Some sources suggest that copper becomes maximally available at PH 5 and then reduces as the PH increases / becomes more alkaline (up to the point where the metals hard precipitate out of solution).
Nor have I. Or, maybe, no one has noticed or the symptoms are misinterpreted as a different excess/deficiency (I mean who really thinks much about Cu). Majority of the Cu accumulation in the plant species I’ve read about occurs in the roots, and, hence damage is most likely to occur there. There were a few species that do tend to evenly distribute the Cu into both the leaves and the roots but they seem to be outliers. Whether Canna may be one of these outliers, I am not certain. One thing does seem certain is that copper does have an effect on root growth. Most papers focus on copper in solution as opposed to localized root contact, though.
The intrigue is fascinating and suggests that tissue analysis might be the way to determine suitable Cu levels. Otherwise, I have not as of yet been able to determine how the micronutrient levels in those nutrient formulations are being determined. They have wildly differing ratios which could be the result of how the test runs were performed and information gleaned from tissue analysis.
And, some more papers for reference:
This is an interesting paper. No excerpts as I’m actually going to read this first as opposed to simply skimming, The Responses of Plants to Metal Toxicity: A review focusing on Copper, Manganese and Zinc , S. M. Reichman.
“Effect of Soil Contamination on Some Heavy Metals Content of Cannabis sativa”, MURAD ALI KHAN, et. al.
The most common sources for copper distribution on soils are pesticides, fertilizers, industries and sewage sluges. Critical concentration for copper in plants is between 20 and 100 mg/kg.
…Thus the copper concentration in plant parts was in the order roots > leaves > stem.
“Copper localization in Cannabis sativa L. grown in a copper-rich solution”, Laura Arru, et. al.
In this study we examined the ability of Cannabis sativa to tolerate Cu salts.
Plants grown on 1 mM CuSO did not show any phytotoxicity symptoms, while dramatic toxicity was observed when plants were grown on a 2.5 mM CuSO4 solution. On the basis of these observations, a 1 mM threshold was chosen for subsequent experiments.
Organic acids also seem to play a role in heavy metal tolerance, transport or storage in plants; levels of citric, malic, malonic and oxalic acids have been correlated with elevated concentrations for example of Ni or Zn in the biomass (Lee et al., 1978; Tolr ́a et al.,1996) of hyperaccumulators.
…hemp demonstrated to possess the ability to transfer Cu from the root to the shoot…
“Effect of Copper Toxicity on Root Morphology, Ultrastructure, and Copper Accumulation in Moso Bamboo”, Junren Chena, et. al.
The composition of the nutrient solution was as follows (in mmol /L): NH4NO3 (714), NaH2PO4 2H2O (161), K2SO4(256), CaCl2 (499), MgSO4 7H2O (823), Na2EDTA (13), FeSO4 7H2O (13), MnSO4 H2O (5), (NH4)6Mo7O24 4H2O (0.04), H3BO3 (9), ZnSO4 7H2O (0.08), CuSO4 5H2O (0.08), citric acid (monohydrate) (35), and concentrated H2SO4 (0.025 L).
Excess copper was applied as CuSO4 5H2O at (1) 0 mM (control), (2) 10 mM, (3) 25 mM, (4) 50 mM, (5) 100 mM, (6) 200 mM, (7) 400 mM. The pH value of the nutrient solution was adjusted to 5.8 with 0:1 M NaOH or 0:1 M HCl. The nutrient solution was continuously aerated and renewed after every 5 d.
Copper, as an essential microelement, plays an important role in growth and development of plants, but at the same time plants are very sensitive to excess Cu. Exposure of plants to slightly elevated Cu levels provokes symptoms of stress-induced morphogenic responses (SIMR) such as inhibition of cell elongation, local stimulation of cell division, and alterations in the cell differentiation status (Feigl et al., 2013).
“Copper phytotoxicity in native and agronomical plant species”, Dane T.Lamb, et. al.
Critical exogenous Cu concentrations (50 percent reduction in roots) for E. camaldulensis, D. sericeum, A. richardsonii, B. macra (dilute), L. sativa, B. macra (concentrated), R. sativa and A. decurrens were, respectively, (μg/L) 16, 35, 83, 88, 97, 105, 128 and 186. > (e.g. ppb)
On B. macra, specifically:
Copper tolerance in B. macra was observed to be higher in the more concentrated nutrient solution despite the estimated Cu2+ concentration being very similar in treatment solutions.
Additional short-term rhizo-accumulation studies showed that neither Ca2+ not K+ was responsible for reduced uptake at the roots. However, the estimated maximum shoot Cu was reduced from 41 to 24 mg/kg in the more concentrated solution.
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