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Photosynthesis is not a ‘constant’

Photosynthesis does not occur at a constant rate of speed. It varies from moment to moment dependent on the availability of light, carbon dioxide, water, temperature, chlorophyll concentrations, plant nutrition and genetics. This seems obvious on the surface, yet is almost always missed during research.

We understand that limitations on water, or nitrogen, or temperature extremes can have a pronounced impact on photosynthesis and consequently on plant growth and yield.

In contrast to this ‘downside potential’ of photosynthesis limitations, there is also an ‘upside potential’.

When environment and nutrition is optimized, plants can photosynthesize much more rapidly than what is ‘common’ or ‘normal’ (depending on how you define normal).

An extreme example is tomato production in greenhouses in the Netherlands, where yields are reaching up to 100 kg per square meter, equal to 890,000 lbs per acre. (No, that is not a typo, and it does not include an accidental additional zero.) Field grown fresh market tomato yields in the US range from 30,000 to 50,000 lb per acre, or about 6% of the yields in the greenhouses. To produce those results, lighting, CO2, and nutrition are all being managed very tightly.

This perspective on managing photosynthesis is very valuable when we think about how to increase yields and crop performance, and is often overlooked.

Very importantly, photosynthetic variability is completely overlooked in carbon sequestration research.

Research reports that this or that ecosystem can sequester xx amount of carbon. Grasslands at a certain level, forests at a certain level, farmland at a certain level.

The research, and the predictions coming from that research, contain the flawed assumption that the rate of photosynthesis is a constant from season to season.

Some fields/regions will photosynthesize less and sequester less carbon than the research indicates, because of a challenged environment.

Some fields and regions have the capacity to photosynthesize and sequester carbon at rates multiples higher than the research indicates.

As photosynthesis varies, so does root exudation, carbohydrate partitioning, disease resistance, insect resistance, crop response to microbial inoculants, fertilizers, and sprays.

All research evaluating the performance of products or practices on crops should contain the parameter, “what was the rate of photosynthesis in the plants contained in the study?” When this highly variable parameter is ignored, research does not translate consistently to other fields and farms.

Nutritional integrity is needed to increase photosynthesis

We know it is possible to increase the quantity of sugars produced in each 24 hour photoperiod as much as three to four times higher than the baseline of what is considered ‘normal’ or common in most crops today. In addition, it is also possible to increase the ‘quality’ or the complexity of carbohydrates produced in each photoperiod. Plants with limited nutritional integrity produce lower volumes of simple sugars. Healthy plants produce much larger volumes of more complex sugars.

When the plant begins producing larger volumes of more complex sugars, the crop begins behaving differently. There isn’t a good way to describe this. Internode lengths become shorter, while growth is faster. Clusters of fruit or heads of grain have more kernels or fruit, and mature earlier. Very importantly, the crop begins contributing more carbon to the soil than it removes, even when 100% of the above ground biomass is removed.

Here are some thoughts Don Huber shared when I asked him about photosynthesis during our first podcast interview.

John: Don, one of the things I believe is quite important that we haven’t spoken about is the general impact of photosynthesis and the quality of photosynthesis—how photosynthesis can vary in crops and cover crops and how that influences the volume of root exudates. How can a grower increase the quantity of photosynthesis and increase the quantity of root exudates in the soil profile?

Don: You’re not going to have any photosynthesis if you don’t have manganese. Manganese is critical for splitting water; it provides the hydrogen that can then combine with carbon dioxide. You’re not going to have any photosynthesis without magnesium, which is part of the chlorophyll molecule. You’re not going to have a very efficient photosynthesis without iron and sulfur and all the other minerals, because your physiology is all tied together.

If you want to improve the efficiency of photosynthesis, the first place to look is mineral availability—having that system work. So, if you don’t have a backlog of sugar as fructose or glucose, you want that sugar to be stored as sucrose. That changes the osmotic relationship; it changes the overall physiology of the plant. You’re also not going to have any sucrose if you don’t have manganese, because manganese is responsible for your sucrose-phosphate synthase enzyme as a cofactor.

It’s a system that works together. If you don’t have sulfur, you won’t have enzymes, because most of your proteins are initiated with either cysteine or methionine—your sulfur amino acid. C4 plants have a more efficient photosynthetic pathway. They have PEP carboxylase, as well as rubisco enzymes—after the carbon dioxide from the air binds with the hydrogen that is split off of the water by manganese. So, you have C4 plants and C3 plants, and the physiology that’s involved—but all of them require the mineral nutrients. And any one of those deficiencies influences the overall efficiency of the whole process. 

What healthy peas actually look like

These are fresh market hand-picked peas grown with Advancing Eco Agriculture nutrition and biology management systems, after three months of treating compacted mostly dead soils.

We know that plants routinely are only photosynthesizing at 15%-20% of their inherent capacity. Increasing this performance level to 60+% is a realistic objective for field-scale agriculture. The steps to achieving these results include expanding leaf width, increasing leaf thickness, increasing chlorophyll concentrations, ensuring generous levels of manganese for water hydrolysis, and supplying adequate CO2, in addition to the obvious needs for water and sunlight.

How much more photosynthates would you expect these leaves to produce above the average in a 24-hour photoperiod?

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