“Magnesium is an essential macronutrient for plant growth and metabolism. It takes part in key physiological functions and metabolic processes of the plant, which may be disrupted under Mg deficiency or toxicity conditions. Yet there is a lack of information pertaining to the response of cannabis plants to Mg nutrition, which limits the development of optimal fertigation practices.” Therefore, Prof. Nirit Bernstein and Dalit Morad of the Volcani Center in Israel studied the effects of five Mg treatments on plant development and function and distribution of minerals in medical cannabis plants, at the vegetative growth phase. “Similar to other nutrients, a defined supply level of Mg is required for optimal plant development and function, and higher or lower application rates result in deficiency or toxicity damages.”
Results
The medical cannabis cultivar ‘Anafurna’ (Canndoc Ltd., Herzliya, Israel) was used as the model plant for the study. The plants were randomly separated into 5 treatments of increasing Mg concentrations: 2, 20, 35, 70, and 140 mg L−1 (ppm); 5 plants per treatment.
Plant growth and visual appearance
What were the results? First of all, the visual appearance of the shoot, young leaves, mature leaves, and roots reflects the plant response to the level of Mg supplied, the researchers explain. “The plants supplied with 2 mg L−1 Mg showed interveinal chlorosis. Plants provided with higher Mg concentrations (20–140 mg L−1) appeared similar and unharmed, except that under the high concentration treatments of 70 and 140 mg L−1 Mg, symptoms of toxicity, e.g., necrotic spots, were apparent on old leaves from the bottom of the main stem.”
The results showed that plant height at the termination of the experiment was significantly lower in plants that received 2–20 mg L−1 Mg than in the 70–140 mg L−1 plants, but the effects were small with differences between treatments ~6 cm. “Plant biomass was lowest in plants that received 2 mg L−1 Mg compared to the other treatments; a slight significant reduction in plant biomass was also observed under 20 mg L−1 Mg, and there were no significant differences in plant biomass between plants that received 35–140 mg L−1 Mg."
Physiological parameters
“Membrane leakage, an indicator of tissue stress, was lower in the young-mature leaves under the range of 20–70 mg L−1 Mg than under a lower or high Mg supply. This demonstrates the sensitivity of the plant tissues to Mg deficiencies (2 mg L−1 Mg) and toxicity (140 mg L−1 Mg) in young-mature leaves. However, in older-mature leaves, sensitivity was observed only under high concentrations (70–140 mg L−1 Mg).” Mg supply also affected photosynthesis and gas exchange parameters in both young-mature and older-mature leaves. “In the young-mature leaves, photosynthesis rate, transpiration rate, stomatal conduction, and internal CO2 concentrations were lowest under limited Mg supply (2 mg L−1) and increased with Mg supply up to the concentrations of 20 mg L−1. In the older-mature leaves, the opposite trend was observed. Photosynthesis rate, transpiration rate, and stomatal conduction were highest under the low Mg supply (2 mg L−1) and decreased with an increase in the Mg supply.”
Nutrient concentrations
Mg supply affected the uptake of Mg into the root and its translocation to the shoot, as there was a general increase in Mg concentrations in the roots, leaves, and stems with the increase in Mg supply throughout the concentration range tested. “Ca uptake and translocation showed an opposite trend. Under the lowest Mg supply (2 mg L−1), Ca accumulated to the highest concentrations in all plant organs, and it decreased with the increase in Mg supply up to the concentration of 140 mg L−1, suggesting competition for uptake between the two cations,” the researchers explain. “Higher Ca concentration was observed in leaves compared to stems and roots. K accumulation was highest in the shoot organs under the lowest Mg supply, and it decreased with the increase in Mg concentration up to the concentration of 70 or 140 mg L−1. Unlike leaves and stems, K concentration in the root increased with the increase in Mg supply above 35 mg L−1, which indicates accumulation of the cation in the root under high Mg concentrations. N and P concentrations in the plant organs were affected as well by Mg supply. The concentrations of both nutrients were highest under Mg deficiency concentrations, with N presenting a high level of shoot accumulation, in contrast to P, which showed higher accumulation in the root.”
What is the optimal range?
“Our major findings are that growth and function of the cannabis plant are optimal under an Mg supply of 35–70 mg L−1 and impaired under the lower-deficient Mg input of 2 mg L−1. Supply of 20 mg L−1 resulted in but a small reduction in growth compared to the optimal supply range. The highest Mg level of 140 mg L−1 had a small inhibiting effect on plant function (gas-exchange parameters and pigment contents, especially in old leaves), but it did not significantly inhibit morphological development and biomass accumulation. Taken together, the findings suggest 35–70 mg L−1 Mg as the optimal concentration range for cannabis plant development and function,” the researchers conclude.
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