I have read in many thread that for turning certain species in deeper red a small deficiency of KNO3 can induce this. Many time in our tank plants are growing green when they are in the middle of the tank and then when it reach the surface the new growth turn red. That is because at water surface water the light is stronger and accelerate up take. So a small deficiency is created because we are not dosing enough to reach the plant's demand for that amount of light.
Hi mate,
There are lots of reasons for the color change. Plants have lots of different pigments and each pigment has a different function and therefore a different color. There are three main jobs of pigments:
The first job is to absorb the energy of the specific color(s) of light and to pass this energy on for conversion to chemical energy.
The second main job is to protect the plant from radiation poisoning by reflecting away the energy of specific color(s) from the leaf if those colors have too much energy.
The third main job is to change the color of the inbound light and to then reflect that new color on to another set of pigments within the leaf in order to trigger some response in the plant.
Pigments are very complicated molecules and so they are very expensive to manufacture. The plant must analyze the inbound spectral energy, compare the energy to available resources such as nutrients and CO2, take into account it's present state of health and then decide which pigments to produce and how many to build and to allocate.
So there you have a wide variety of conditions that can result in color change. It's not as simple as adding more of this or adding less of that. There is always the equation of survival and reproduction that must be solved.
Chlorophyll is a strongly green pigment as we all know, and this molecule is composed of a high percentage of Nitrogen. The human visual cortex is mostly responsive to green. Therefore, when the chlorophyll content of the leaf is high, then there is a tendency for the green coloration to dominate our perception of the leaf's color. It is entirely possible therefore that under a Nitrogen limitation, which would restrict the population of Chlorophyll, we would perceive less green and other colors. But this condition can happen whether the leaf is at the surface or not.
Nitrogen starvation is one of the reasons we see plant leaves looking yellow. When the chlorophyll density of the leaf is poor, it's color influence is reduced. ANY nutritional deficiency which curtails the production of chlorophyll will result in a color change. When we see Autumn colors on trees for example, this is because the tree pulls the high value chlorophyll away from the leaves for storage, and the remaining pigments, such as Carotene, which are yellow, red or orange, remain in the leaf.
When the leaf is under spectral stress, such as during excessive lighting, then main job number two has to be implemented. The plant analyzes the characteristics of the inbound photons and determines which photons are causing the damage. It then develops pigments in that leaf which reflect as many of those photons as possible. So the pigment choice is very specific to the colors and strengths of the inbound light. It may require several different pigments as well as very high nutrition to manufacture and produce sufficient levels and types of pigments to protect the leaf from destruction. Each leaf will have a different distribution of pigments because their particular location and orientation will be slightly different than another leaf, and the plant will not manufacture more pigments than it needs for each leaf, so that's why the upper leaves will be colored while the lower leaves, which may not be under as much stress, will not color as much.
The manufacture of certain proteins and enzymes, as well as certain behavioral patterns such as flowering, are triggered depending on the inbound wavelength as well as other environmental factors. The plant will then use job number three to capture inbound photons of a certain color and reflect photons in the target color range. Pigments sensitive to that new color will be distributed in specific locations relative to the fluorescing pigments.
So because of these complications, when we see a color change, it's not always obvious WHY it is happening. There are many cases where color changes occur without increasing the light. In a specific case, it may be that increasing the nutrition or CO2 enables the plant to manufacture a certain pigment.
Here is an example case where light was NOT increased, but instead CO2 and nutrients were exaggerated. From this:
To this:
So in that case, even with a super high Nitrogen content, there was no trouble getting color. As this stem reached the surface though the red disappeared, and we might assume that high light stress caused a different set of pigment distribution. Again, that's only an assumption.
Following the progress of this specimen, this is what the stem looked like when it reached just a few centimeters from the surface. So it lost red and was actually more green/yellow under the highest light possible in that tank. So it's not always possible to predict what colors will appear, or when, until we study each species under a variety of conditions that we have total control over.
Here is a totally different scenario. This is nothing special, just a hodgepodge of stems. On the left of the image the light intensity is less than on the right. That's because the end of the bulb is about in the middle of the photo, so the plants on the left are always in shadow, but look at the L. aromatica in the background. The stems on the right stay bright green even though there is more light on the right, while the sister plants on the left, which are in lower light, have more orange color. On the far lower left, those stems have no difficulty turning red. So really it depends on the sum total of all the environmental conditions being faced by the plants, not just on how bright you make the light. That's only part of the story.
Cheers,