First off, i looked at this objectively and went through all possibilities from a scientific stand point.
I will cover the main theories behind the technique and what we know about each.
Air And Light:
A traditional theory is the exposure of air and light to the bud sites, increasing said factors by removing obstructing plant material to facilitate more growth. It makes sense when you first look at it and people have tried it and agree it "appears" to improve things. However a deeper look shows that cannabis is a 'Angiospermae' or an anual flower. Which contributes little to no photosynthate.
Here someone called THC-Farmer describes his highly educated opinion and experience on the subject.
He details an experiment where he had a control and a variable. With the variable he removed all leaves during flower, including the new developing leaves from the flower. He wanted to see if any photosynthetic capacity was viable from the cannabis flower only. To no surprise to myself or him, the plant died within days after. Many other science articles have shown this also, that flowers, even though containing some amounts of chlorophyll, typically do not hold the ability to photosynthesize. Its just not possible. The smaller but underdeveloped leaves do contribute to photosynthesis, however they cannot compensate enough to compare with more mature leaves.
So exposing air or light does not appear to offer any advantage to the flower development. One could argue that reducing foilage will help with air movement and light penetration, however this is only going to end up with the same 'total' photosynthetic yield. All that is changed is where the photons will land. Infact exposing shade leaves could be harmful as i have previously said about shade vs sun leaves.
Stress & Phloem Translocation:
This was the more logical theory for myself, as it does seem possible but only lacks the evidence to support it.
Studies on defoliation have showed that a majority of species of plants, show an ability or buffer to compensate for some type of structural damage during its life. Its almost expected by plants that some event will happen and that it be prepared. What defoliation studies have in common, is that plants are not negatively impacted by the loss of leaves up to a certain point (around 25%). It seems that a plant will typically have more leaves than it requires, almost as if history has taught it to anticipate the loss of leaves through enviromental factors. This was some of the conclusions by people conducting these studies.
Now a plant holds the ability to store nutrients (carbohydrates) in its roots, this is its major sink during early growth and vegetation. However in the event of stress, injury or sickness, a plant may be able to redirect this stored nutrient through a process called phloem translocation. A lot is still unknown about this process but it does appear to function this way in some types of plants. These types of plants are ones where all top growth can be removed but still have enough stored energy to revegetate. Cannabis does not hold this ability but it may be possible that all plants have some degree of stored energy to respond to stressful events.
In the defoliation studies some report the event where the variables grew taller than the controls. Suggesting that the plants response to reduced number of leaves caused them to branch out to create new leaves to sustain its level of performance. This response could be aided by phloem translocation. This also brings up the interesting prospect of why supercropping (i like to call it scropping) may increase growth and vigor.
However, to all extent there has been no evidence to support this.
Leaf Age & Exposure To Light:
This is also a possibility however i would not consider this part of traditional defoliation and more selective that would really be classified as pruning.
As a leaf ages, its ability to photosynthesize follows almost like a bell curve with a peak around 20 days.
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Here it can be seen how leaf age is very impacting on its photosynthetic capacity. As the leaf gets older, certain fundamental building blocks responsible for key aspects in photosynthesis, start to be reduced and focused on younger leaves. The amount of chlorophyll does not change. Its just the priority by the plant has shifted. This is why older leaves start dropping off at the bottom when the plant decides it is no longer needed.
With more selective pruning of leaves, it is likely that energy wasted keeping leaves alive that provide very little in return may help develop a plant faster by directing energy where it is most efficient. To some extent plants do this themselves, like how leaves are abscent inside a bushy tree. We do this also by methods such as lollipopping or plant training. By developing a structure that is more efficient for a specific environment. As said however this is a proven method and is practiced throughout horticulture. The traditional method of defoliaiton is a non selective process that removes specific amounts of leaves at specific times. Whats usually practiced and is learned through experience is how much is too much, when & how often such a process should be used during a plants life.
Evidence:
This has been the most dissapointing aspect of my findings. Although then again maybe not.
I looked all over for any and all controlled comparisons, comparing defoliated plants to controls. Unfortunately i found very few. The ones i did find showed clear indications of no improvement but infact a decline in yeild. You would think of all the people who have grown, they would have done such a simple experiment. However it seems people keep insisting on doing the method without controls and make subjective opinions on the matter. The most surprising one is where people defoliate and somehow are surprised that a few days later there is more growth. Then conclude the success of their experiement. I cannot enough make it clear how important controlled side by side comparisons are. This is to remove any and all variables that you 'may' or 'may not' expect, regardless of what you have experienced in the past. Visial bias alone is enough to convince somone of something that is not true, i have much experience with this from my audio engineering side. Not everything is as it appears.
I have been through many forums and threads where people claim once and for all to answer this issue and would conduct a controlled grow. Only to almost always fall through and never return, countless threads encouraging people to post their results. Nothing, ever at all comes to light. Only the select few that seriously go through with it, end up showing the failure of defoliation.
Most who are educated in botny or horticulture almost always seem to disagree with the method. Field experts in commercial grows do not employ techniqies, even though there are ways of doing it efficiently. Studies find no connection, no evidence of any kind. It can only really indicate one thing. We like cheeseburgers.
If anyone has any other possible theorys or further information on the subject that may be interesting, please let me know. If there is anything that may show otherwise i am happy to know, however as it stands i am unconvinced. This is not the first time i have gone away on this subject and come back with the same conclusion.
