By managing specific nutrients — particularly calcium — growers can produce high-quality reproductive growth while still driving strong vegetative growth
There are some nutrients that drive strong vegetative growth, and there are others that drive strong reproductive development. And some of the nutrients in each category interact with and synergize different groups of plant hormones.
Growers can apply specific nutrients in order to synergize specific hormones and antagonize others. They can shape how plants trigger reproductive bud development and fruitwood development. By doing this, they can produce trees and plants that maintain good vegetative growth while also producing high-quality reproductive growth.
Nutrients that Drive Vegetative and Reproductive Growth
There are four nutrients that drive strong vegetative growth: nitrate nitrogen — not ammonium or urea or other forms — plus potassium, chloride and calcium.
For many grassy plants such as corn, sorghum, etc., growers commonly use large doses of nitrate to produce strong vegetative growth. This is true in tree fruit production as well. And then there are other crops where we use potassium to get a very strong vegetative growth response, such as alfalfa. Chloride also contributes a very strong growth response because of the energy similarities that it carries to potassium and some of the ways that it functions in cells and in cell membranes.
Essentially every other nutrient, excluding those four, will trigger a slight reproductive response in plants. But there are three in particular that are the heavyweights of the reproductive category, helping drive bud initiation. These three are manganese, phosphorus and ammonium — in that order of priority.
There is both vegetative growth and reproductive growth happening all the time in every single organelle in every cell within the plant. There is never a complete dominance of one function over the other — e.g., 100 percent vegetative growth and 0 percent reproductive. Rather, there is a relatively sensitive fulcrum balance between the two; the ratio might be 55 percent vegetative energy and 45 percent reproductive energy, or vice versa.
Because there is both vegetative and reproductive growth happening all the time, very small nutrient applications can trigger a switch from one side of the balance to the other.
For example, one of the common challenges that we see in some varieties of tomatoes under some management systems is that an over-application of nitrogen, which is converted in the soil to the nitrate form, produces tomato plants that are lush and green and have a lot of vegetative energy, but no fruit. You have this beautiful green plant, but it doesn’t have a single blossom on it. That’s a plant that is vegetatively dominant.
You can switch a plant like that from being vegetatively dominant to reproductively dominant with a single foliar application of manganese and phosphorus — the nutrients that have a very strong reproductive trigger. You can literally paint a tomato field from green to yellow in a matter of about seven to 10 days with the right foliar application.
The Plant Hormone Level: Auxins and Cytokinins
But there’s another layer here, and when we understand how these two layers interact, we can begin producing reproductive-dominant plants that also have high-quality vegetative growth.
This second layer is understanding how plant hormones interact with these base nutrients. There are five major groups of plant hormones: auxins, cytokinins, gibberellins, ethylene and acetic acid. But the two that we need to focus on — the two foundational groups for plant development — are auxins and cytokinins.
Auxins drive vegetative growth, and they are produced in two locations inside the plant. First, they’re produced in growing shoot tips — the apical meristems. As the shoot tip is growing, it is producing auxin. The second place auxin is produced is in the developing seed. Once you have blossoming and pollination and the seed begins developing, auxin begins to be produced in the seed.
Auxin is a sugar magnet. Wherever auxin is in the highest concentrations, that is where sugar goes. Photosynthesis in the leaves of the plant produces sugars, and those sugars move out of the leaves in the afternoon and evening toward the sugar sinks — and those sugar sinks are determined by auxin concentration. So, when we have high auxin concentrations in developing seeds, or in the new shoots, that’s where most of the sugar is going to go.
Now, the interesting thing is that auxin, when it’s produced in the shoots or in the seeds — but particularly in the shoots — will move out of the shoot, down through the plant, and out through the root system to the growing root tips. And when it gets to the growing root tips, it shuts down cytokinin production.
Cytokinins, unlike auxins, drive reproductive growth, and they are produced in growing root tips. This means that we need to have growing root tips every single day for new cytokinin production to occur — in order to maintain a balance between cytokinins and auxins. When cytokinins are produced in growing root tips, they then move to the upper part of the plant, and they slow down shoot growth. They slow down vegetative shoot growth, and they trigger reproduction.
So, we can think of these two hormones, cytokinins and auxins, as having an antagonistic relationship. They’re constantly competing. Each is trying to be dominant inside the plant. Auxins are trying to be dominant — providing vegetative growth and suppressing cytokinin formation. Cytokinins are trying to be dominant — providing reproductive growth and slowing down shoot growth and auxin formation.
The Goal: Fast Growth with Tight Internodes
Not all vegetative growth is equal, though. Some vegetative growth produces large gaps between fruit-bearing nodes. But higher-quality vegetative growth consists of fast growth while also establishing internodes that are spaced as tightly together as possible.
Trees with internodes that are two inches apart instead of six inches apart will have that much more opportunity for reproductive buds in the future — this is much higher quality fruitwood. Wood that has more closely spaced internodes also is going to have a higher carbon content and higher carbohydrate concentrations.
The interesting part is that by managing nutrients, and thereby managing auxin and cytokinin levels, a grower can produce a shoot that extends, say, 18 inches in three weeks, with internodes that are spaced at two inches or internodes that are spaced at six inches. The same amount of lateral growth can happen in the same amount of time, but the number of internodes that are spaced along that shoot are determined by the nutrient ratios and by the hormone ratios that are present inside the plant.
