That while I agree the study was using a terrestrial plant for it's conclusions, I believe there is some physiological coordination between all plants. By no means is a pig considered anatomically perfect to a human, but medical students and biologists use them to study human anatomy.
The biggest difference between a terrestrial plant and a macrophyte is submergence in water.
"Aquatic plants seem to greatly prefer water uptake of potassium (K). Thus, the shoots of Elodea occidentalis absorbed K over 5 times faster than it's roots (6). And Potamogeton pectinatus showed reduced growth and flowering when K was absent from the water, even though sediment contain ample K."
(Ecology of the planted Aquarium)
What does this mean? It means many ions in the water (CO2(aq), HCO3, K, NO3, PO4, Fe, Mn....) interact with leaf surfaces of plants, sometimes being absorbed faster than even the roots.
Sure, the linked article can be thrown out the window and investigators (we are all investigators here, play nice....) can quickly refer to the differences between terrestrial nutrient level in soil, and mass flow in macrophytes, but what about the premise of the actual article?
"Investigation of the problem of iron availability (14, 20) has led to
general acceptance of the theory that it is the soluble form in the plant which
plays the important role in iron metabolism. It is evident that this element,
by being precipitated within the tissues, may become inactivated and therefore
unavailable to metabolic processes of the plant."
(SOMERS AND J. W. SHIVE, Pg 1)
During the discussion portion of the report, the researcher expounds on how this could effect the plant. Why couldn't this happen to any plant, regardless of where it is? Perhaps some plants have better oxidating / reduction protections, and natural chelation to help offset this somewhat, but in the end, given enough of any heavy metal this will happen to any plant:
"As already indicated, if the ratio of soluble iron to soluble manganese in the plant is outside of the effective
range extending approximately from 1.5 to 2.5, pathological symptoms appear in the soybeans here employed as test plants. It is not to be expected,
however, that the same range of the iron-manganese ratio values which has proved effective for good growth and development of the soybean plant
would be effective for all species with reference to either the nutrient substrate or the metabolically active plant."
(SOMERS AND J. W. SHIVE, Pg 15,16)
Note here: as planted aquarium hobbyists, we have another substrate to deal with: water.
"The theoretical explanation of the important role which iron plays in the
plant processes in which manganese functions actively, centers around two
important facts: first, that metabolically reactive iron functions primarily
in the reduced state, that is, in the ferrous condition; and second, that the
oxidizing potential of manganese is higher than that of iron."
(SOMERS AND J. W. SHIVE, Pg 17)
If you continue reading the precise method for these toxic effects based on ratios of Fe and Mn can be found. Once these substances are inside any plant, similar biological processes happen to convert Fe into usable and non usable states. And this is all regulated, at least partially by Mn.
Here's a picture of Blyxia from awhile ago when i was experimenting with Mn. This Blyxia is suffering from Mn toxicity and Fe deficiency. As the researcher has noted, they are one in the same:
This is a monocot plant so it's harder to see, but notice the "paper white" leaves and degenerated old growth that fits with the description of the report. The red areas are not typical anthromycin pigments, either. It's damaged tissue.