Yield potential is the yield per unit land area that a genotype can achieve at a given location in the absence of abiotic and biotic stresses. To a first approximation, it is the product of the amount of solar energy available at a location during the growing season and the efficiencies with which the crop intercepts that energy, converts it into biomass, and partitions that biomass into the harvested product, such as grain. This last efficiency is commonly known as the harvest index.
The Green Revolution achieved large increases in the yields of our major food crops by both genetic improvement of yield potential and improved agronomy. Yield potential was increased by improving energy interception efficiency and almost doubling the harvest index. Examination of modern cultivars growing under optimal conditions shows interception efficiencies of almost 90 percent and harvest indices of 60 percent. These two efficiencies are therefore close to their biological limits since there will always be a period between planting and leaf canopy closure when the crop cannot intercept all of the incoming energy, and some biomass must remain in the stems, leaves and roots.
Approaching these biological limits is a key factor explaining why the rate of improvement in yield per unit land area is declining, as has been shown for wheat and for rice—the world’s two most important crop sources of dietary calories.
In contrast to interception efficiency and harvest index, conversion efficiency has changed very little. Conversion efficiency is determined by crop photosynthesis integrated over the growing season, minus respiratory losses. Typical net conversion efficiencies of intercepted solar energy into biomass energy for modern crops, averaged over the growing season, are around 0.5 percent, yet the biological limits are between 4.5 percent for C3 crops and 6 percent for C4 crops (i.e., plants in which the first products of CO2 assimilation are three-carbon and four-carbon compounds, respectively).
Why has selective breeding failed to improve conversion efficiency? Three reasons exist. First, as practical means to measure leaf photosynthetic rates became available in the 1960s and 1970s, little or no correlation was found between leaf photosynthetic rate and yield. Second, the major food crops appeared to be limited in their genetic potential to set and fill seed or grain. Third, it was reasoned that if photosynthesis is the key to yield, then selection by breeders would have resulted in increased photosynthesis. So what has changed our thinking? Ironically, the answer lies in global climate change.
Global climate change is dominated by rising CO2 levels, and this realization encouraged many field experiments investigating the direct effects of elevated CO2 on crops. CO2 is a limiting substrate for photosynthesis in C3 crops, which include wheat and rice. The primary effect of CO2 elevation is increased photosynthesis. An overwhelming body of data now shows that crops grown with elevated CO2 in field conditions have increased photosynthesis and invariably result in increased yields.
So can photosynthetic efficiency be improved through genetics? Unlike leaf growth and harvest index, there is little genotypic variability in photosynthesis among C3 crops. There is also little variation between species; the mechanism in soy is identical to that in wheat and rice, so there is little to select from. However, synthetic and systems approaches through bioengineering offer new opportunities to achieve variation and thereby increase efficiency.
Over the past 50 years, the mechanism of photosynthesis has been studied to the extent that it is now the best understood of all plant processes. All the discrete steps have been described, and the relevant genes, proteins and metabolites are well-described. As a result of this knowledge, and with the availability of high-performance computers, the entire process has been represented as a complete dynamic model, allowing millions of permutations to be tested to find optimal approaches to increasing efficiency. At the same time, bioengineering has become routine for our major crops, allowing practical testing of computer-based designs.
New areas of research are emerging from these new techniques. For example, cyanobacteria, from which crop chloroplasts evolved, have their own CO2-concentrating mechanism that was lost in the evolution of land plants, which occurred at a time when earth’s atmosphere contained many times the CO2 concentration of today. Re-introducing this mechanism to crop chloroplasts could, in theory, increase photosynthetic efficiency by 60 percent.
Computer-based design can also be applied at the level of the leaf canopy. A recent analysis has shown that altering the distribution, angles, and albedo of leaves within a canopy substantially increases the photosynthetic efficiency of solar energy, water and nitrogen use. In total, combined improvements at the cell, leaf and canopy level could more than double the conversion efficiency of today’s major C3 crops, which is no longer possible with interception efficiency and harvest index, the other two efficiencies that govern yield potential.
Although we are far from achieving these improvements in practice, bioengineering of model species, including tobacco, has shown significant and reproducible improvements in conversion efficiency in controlled environments. It now remains to be seen if these improvements can be replicated in realistic field conditions with major food crops. Recent analyses show that, given current improvement trajectories, future global yields of the four largest food crops will fall far short of projected 2050 demand, possibly by as much as one-third. Genetic improvement of photosynthetic efficiency is an unexplored opportunity to deliver significant yield increases before that happens.
Reprinted from the November/December 2014 issue of Resource, the magazine of the American Society of Agricultural and Biological Engineers, with permission from the publisher. View the full issue online at www.asabe.org/publications.aspx.
Stephen Long is a Gutgsell endowed university professor of plant biology and crop sciences at the Institute for Genomic Biology at the University of Illinois at Urbana-Champaign.
Xin-Guang Zhu is a plant systems biology professor at the CAS-MPG Partner Institute for Computational Biology at the Chinese Academy of Sciences in Shanghai.