According to How Plants Work, anthocyanins, the pigments that give plants their reddish color, can protect plants from heavy metal toxicity. The anthocyanins bind to heavy metals, detoxifying them and preventing damage to cells (p. 125).
Is this the reason aquatic plants, such as Ludwigia “red” and Rotala “Singapore”, become more red in the presence of extra trace dosing? If so, then this is an indication of heavy metal stress, not actually a sign of plant health, since these plants grow well even without extra traces dosing.
I speculated about this a few months ago when I observed L. “red” become less red as traces were reduced. I knew that plants couldn’t use most of the trace metals that were added so something was happening to cause them to increase their anthocyanins content and reduce them as traces were reduced.
Since anthocyanins reflect red light, which is why they appear red, and high light intensities with sufficient red light is capable of inducing the production of these pigments, the increased metabolic activity may cause the excess production of free radicals, which are toxic oxygen molecule species. In order to protect their cells from these free radicals – they are free because the oxygen is not bound to any other atom or molecule – plants increase anthocyanin production to bind them, thus preventing oxidative stress which can damage leaves.
Oxidative stress can result in obvious symptoms like leaf burn, such as the necrotic spots on this Golden Pothos plant which was subjected to toxic concentrations of iron:
These are the same symptoms in Rotala rotundifolias under iron toxicity:
The excess iron in plant tissue in the presence of high light intensities causes the rapid production of free radicals which kills the cells resulting in necrotic spots in the leaves. If light intensity were lower, the plant’s physiological mechanisms would be sufficient to process the free radicals and no visible symptoms of toxicity would occur.
Not all plants are capable of producing anthocyanins. Rotala rotundifolia “green” is such a plant and it also happens to be the most sensitive to poor growing conditions. Here it is with small, stunted and deformed leaves:
Does this kind of growth look familiar?
If it is true that plants that are capable of producing anthocyanins in the presence of toxic metal concentrations are also the most tolerant of heavy metal stress, then it can be concluded that these plants can be used as a visual indication of excess trace dosing. But since the increased redness of plants is considered by many to be desirable, then what most of these people believe are healthy plants are actually a sign of poor growing conditions.
There are many other reasons for the production of anthocyanins. The author of the book, Linda Chalker-Scott, states that plant scientists still don’t know the full extent of the function of these pigments. They can be produced in the presence of toxic concentrations of nutrients but they can also be produced in the absence of certain nutrients such as nitrogen, phosphorus, boron, zinc, and others. They can also be produced when suffering from diseases such as fungal infections.
As for limiting nitrogen as a way to “bring out the reds” in certain plants, this method is actually subjecting the plants to a state of stress. So I wonder, do people prefer the look of stressed plants over healthy ones? If so, that’s a problem for the planted tank hobby as they purposefully subject their plants to poor growing conditions only for the aim of aesthetic appeal.
Classical Aquascaping 