiod-sensitive hemp grown hydroponically for the medicinal market
Journal	Water
Author(s)	Marebesi, Amando O.; Lessl, Jason T.; Coolong, Timothy W.
Author affiliation(s)	University of Georgia
Primary contact	Email: aom at uga dot edu
Year published	2023
Volume and issue	15(12)
Article #	2176
DOI	10.3390/w15122176
ISSN	2073-4441
Distribution license	Creative Commons Attribution 4.0 International
Website	https://www.mdpi.com/2073-4441/15/12/2176
Download	https://www.mdpi.com/2073-4441/15/12/2176/pdf?version=1686282553 (PDF)

Abstract

Heavy metal contamination of agricultural soils is potentially concerning when growing crops for human consumption. Industrial hemp (Cannabis sativa L.) has been reported to tolerate the presence of heavy metals such as cadmium (Cd) in the soil. Therefore, the objectives of this study were to evaluate Cd uptake and translocation in two day-length-sensitive (DLS) and two day-neutral (DN) hemp varieties grown for the medicinal market and to determine the impact of Cd exposure on cannabinoid concentrations in flowers. A hydroponic experiment was conducted by exposing plants to 0 mg·L⁻¹ Cd and 2.5 mg·L⁻¹ Cd in the nutrient solution. Cadmium concentrations ranged from 16.1 to 2274.2 mg·kg⁻¹ in roots, though all four varieties accumulated significant concentrations of Cd in aboveground tissues, with translocation factors ranging from 6.5 to 193. Whole-plant bioconcentration factors ranged from 20 to 1051 mg·kg⁻¹. Cannabinoid concentrations were negatively impacted by Cd exposure in DN varieties but were unaffected in DLS varieties. Biomass was reduced by Cd exposure demonstrating that these varieties might not be suitable for growth on contaminated soil or for phytoremediation. There is potential for Cd accumulation in flowers, showing the need for heavy metal testing of C. sativa consumer products.

Keywords: auto-flower, bioconcentration factor, cannabinoid, heavy metal, hemp, metal stress, translocation factor

Introduction

For centuries, humans have cultivated hemp (Cannabis sativa L.) for its fiber, seed, therapeutic, and psychoactive properties. During the domestication process, wild Cannabis populations have been subject to selection, giving rise to the multiple varieties that exist today. [1] The term "industrial hemp" is commonly used to refer to C. sativa plants [2] with total tetrahydrocannabinol (THC) concentrations below 0.3%. Plants with total THC concentrations above 0.3% are classified as marijuana and subject to federal prohibition in the United States (U.S.) [3] Current industrial hemp breeding efforts target fiber, seed, or cannabinoid production, based on the end user. Hemp varieties with high cannabidiol (CBD) concentrations are often selected for medicinal and therapeutic uses. There is also a focus on developing varieties that are day-neutral (DN) or minimally sensitive to photoperiod in order to expand production opportunities. [4]

Hemp is generally considered a qualitative short-day plant that flowers in response to decreasing photoperiods. Hemp selections that flower in response to photoperiod are known as day-length-sensitive (DLS). After emergence, hemp undergoes a photoperiod-dependent vegetative phase maintained by exposure to approximately 14–18 hours or more of light daily. [5,6] When hemp is planted during periods of short days (<13 hours of light), it may flower prematurely. Premature flowering, prior to complete vegetative development, can result in yield reductions. [7,8] In contrast, some hemp varieties exhibit DN flowering tendencies known colloquially as “auto-flower” hemp. These DN varieties are relatively insensitive to photoperiod for flower induction. The DN trait is speculated to arise from Cannabis ruderalis (C. sativa ssp. ruderalis) and may have originated from hemp located in high latitudes where photoperiods can be long and growing seasons are typically short or regions with relatively short and constant photoperiods. [5,9,10] Advantages of DN hemp varieties include the ability to flower in regions that have little variation in photoperiod throughout the year (tropics) or during times of the year when photoperiods may be inadequate to grow DLS varieties. However, many DN types of hemp have been reported to be particularly sensitive to environmental stressors such as high temperatures and may have lower yields than comparable DLS varieties. [11]

In addition to uses for fiber, seed, and medicinal purposes, hemp has also been proposed as a candidate for use in phytoremediation, which utilizes plants to remove contaminants, such as heavy metals or other chemicals from soils. [12,13] Accumulator plant species can uptake heavy metals from soils, even at low external concentrations, and concentrate them in plant tissues. [14] By growing accumulator plants in contaminated soil, it is possible to realize in situ decontamination, an economically viable approach that preserves physicochemical soil characteristics, while removing contaminants. [15] The morphophysiological characteristics of hemp, such as high biomass production, deep roots, and short life cycle, make it a potential candidate for phytoremediation. [16,17,18,19,20]

Heavy metal contamination of agricultural soils is a concern when growing crops for food or medicinal purposes, due to potential harm to human and animal health. [21,22,23] Cadmium (Cd) contamination in the environment has been linked to anthropogenic activities, such as mining and smelting. Further, Cd can be introduced to soils via contaminated manure, sewage sludge, and phosphate fertilizers. [24] Cadmium is known to cause health issues when ingested in amounts greater than the provisional tolerable monthly intake (PTMI) of 25 μg·kg⁻¹ of body weight. [25] In previous studies utilizing naturally and artificially contaminated soil and substrate containing from 0 to up to 200 mg·kg⁻¹ Cd, hemp varieties grown for fiber production accumulated Cd in aboveground tissues at levels that could be harmful to human health. [26,27,28,29] For instance, the hemp fiber variety Silistrinski grown in naturally contaminated soil containing 12.2 mg·kg⁻¹ Cd accumulated 1.22 mg·kg⁻¹ Cd in its flowers. [27]

There are multiple indicators that can be used to determine the accumulation potential of a plant species. Bioconcentration factor (BCF) is the ratio between the metal concentration in plant tissues and the initial metal concentration in the soil or growing solution. [20,30,31,32,33] This indicator has also been used interchangeably with terms such as accumulation factor (AF) [28], biological absorption coefficient (BAC) or index of bioaccumulation (IBA). [23] A separate indicator of accumulation potential is the translocation factor (TF), which is the ratio between the metal concentration in the above ground biomass and the metal concentration in the roots. [31,32] Additionally, plant growth parameters can be assessed to determine the tolerance index (TI), calculated as the ratio between growth in contaminated and non-contaminated soils. [20,30] There is significant variability in BCF among plant species and chemical elements. It has been proposed that plants with BCF >100 mg·kg−1 Cd on a dry weight (DW) basis in its leaves could be referred to as hyperaccumulators [14]. Conversely, [12] suggested that true hyperaccumulators are able to accumulate higher concentrations of metals in leaves than in roots (TF > 1).

Notes

This presentation is faithful to the original, with only a few minor changes to presentation. Some grammar and punctuation was cleaned up to improve readability. In some cases important information was missing from the references, and that information was added.