Note: diels alder was instrumental in helping turn this very technical subject into something much easier to understand. This is by no means a complete, technical explanation of Botany, rather it's an understanding of the basic principles.
Many times I've seen in posts that "roots grow during the dark.". If this were true, the 24/0 photoperiod would result in a plant with a tiny root structure, if one at all! We know this is not the case - so how does it actually work?
To simplify things, lets use an analogy. Try to think of a plant as a building... one constantly under construction. The plant needs raw materials, (fertilizers and water), and energy (light) in order to "build itself". The raw materials are the "bricks and mortar" of the building. The energy is the workers, vehicles and power tools used to assemble the building.
The Plant is capable of storing some raw materials and some energy for use later, but the amount is limited...think of a warehouse and a battery.
During the day, (Lights ON) the plant is collecting and storing light energy, and is using and storing raw materials. The plant is stockpiling raw material, and is charging it's batteries... it is ALSO using raw materials and using the energy it is collecting. It's building itself, literally putting itself together.
During the day however, the plant is not as efficient at building itself, as it is at night (lights OFF.) It can build itself, but not as quickly.
While the lights are OFF, the plant is using energy and raw materials to build itself.... the plant is more efficiently using the raw materials that it stored during the day. The plant is better at transporting and assembling the raw materials.
The bad news: since there is no light energy, the plant must rely on energy it stored while the lights were ON (its stored energy). (Essentially, the plant is running on batteries, and using raw material from the warehouse.)
There is no light energy to collect. Since the plant needs energy to absorb more raw materials, it is easier for the plant to use raw materials that it stored during the day than it is to absorb raw materials through its roots.
Although the plant IS capable of "doing it all" with the lights on, (Collect, store and use energy & raw materials) it does a better job of actually doing the work (using the energy and raw material) while the lights are out. During the dark however, it relies solely on its limited supply of stored energy and stored raw material.
One last thing to remember is the fact that a plant will always strive to maintain a balance between the size of its roots and the size of its canopy (Leaf mass.) The roots must be big enough to supply as much raw materials as the canopy can use, and the canopy must be big enough to provide the energy required to store those raw materials....
Trim the roots on a healthy plant, and canopy growth will slow to a crawl until the roots have grown big enough to again support the canopy.
Trim the TOP of a healthy plant, and root growth will slow similarly, until the canopy has grown big enough to again support the roots.
If the plant is already in balance, the canopy and the roots will grow at the same rate.
If you actually measured them several times daily over several days, you'd notice that they actually DO get bigger at night, but roots and canopy at the same rate, unless either has been trimmed, and as long as the "batteries" hold out.
