An Introduction to Critical Points of Influence

“Critical Points of Influence” is a phrase coined by John Kempf to describe the narrow windows during a plant’s development when it undergoes a significant hormonal shift. At these points, a great deal of the plant’s future yield potential or disease susceptibility is determined. The points are “Critical” because they present opportunities for a grower to take actions that can have outsized effects on yield and quality. The plant relies on a few key nutrients at each Critical Point of Influence; delivering the right amount of the right nutrition in the right form at these points will help growers influence their bottom line.

The physiological effects at these Critical Points of Influence are similar throughout all plants and crop types. They may happen in different orders or at different times of the year, but the general principles apply to all crops, whether they are annuals or perennials, single- or multi-fruiting, woody or herbaceous.

In this series of blog posts, we will describe 6 of the Critical Points of Influence, the ways we can influence the crop at each point, and the nutrients that the savvy grower needs to manage at each of them.

Those 6 Critical Points of Influence are:

1. Bud Initiation
2. Bloom / Pollination
3. Cell Division
4. Fruit / Grain Fill
5. Ripening
6. Vegetative Growth

Vegetative vs. Reproductive Dominance

Generally speaking, as a plant grows, it shifts between two dominant modes: vegetative and reproductive. There are a variety of ways to think about these two modes: expansion and contraction, masculine and feminine, shoots and roots. 

A Critical Point of Influence occurs when the plant shifts from one mode to the next.

It’s important to note that both of these modes exist simultaneously throughout the plant’s life, and that “dominance” generally involves a slight advantage to one of them. For example, it may be 55% vegetative to 45% reproductive energy: both exist strongly within the plant, but in this case, the vegetative energy will be dominant. Because the edge is so tight, often it doesn’t take much to push the plant into the other mode, which is power for the grower who wants to influence their plants.

Auxin & Cytokinin

The vegetative and reproductive modes are controlled by two primary plant hormones: auxin and cytokinin. It’s worth diving a bit into how these two hormones work, as it will greatly enhance our understanding of the Critical Points of Influence.

Auxin is produced in two locations in the plant:

• Growing shoot tips
• Developing seeds

Auxin acts as a magnet for sugar within the plant; wherever auxin concentrations are highest, that’s where the plant will send its sugars from photosynthesis.

By contrast, cytokinin is produced in:

• Growing root tips

Auxin and cytokinin are to some degree competitive; each is trying to gain dominance within the plant. Auxins produced at the shoot tips will move down through the plant into the roots, where they will shut down cytokinin production, and cytokinin moves up the plant and slows down auxin production. Cytokinin will trigger reproduction, while auxin will trigger growth.

In the coming blog posts in this series, we’ll discuss how various nutrients synergize with auxin and cytokinin, and how to use those nutrients to alter plant behavior to your benefit.