As mentioned above, auxins are produced in two locations: in the shoots and in the seed. In many crops, as the seed begins developing and as the fruit begins developing, that fruit becomes the dominant auxin source, and it becomes the dominant sugar sink. So, most of the sugars that are being produced are moving into the fruit. This is appropriate — as long as not so many sugars are moving into the fruit that they are sabotaging root growth and sabotaging shoot growth.
In other words, when fruit is developing, roots and shoots begin to receive less sugar. Sugar is always going to move into the fruit because the plant has a desire to reproduce. We can thus monitor sugar production — photosynthesis, and the overall energy of a plant or a tree — by monitoring the shoot growth while we have fruit on the plant or on the tree.
If we’re growing blueberries or cherries or stone fruit or apples, for example, we want to have new shoot tips constantly growing when there is fruit on the tree. It doesn’t need to be growing rapidly, but we never want it to set the terminal bud. The moment the growing shoots set its terminal bud, we know this has happened because the plant did not have enough energy to both fill the fruit and maintain shoot growth. Shoot growth is thus an indicator for whether a plant has enough energy.
But there’s another aspect to this: shoot growth is actually not the first to drop off and to decline. It’s actually root growth that is the first to be sacrificed. And remember that shoots and seeds are producing auxins, but the roots are not. So, the auxin concentration in the root system is the first to decline, which means that it is the sink that will get the least amount of sugar. Roots will no longer receive sugar once the plant cannot produce enough to fill its fruit load.
What this means — for annual plants in particular, and to some degree for perennials — is that when the root system no longer gets enough sugars in every 24-hour photoperiod, it no longer has the energy to keep growing. Root growth declines, or in many cases stops entirely. But remember that the plant needs cytokinin production, which takes place in growing root tips. So, when the growing root tips stop, you no longer have any new root tip extension, and that means you also halt the production of cytokinins. At this point, there are no cytokinins moving to the upper parts of the plant to trigger the initiation of reproductive buds.
Take tomatoes, for example. It’s very common to see indeterminate tomato varieties that are staked and tied, and the field looks picture perfect — every row looks like a hedge. They’re 40 to 42 inches tall, and the top is very dense. There’s very tight internode spacing — three to four inches apart.
But, as the fruit begins sizing and the seed begins producing a lot of auxin, the root system is no longer getting enough sugar because now the sugar is moving into the fruit. All of a sudden there will be rapid shoot extension out of the top of the plant, with long internodes. For the first 10 to 12 weeks of the plant’s life it produced tight internodes that were three to four inches long, but now there is rapid shoot extension, and the internodes are six to eight inches, or even 10 to 12 inches, long. The tight, clipped hedge suddenly becomes very ragged because of this rapid shoot extension.
This is the first indication that the root system has gone into decline. There is no longer any active root growth because sugars are all moving into the fruit and are no longer moving down to the root system. This also triggers a lot of root disease susceptibility. Challenges with Phytophthora or Rhizoctonia or Anthracnose, or any number of various soilborne diseases, are exacerbated and begin developing quickly when you reach this stage of fruit development because the plant is not producing enough sugars to sustain both the fruit and the root system.
This doesn’t need to be the case, though. This is what is common — this is unfortunately normal — but it isn’t inevitable. Plants have the genetic capacity to photosynthesize to much higher levels and to be able to fill a fruit load and simultaneously sustain constant root growth and root development throughout the entire fruit-fill period.
Driving Growth with Calcium
This brings us back to our discussion of the four nutrients that drive vegetative growth and how they combine with these two hormones.
Of the four vegetative nutrients, three have a synergistic relationship with auxin. They drive rapid vegetative development, and they drive auxin formation. Those three nutrients are nitrate, potassium and chloride. When a plant has a generous supply of nitrate, potassium and/or chloride, this can trigger the response I just described. The plant has very rapid growth, but with auxin dominance. And because you have auxin dominance, you have very wide internode spacing — internodes that are six to eight inches apart or longer, depending on what plant and crop we’re talking about.
But the fourth of the vegetative nutrient drivers, calcium, does not have a synergistic relationship with auxin. Instead, it has a synergistic relationship with cytokinin. This, then, is the key. You can achieve the same amount of shoot extension when you use calcium to drive your vegetative growth as you can when you use nitrate or potassium or chloride. But instead of having internodes that are widely spaced, you now have internodes that are very tightly spaced because you have cytokinin dominance instead of auxin dominance. This is also how you produce high-carbon wood on perennial tree crops and berries.
If you want to produce tree structure and plant structure that can support and sustain a heavy fruit load and be extremely reproductive and extremely healthy, while also getting strong, high-quality vegetative growth at the same time, you want to balance plant nutrition in such a fashion that you have cytokinin dominance. In other words, you want to get your vegetative growth energy from calcium instead of from nitrate and potassium and chloride.
You still need those nutrients on some crops — nitrate and potassium in particular — I’m not suggesting you discontinue them. But instead of using those nutrients to get vegetative growth response, use calcium. You can achieve the same quantity of growth with a much higher quality of growth.
John Kempf is the founder of Advancing Eco Agriculture and the executive editor of Acres U.S.A. magazine.
