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A recurring series focusing on plant cultivation by university researchers

When most people think of iron, it’s in the context of construction and manufacturing, and considerations focus more on quality and stress ratings. But iron plays a crucial role in both human and plant physiology. (The metallic taste in blood is from its iron content, as anyone who has bit their tongue will attest.) In the plant world, iron (Fe) is an important part of protein synthesis, chloroplast development, and the photosynthetic process of energy storage in cannabis.

In plants, Fe is an immobile element, meaning it cannot be moved (translocated) within the plant from older portions of the plant to satisfy the demand in newer developing plant portions. This is important information given that nutrient deficiency symptoms will be seen on the new and expanding leaves.

Figure 1. Iron deficiency manifested as an overall yellowing of the leaflets, especially around the margin and base. This yellowing continued and resulted in a dark green coloration of the veins and the regions within the major veins to be yellow (interveinal chlorosis.) Figure 2. As iron deficiency continued, the interveinal chlorosis became more apparent and widespread.
Photos by Paul Cockson

Deficiency Symptoms

In earlier research conducted at North Carolina State University (NCSU), we induced nutrient disorders in cannabis to study the results and discover optimal levels of key nutrients. Iron deficiency first manifested as a slight yellowing of the leaflet margins and interveinal regions of the leaves (Fig. 1, above). As symptoms progressed, the veins developed a dark green coloration, and the interveinal regions of the leaflets became severely yellow (Fig. 2, above). Upon the initiation of reproduction and flowering, the reproductive leaves on the developing buds resulted in interveinal regions with dark green veins (Fig. 3, below). The new and expanding leaves within these buds developed a yellowing of the leaf margin (Fig. 4, below) and in the most advanced cases resulted in severe interveinal yellowing where the secondary leaf veins were seen (Fig. 5, below).

Figure 3. Upon reproductive induction, the reproductive leaves showed severe signs of interveinal chlorosis.

Iron Fertility Rates

In a second research study, we looked at six Fe fertility rates (0, 1, 2, 3, 3.5, and 4 parts per million (ppm)). We explored the impacts of fertility rates on above-ground plant growth, leaf tissue nutrient accumulation and cannabinoids. The recommended rate for Fe fertility depends largely on your end product goals.

Biomass: Optimization of vegetative biomass production, such as what occurs in a fiber operation or mother stock management, was maximized early in the production cycle (first four weeks after transplant) at the highest (4 ppm Fe) fertility rate when compared to 0 ppm Fe treatment in the vegetative stage. After floral induction, biomass was generally similar during the reproductive stage regardless of Fe fertility. Thus, rates lower than 4 ppm can be used.

Leaf tissue: The upper ranges of Fe fertility (3.5 and 4 ppm) in the vegetative stage resulted in the greatest leaf tissue accumulation and appeared to level off given the values were statistically similar. For the pre-flowering and flowering stages, the highest fertility treatment (4 ppm Fe) resulted in the greatest leaf tissue levels, though a rate lower than this may be adequate given the high leaf tissue concentration present in the flowering stage was also within the recommended range.

Figure 4. The reproductive leaves also had pale leaflet margins, in addition to the interveinal chlorosis. Figure 5. Eventually, the interveinal chlorosis became so severe that the secondary and tertiary veins became dark green, and the smaller, angular portions of the interveinal regions became yellow, as seen on the leaflet margins above.

Cannabinoids: When cannabinoids were analyzed, (CBDA, CBGA, THCA, Delta-9-THC), no clear trend was seen regarding an increase or decrease in concentrations when it came to Fe fertilization rate (Graph 1, p. 29). Given the variability seen in the cannabinoids data regarding Fe fertility, our recommendation for growers optimizing cannabinoids would be to utilize a fertility rate between 3 and 3.5 ppm given the adequate levels of Fe accumulation in the leaf tissue as reported in Graph 1 (below).

(Samples included the shoot apical cola bud, three terminal axillary buds, and three interior branch nodal buds. The combined buds were pooled together to produce a sub-sample of buds, and their fresh weights recorded at time of harvest.)

Graph 1. Total concentration (mg / L) of the acid pools for CBD, CBG, and THC under varying iron (Fe) fertility concentrations (0, 1, 2, 3, 3.5, and 4 ppm). The graph above plots how increasing Fe fertility (increasing Fe from left to right in ppm) will impact the different cannabinoid pools. For example, this graph indicates that with increasing Fe fertility, CBGA, CBDA, and THCA increase in the floral material. While these data represent general trends, more data is needed to determine true trends by analyzing full floral material weight and comparing this back to the composite numbers. These data should be utilized as rough guidelines for the cultivar and will not necessarily apply to all types.

Conclusions

Biomass was optimized at the vegetative stage under higher Fe fertility; however, after reproduction (flowering), our research results indicated a similar response to plant biomass production at lower rates than 4 ppm Fe. Leaf tissue Fe was optimized at the two highest fertility treatments (vegetative) and later at the highest fertility rate during reproduction. However, given the high Fe values in the flowering stage leaf tissue, a lower rate may be suitable. Cannabinoids resulted in no discernible trend, though a recommended level between 3 and 3.5 would be closer to a recommended fertility range.

Additional research is being conducted at NCSU on optimal fertility rates for cannabis exploring micronutrient toxicities. More information on life stage demands and rates will help add to the body of knowledge and get us closer to optimal fertility rates for cannabis production.

Paul Cockson is a Ph.D. student at the University of Kentucky’s Horticulture department. He is a part of the controlled environment horticulture (CEH) lab and is conducting research on plant nutrition and abiotic stress impacts on greenhouse vegetable quality and fruit development.

Patrick Veazie is an undergraduate researcher in the Department of Horticultural Science at North Carolina State University.

David Logan is an undergrad research assistant in the Department of Horticulftural Science at North Carolina State University.

Dr. Brian E. Whipker, Ph.D., is a professor of floriculture at North Carolina State University specializing in plant nutrition, plant growth regulators and diagnostics.