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I know this may be TLDR for some of you, but lots of smart heads here and I’d value your input, corrections or additions to the study summary I wrote below. I’d really love to get GURUs’ opinion on this and if he is aware of any negative consequences to increasing sugar load in the nutrients / feed. Is adding more sugar the magical unicorn?

#ChadWestport #HighScience #CannabisGrowing #TPC

Publication: Frontier Plant Science
Peer Reviewed: Yes
Authors: Tesfamichael H. Kebrom
Published online 2017, Oct. 31
Title: A Growing Stem Inhibits Bud Outgrowth – The Overlooked Theory of Apical Dominance

Most of the growers who are aware of Apical Dominance, point to it as the impetus for why they trim or train their plant. The general understanding is that the plant sends auxins (which promotes growth) to the tallest branch, which is why we aim to get a nice level canopy on our plants. Apical dominance is also cited as a reason for topping the plant, based on the knowledge the plant will redistribute those auxins to other, lower branches and promote their growth. But what if I said it wasn’t so simple?

Background:
Three theories for apical dominance have existed since the 1930s, Direct, Diversion and Indirect. Over eight decades, the first two theories have been heavily studied and were adapted to Auxin Transport Canalization and Second Messenger theories, respectively. However, neither can fully explain the phenomenon of apical dominance. The indirect theory developed in the 1930s has thus far been largely overlooked, but this study brings new insight to the inner-workings of the indirect theory.

What is the Indirect Theory?
Recent discoveries have been made indicating that “a growing stem is a strong sink for sugars, and sugar is indispensable for shoot branching, the indirect theory of apical dominance might now be explained as auxin-induced stem growth inhibits bud outgrowth by diverting sugars away from buds”.

***When they say “buds”, they are referring to new growth of lateral branches, not the big stinky green buds we all love. But it’s from their buds that we grow our buds.

How does it work?
In peas, the sugar level in a dormant bud increases when the bud is stimulated to grow by decapitation that removes a growing shoot tip. In addition, dormant buds in intact pea plants grow when directly fed with sucrose providing conclusive evidence for the significance of sugars for bud outgrowth.

Sink organs, such as the stem and newly formed leaves can limit sugar availability to buds. In the presence of strong sink organs such as a growing stem and limited sugar production in the main shoot, buds may become dormant. Auxin-induced stem growth indirectly inhibits buds by depriving sugars necessary for their growth. Size of internodes adjacent to the buds determines the sink strength of internodes for sucrose utilization and storage, and indirectly regulates availability of sugars to buds.

One of the ways for there to be limited sugar production, is by the removal of leaves and the loss of sugars generated through photosynthesis. This study states “A small reduction in photosynthetic leaf area due to disease or herbivory could result in the inhibition of bud outgrowth in particular during the early stage of plant growth and development”.

***Peas, like Cannabis are a dicot. Both monocots and dicots were used in this study, but I tend to reference the material regarding dicots.

Why is this important?
This brings more depth to the function of apical dominance, something we use as a tool to manipulate the growth of plants. Plants create sugars through photosynthesis but they are also able to absorb sugars from the rhizosphere. Using this theory, adding more sugars to the nutrient mix would increase your lateral growth when you have a healthy rhizosphere with microbial activity. The study in its summary states “Shoot branching might be an unintended consequence of source-sink relationships and result from an overflow of sugars to auxiliary buds that cannot be utilized by the main shoot”.

Key Take-aways;

  • Minor defoliation of a young plant could inhibit growth of lateral branches
  • Sugars play a large role in the shoot growth
  • Application of cytokinins directly to bud sites will encourage shoot growth

I didn’t include mention of Cytokinins, although this study does report that when cytokinin levels are higher than auxin levels, the plant will focus more on shoot development instead of root growth. Cytokinins are a PGR and I’m unaware of their systemic values and chemical stability when combusted, which is why I didn’t include much information.

One interesting note was that “buds in cytokinin deficient Arabdopsis plants (a dicot as well) grow in response to decapitation”. This shows that “factors that control shoot branching by acting outside the bud override those that act within the bud and induce or inhibit bud outgrowth”. However “cytokinins promote bud outgrowth when applied directly to the bud”.

Summary;
Keep in mind, the Indirect theory is one of three working theories on apical dominance. All environmental controls and nutrient levels are presumed to be at optimal levels. In the future, hopefully we will have similar tests using Cannabis plants, but much of plant science world has yet to take on the task.

It does however beg the question of should I be adding more sugars into the food and using microbes as the intermediary to the roots in order to promote better bud growth on my plants? “When investigating plant growth and development, sugar demand and supply should be taken into consideration”.

Link to study – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5671643/