Accumulation and Histochemical Localization of Cadmium in Hemp (Cannabis sativa L.) Leaf and Root Tissue

As anthropogenic activities have facilitated the spread of heavy metals into the environment, plants have been identified that may have the ability to tolerate exposure to high levels of these metals. Industrial hemp (Cannabis sativa L.) has been suggested as potentially having the ability to accumulate elevated concentrations of some contaminants from soils, including cadmium (Cd), a heavy metal that has been linked to serious health risks to humans. Therefore, there is an interest to document the ability of hemp to tolerate exposure to Cd to determine if there is a risk when products for medicinal consumption are made from the hemp plant. The objectives of this study were to perform histochemical localization of Cd at the cellular level and document potential changes in plant tissues in response to hemp exposure. Plants were grown in hydroponic solutions for 4 weeks and then exposed to either 0 or 10 mg·L−1 Cd for 17 days and harvested. There were no differences in dry weights (dw) of leaves, stems, and roots among treated and control 17 days after treatment (DAT). However, plants exposed to 10 mg·L−1 Cd had Cd concentrations of 1448.0 mg·kg−1 dw in roots and 23.2 mg·kg−1 dw in leaves at 17 DAT, whereas Cd was not detected in roots or leaves of control plants. On staining with dithizone, Cd was primarily localized in leaf epidermis, mesophyll, and trichomes, and in root rhizodermis, cortex, and pericycle in hemp plants exposed to 10 mg·L−1 Cd. Image analysis was able to further quantify these results based on area stained. Our results suggest that Cd is primarily accumulated throughout hemp roots, with lesser amounts translocated to the leaves, where it may be localized in the epidermis and trichomes

Cadmium exposure is associated with increased transcript abundance of multiple heavy metal associated transporter genes in roots of hemp (Cannabis sativa L.)

Industrial hemp (Cannabis sativa L.) has demonstrated promise for phytoremediation due to an extensive root system, large biomass, and ability to survive under relatively high levels of heavy metals. However, little research has been conducted to determine the impact of heavy metal uptake in hemp grown for medicinal use. This study evaluated the potential for cadmium (Cd) uptake and its impact on growth, physiological responses, and transcript expression of metal transporter genes in a hemp variety grown for flower production. The cultivar ‘Purple Tiger’ was exposed to 0, 2.5, 10, and 25 mg·L-1 Cd in a greenhouse hydroponic study in two independent experiments. Plants exposed to 25 mg·L-1 Cd displayed stunted plant growth characteristics, reduced photochemical efficiency, and premature senescence suggesting Cd toxicity. At the two lower concentrations of Cd (2.5 and 10 mg·L-1 Cd), plant height, biomass, and photochemical efficiency were not affected, with chlorophyll content index (CCI) being slightly lower at 10 mg·L-1 Cd, compared to 2.5 mg·L-1 Cd. There were no consistent differences between the two experiments in total cannabidiol (CDB) and tetrahydrocannabinol (THC) concentrations in flower tissues at 2.5 and 10 mg·L-1 Cd, compared to the control treatment. Root tissue accumulated the highest amount of Cd compared to other tissues for all the Cd treatments, suggesting preferential root sequestration of this heavy metal in hemp. Transcript abundance analysis of heavy metal-associated (HMA) transporter genes suggested that all seven members of this gene family are expressed in hemp, albeit with higher expression in the roots than in the leaves. In roots, CsHMA3 was up-regulated at 45 and 68 d after treatment (DAT), and CsHMA1, CsHMA4, and CsHMA5 were upregulated only under long term Cd stress at 68 DAT, at 10 mg·L-1 Cd. Results suggest that expression of multiple HMA transporter genes in the root tissue may be upregulated in hemp exposed to 10 mg·L-1 Cd in a nutrient solution. These transporters could be involved in Cd uptake in the roots via regulating its transport and sequestration, and xylem loading for long distance transport of Cd to shoot, leaf, and flower tissues

Sparingly-Soluble Phosphate Rock Induced Significant Plant Growth and Arsenic Uptake by <i>Pteris vittata</i> from Three Contaminated Soils

We evaluated the ability of As-hyperaccumulator <i>Pteris vittata</i> (PV) to remove As from As-contaminated soils over five harvests in 2.5 years in raised beds (162 kg soil/bed). We tested the hypothesis that a P-limiting environment would enhance PV growth and As uptake owing its unique ability to uptake P under As-rich environment. In Dec. 2009, PV was transplanted to three As-contaminated soils (pH of 5.5–7.2) containing 25–129 mg kg^–1 As, which was amended with sparingly-soluble phosphate rock (PR-soil) or soluble P fertilizer (P-soil). During the 2.5-year, PV obtained sufficient P (1882 vs 2225 mg kg^–1) from PR-soils, with increased root biomass (33%) and root exudation (53%) compared to P-soils. In addition, its frond biomass increased by 20% consecutively with each harvest (six month interval) from 18 to 36 g plant^–1. Its frond biomass in PR-soils (52.2 g plant^–1 year^–1 or ∼12 mt ha^–1 year^–1) averaged 39% more than that in P-soils. To our knowledge, this represented the largest PV frond biomass reported, demonstrating the unique ability of PV in using insoluble P from PR in alkaline soils. In addition to biomass increase, PV from PR-soils had ∼1.5 times more As in fronds (2540, 780, and 920 mg kg^–1) than those from P-soils (1740, 570, and 400 mg kg^–1), with soils containing 129, 25, and 30 mg kg^–1 As, respectively. The low available P in PR-soils induced substantial plant growth and As uptake by PV. This translated into significantly more As removal from soil, averaging 48% reduction in PR-soils and 36% in P-soils in 2.5 years. With multiple harvests and PR amendments, our results showed As removal by PV from contaminated soils was ∼7 times faster than published studies

Cadmium Bioconcentration and Translocation Potential in Day Neutral and Photoperiod Sensitive Hemp Grown Hydroponically for the Medicinal Market

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&middot;L&minus;1 Cd and 2.5 mg&middot;L&minus;1 Cd in the nutrient solution. Cadmium concentrations ranged from 16.1 to 2274.2 mg&middot;kg&minus;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&middot;kg&minus;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