Good stuff mate,
Food production is an expression that not many people associate with aquatic plants, yet if I said we were talking about a potato plant or an apple tree it would be a little easier to grasp right? The whole point of plants using light is to produce food. That food is the end product of photosynthesis. The Carbon that is in Carbon Dioxide (CO2) is combined with Nitrogen, Phosphorous and other nutrients to make a carbohydrate - sugar.
Sugar is the end product of photosynthesis. The plant then eats the sugar and
that is what fuels growth. So when you think about what a plant does during the photoperiod you should always think about sugar production first, because this is the key. If a plant suffers a "nutrient deficiency" the result is a reduction in sugar production, which equates to loss of food and results in malnutrition.
Again, few people think about the fact that plants eat sugar and breathe oxygen to burn that sugar in order to produce the energy required for growth. This is exactly the same as for us or for animals. Have you ever forgotten that you had a potato in a cupboard somewhere, and discovered it months later? What does it look like? Most likely it has turned into a plant, and what was the potato has shriveled in nothing more than skin. The plant ate the potato and used the carbohydrate energy to grow leaves and roots in order to start the cycle all over again. And grew in complete darkness, but once the potato is consumed, there needs to be more food and the job of chlorophyll is to use light energy to produce food, so in the dark the plant begins to shrivel because it cannot produce carbohydrates without the light energy.
A more technical explanation.
Fruit trees and nut trees are particularly talented in making sugar, so much so that there is a surplus of sugar, and this is what we pick from the tree to eat. When we eat the carbohydrates in the fruit, our cells burn the sugar using Oxygen and the carbon from that very same sugar is recycled as waste and is expelled from our lungs as CO2. So Carbon and Oxygen are caught in an infinite loop where they are taken from air, by plants, algae and some bacteria, hydrated with water turned into sugar, burned for energy and expelled as CO2. It's a miracle.
Light is used to power the reactions of sugar production. When you plug in you light bulb and shine it at the plant the light that reaches the plant leaves is, in a way, "plugging in" the plant, because light and electricity are actually two sides of the same coin. Light produces electricity and electricity produces light. But what happens if you add too much electricity to a bulb for example? The filaments in the bulb can only handle so much electron flow. Adding too much burns out the bulb, and so it is with plants that they can only handle so much electron flow. So if you add too much light, then the areas of the plant that handle the electricity created by the light also burn out and this stops sugar production. If sugar production stops it's only a matter of time before the plant uses up it's reserves of sugar and after that it starves to death.
When you think of light therefore, you must think in terms of electricity which "plugs in" the plant and allows it to power the chemical reactions used to make sugar:
Therefore, we have to regulate how much electricity is used to energize the plant, by regulating the amount of light being pumped into the leaves. If we don't pump enough light then the plant cannot generate enough electricity and it starves. If we pump too much light then the plant "circuits" burn out and it starves. The good news though is that if we can provide enough nutrients and CO2 then it makes these food production "circuits" more robust so that the plant can use the high light levels. Add more flow/CO2 and nutrients give the plant a much higher capacity to absorb and use the energy of the light.
So any time you see leaves falling off, or melting, or translucent, or stems rotting, or holes in leaves - any structural fault in the plant, then this tells you immediately that there is too much light/electricity for the plant to handle and that it is starving. That is fundamentally the root cause. How you fix the problem is up to you:
1. You can reduce the electricity.
2. You can increase the CO2 to bolster the electricity handling circuits.
3. You can make the plant more efficient at breathing the amount of CO2 that is there by improving flow and distribution.
As far as photoperiod relationship to dosage, it's not really something that you can easily adjust for. Primarily because plants can uptake nutrients even when there is no light. Nutrient uptake mechanism is a completely different beast than the way in which those very same nutrients are used.
Also, photosynthesis/food production itself has a time limit and is self regulated by the plant. This can only happen for about 8-10 hours a day maximum. Furthermore, when you dose a liquid carbon product, you are interested in increasing the
concentration and therefore the
availability of CO2, so there is no point in lowering the dosage just because you will only have the lights on for 5 hours because if you do that then you will have less CO2 for those 5 hours than you would have if you had dosed the proper amount. In fact, the situation is usually just the opposite. You often need to add more than what they suggest, because they assume that everything else in your tank is perfect, which is almost never the case.
As a result, you would be better off NOT trying to make dosage adjustments based on photoperiod because that would be like opening a can of snakes.
Cheers,