Simple models for carbon assimilation by plants

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Courtesy of METER Group, Inc.

The detailed processes in photosynthesis are complicated and hard to model. In many cases, however, it’s possible to simplify the model by focusing on one or more of the limitations to assimilation.

Carbon Assimilation Simplified: Light and Water
In simplest terms, carbon assimilation involves the chemical transformation of carbon dioxide and water to carbohydrate and oxygen within the leaves of plants. The process requires energy to proceed, and that energy is supplied by light, usually coming from the sun. The CO2 comes from the atmosphere, and must diffuse into the leaf mesophyll cells to be fixed. Since the inside of the leaf is much wetter than the atmosphere, water diffuses out as CO­2 diffuses in. The amount of water used in the actual photosynthetic process is miniscule, but the water lost in connection with CO2 uptake is substantial.

Limited by Light, Limited by Water: Two Separate Approaches
Based on this simple description, we could postulate situations where light would be the limiting factor in assimilation, and others where water would be the limiting factor. Our models, in words, might be: assimilation is proportional to the plant’s ability to capture light, or assimilation is proportional to the plant’s ability to capture water. Both approaches can be useful in modeling biomass production.

Light Based Model
In an earlier newsletter article we discussed the light-based model. In equation form it is

A = efS (1)

where A is the net dry matter assimilation, S is the total incident radiation received during the time the crop is growing, f is the average fraction of radiation intercepted by the crop, and e is a conversion efficiency. If A and S are both expressed in mol m-2s-1, then e is a dimensionless conversion efficiency. In light limiting situations, the value of e is quite conservative for a particular species, and in the range 0.01 to 0.03 mol CO2 (mol photons)-1 Campbell and Norman (1998, p. 237 give additional information and references to do a more complete analysis).

Measuring f with the AccuPAR LP-80
It is clear that f, the fraction of incident light intercepted by the plant canopy is a critical factor in determining assimilation. This factor is directly measured with the AccuPAR LP80. In light limited environments one can predict dry matter production knowing the amount of incident PAR and the light conversion efficiency, e, and then measuring f over time with the LP80.

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