Soyabeans Photo by Scott Bauer/US ARS

Protein versus Yield (and other plant factors affecting yields) In general, high protein grain crops such as soybean (circa 38% protein) have much lower yields - the tonnage of harvested product per hectare - than lower protein crops such as maize (circa10% protein). Even within one crop, such as wheat, yields of high protein varieties are commonly lower than those of lower protein varieties. These differences are attributable to a a well known fact about the biochemistry of plants: it takes more energy to synthesize protein than it does to synthesize carbohydrates.

Maize Photo from CGIAR Photgallery

Based on a knowledge of metabolic pathways, plant physiologists have calculated the costs of synthesizing different plant compounds from glucose (Table 1).

Table 1. The amount of product that can be produced from 1 gram of glucose for different classes of plant compounds. (Source: Loomis and Connor, 19921)
Class of Compound	PV (Production Value) g product g-1 glucose
Carbohydrate	0.83
Protein (from NH4-N)	0.62
Protein (from NO3-N)	0.40
Lipid	0.37
Lignin	0.52
Organic acids	1.10

The Production Value (PV) is the quantity of a compound that can be synthesized at the cellular level starting from one gram of glucose. Glucose produced by photosynthesis is used both as a source of energy for metabolism, and as a building material for other compounds - hence the more glucose that is required to drive metabolic reactions in synthesizing a compound from glucose, the lower is the amount of that compound that can be produced. That's at the cellular level. This effect will carry over to the crop level if other factors affecting yield are more or less the same (length of the growing season, the harvest index, degree of water limitation) and a crop with a higher protein content could in general be expected to produce less bulk or biomass, than a crop with a lower protein content.

Note in Table 1 that the Production Values for lipids are even lower than for protein - hence we would predict that crops with high oil content will in general have lower yields than those with low oil content.

Table 2 below shows the calculated PVs for seeds of different crops, given different proportions of the major constituents. PVs are calculated for the different N sources as well, including for legumes, N₂ (nitrogen gas in the atmosphere). PV for crops growing on nitrate (NO3^-) is lower than PV for crops growing on ammonium (NH4⁺) because some glucose has to be used to reduce nitrate to ammonium before it can be used. PV for crops growing on N2 is very low. It takes a lot of energy to break the highly stable bonds of the dintrogen molecule. It seems likely that high energy costs explain why nitrogen-fixing symbioses such as are found in legumes are not widely distributed in the plant kingdom.

The final column above are the average yields for 4 of these crops in the province of Ontario, Canada for 1999. Note the low yields of the high protein and high oil crops compared to rice, wheat and maize.

Other important crop factors influencing yield are the length of the growing season, and the type of photosynthetic metabolism (C3 or C4). In general the longer the growing season, the higher the yield which probably explains why winter wheat has a higher yoield than spring wheat. In general C4 plants have higher yields than C3 plants, a difference which is due in part to C4 plants exhibiting a higher rate of photosynthesis per unit of CO2-fixing enzyme, which means they need to synthesize less protein. Only maize amongst the plants in Table 2 is a C4 plant. Maize also has a longer growing season than the other species (except winter wheat), and it has relatively low oil and protein content - the combination of these factirs make it a very high yielding crop.

Thinking about other types of crops... what might you expect for long season, starchy root crops such as potatoes and yams - high or low yields? For sugarcane? One goal of biotechnologists is to introduce nitrogen-fixing genes into cereals - how might that affect yields?