Can you shed more light on this subject? Any scientific evidence or explanation we could all benefit from?
Well, this is a difficult question to answer definitively. Scientific journals do not address nutrient deficiency in aquatic plants because the journals will only address issues that are within the context of natural environments. Quite to the contrary, scientist attempt to determine the causal factors and results of eutrophication of natural systems.
If you search the web for aquatic plant deficiency symptoms what you will find is that because there is no scientific data available, the authors simply copy the data from farming guides or other terrestrial gardening documents. Case in point; that Plagron site you referenced above? Have you not wondered why every article on deficiency only ever shows an image of a farm or garden plant? Never an image of an aquatic plant. Has it never occurred to you that the physiology of an aquatic
So this creates an illusion when someone searches because all they will find is a repeat of what the author copied and relabeled.
On the other hand, we here at UKAPS as well as Tom Barr have experimented with nutrient deficiency and resolved many cases and we have determined, with a fair degree of accuracy, which causal factors are associated with which syndromes. Since there is very little scientific documentation we rely on our experience over the past decade or so.
Do you mean that aquatic plants can selectively choose what amount and which ions they need? I'd love to know about this subject a littler bit more, if possible.
Yes, this is nothing new. They use the same basic mechanism that is internal and which is also available to land plants, but merely extend the scheme to the outer membrane of the leaf. This is simply a set of "tunnels" through which the selected ion travels from outer to inner. These are called "ion channels" and this is the fundamental mechanism by which leaves and roots uptake and transport nutrients and water, both from external sources as well as internally to individual cells. The channels are selective based on their electronegativity and how that matches the charge of the ion in question.
Here is an internal snapshot of the scheme (this is of a land plant):
Here you can see the representation of inside a vacuole (light blue area)which can store fluid which may contain nutrients and other ions.
You can see that the brightly colored objects which represent specific channels embedded in the membrane of the vacuole.
The specifi ions are attracted to and travel down the appropriate channel. There is also an electric charge on the inside of the cytoplasm. if the cytoplasm has it's limit of that ion then the charge builds up and produces a potential in the opposite direction, limiting the rate at which the ion can flow from the vacuole to the cytoplasm.
There are all kinds of channels based on which genes that plant has to produce them, again, based on their environment.
Here is a discussion regarding the ability of a certain type of sea grass to resist the toxicity of Sodium in sea water by using ion channels to attract ions and to build up an electric charge to prevent the penetration of Sodium (Na+):
https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/S0014-5793(97)00786-2
Here is a study of the ion transport mechanism using Egeria densa.
https://link.springer.com/article/10.1007/s002329900136
Cheers,