# Photons and Yield Light incidence over our crops will have a direct effect on yield. Photosynthesis is the most important process in plants to produce energy. But how can we relate photons of light to yield? Based on research we know now one mole of photosynthetic photons is required in order to produce one gram o dried biomass (yield).

1 gram / mole

In order to understand how light promotes a specific amount of biomass we need to learn about: Photosynthesis, Quantum yield, Conversion efficiency, Respiration, and Harvest index.

Photosynthesis:

Photosynthesis is the process where plants utilize CO2 and water to produce energy. These simple molecules are able to produced energy with the help of light. Plants use photons of light in order to convert Carbon Dioxide and Water into Carbohydrates and, release oxygen in the process.

But, how much light is required?

We can measure the number of photons required to produce plant biomass by understanding quantum yield:

• Quantum yield: Refers to the moles of carbon fixed per mole of photons absorbed (mol mol-1). We know the minimum amount of photons required to produce carbohydrates is 10. Meaning the quantum requirement to get 1 molecule of carbohydrates is 10 photons. However, 10 is a quantum requirement we can get at low light intensities. Previous research done by Dr. Bugbee at Utah State University has demonstrated that under high CO2 levels (1600 ppm) and light intensity around 400 μmol m-2 s-1 common crops such as tomatoes and spinach require 12 photons of light (Quantum yield = 1 mol / 12 mol of photons = 0.08 mol mol-1).

Based on research we can then conclude that about 12 photons of light are required to produce 1 molecule of carbohydrates.

In our mission to understand how photons are used to produce plant biomass we need to consider conversion efficiency

Efficiency

How efficient can photosynthesis be? Efficiency is equal to output divided by input.

Efficiency = Output / Input

Carbohydrates are equal to 30 g per mole (CH2O, Where the atomic weight of C=12, H2= 2, O=16). Remember, 12 moles of photos are required to produce carbohydrates. Then the efficiency will be calculated as follow

Efficiency =  30 grams / 12 mol = 2.5 g

2.5 g per mole is a little far from the first number shared above where we mentioned one mole of photosynthetic photons is required in order to produce one gram o dried biomass. Why? In order to correctly link the use of photons to the production of plant biomass, we need to consider how the carbohydrates produced are used. Plant respiration is a process utilizing carbohydrates in order to produce energy.

Plant respiration and yield

When working with plants we usually hear about the Net photosynthetic rate. The net photosynthetic rate refers to the total rate of fixation of carbon without considering the CO2 lost during respiration. This is the correct way to measure photosynthesis.

When trying to link photons to yield, we need to consider that some percentage of carbohydrates will be used for plant respiration, a process that of course will affect how carbohydrates are used to produce plant biomass or yield.

In the need to consider respiration in the total energy available to produce plant biomass, we need also to learn about respiration efficiency. Previous research has shown that respiration efficiency is around 60%. In order to consider the impact of respiration efficiency and photosynthesis efficiency we need to calculate what we call: Photon conversion efficiency

Photon conversion efficiency is equal to photosynthesis efficiency multiplied by respiration efficiency.

Photon conversion efficiency = (2.5) (.60) = 1.5

This calculation gives us a total of 1.5 grams of biomass produced for every photon of light. Meaning that in a perfect scenario where all photons emitted are absorbed by the plant 1.5 grams of biomass will be produced per every photon of light. Nevertheless, perfect scenarios are really hard or impossible to get. This is why we usually assume: One mole of photosynthetic photons is required in order to produce one gram o dried biomass.

1 gram / mole

When understanding how plants use the light we are able to analyze our system better and learn about the importance of light incidence over our crop. With a good design in our growing systems, we can optimize light incidence. Also, by learning about the impact of light incidence is possible to estimate how much can cost to produce different crops in indoor facilities. But in order to be able to calculate this, we need to know about the concept Harvest Index.

Harvest Index

The harvest index is the ratio of usable or edible product to total plant biomass. For example, many crops are considered to have a harvest index of around 50%. This is because we grow a plant and harvest the fruit. Therefore we are consuming about half of the total plant biomass. This can be a good estimation for cucumber, tomatoes, etc. Leafy greens such as lettuce have a higher harvest index (Around 80%) because we consume almost the whole plant.

Harvest index can be really important when trying to relate light, yield, and cost of light in indoor facilities.

Example: Based on the number of photons and harvestable material in ideal conditions, the cost of photons as a percentage of the market price for lettuce will be around 5%. In comparison to plants with a lower harvest index such as tomatoes where the cost of photons can be around 18%. For plants where harvested material is low in comparison to total plant biomass produced, the cost of production based on use and cost of light will be higher. This is why most indoor facilities are focused on leafy greens. This is also why optimization of light incidence over the crop should be crucial for every growing system.

Of course, we do not forget that artificial lighting can also be used as supplemental lighting. Where we are able to use free sunlight but improve the growth and quality of plants by improving light intensity, incidence, and quality with supplemental lighting.

Photosynthesis and the use of photons can be complex topics. However, if we really want to have control over our crops we need to pay attention and understand photosynthesis which is the most important process in plants to get energy and with that our products.
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