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Varietal Responses of Crops to Atmospheric CO2 Enrichment
Volume 11, Number 40: 1 October 2008

Determining how different crop cultivars respond to elevated concentrations of atmospheric CO2 is an essential first step in breeding varieties to best profit from the ongoing rise in the air's CO2 content, which undertaking is an absolute necessity if we ever hope to be able to produce enough food to feed the planet's burgeoning human population without usurping all of the land and freshwater resources that currently provide habitat and sustenance for the rest of the planet's biota, or what we could call "wild nature." However, in the words of James Bunce of the USDA's Agricultural Research Service, "much of the work on intraspecific variation in crops has been done in controlled environment chambers, in glasshouses or with plants in pots, and it is uncertain how those results may extrapolate to field conditions."

In a four-year exploratory study that went a long way toward overcoming these various experimental limitations, Bunce grew adequately fertilized plants of four varieties of the common garden bean (Phaseolus vulgaris L.) -- Matterhorn (a great northern bean), Jaguar (a black bean), Red Hawk (a kidney bean), and Brown Beauty (a snap bean) -- from seed to maturity under standard field conditions at Beltsville, Maryland (USA) within open-top chambers, where photosynthetic measurements of mature upper-canopy leaves were made in full sunlight at midday during the pod-filling stages of four growing seasons, and where final seed yields and other plant characteristics were determined at harvest.

This work revealed that the extra 180 ppm of CO2 in the CO2-enriched chambers (a concentration increase of close to 50% during daylight hours) resulted in a mean long-term stimulation of midday net photosynthesis of approximately 18% in the Matterhorn and Jaguar bean varieties, but an increase of fully twice that much (36%) in the Red Hawk and Brown Beauty cultivars. In terms of dry mass seed yield, however, the Matterhorn variety led the way with a CO2-induced increase of about 39%, followed by Red Hawk at 21%, Brown Beauty at 18%, and Jaguar with an actual 10% decline in seed yield. What is more, as Bunce reports, "the highest yielding variety at ambient CO2 [Jaguar] was out-yielded by a different variety at elevated CO2 [Matterhorn]."

In light of these several observations, it is clear there is significant variability in seed yield response to atmospheric CO2 enrichment among the four bean varieties tested by Bunce. In addition, it is equally clear there was no a priori way of knowing which of the four cultivars would prove to be the best responder to an increase in atmospheric CO2 concentration, or that one of them would actually respond negatively to an increase in the air's CO2 content. Consequently, Bunce's experiment demonstrates the great need we have to perform a host of such experiments on our most important crop plants, in order to identify which of their many varieties should be selected for crop breeding work, in order to take full advantage of the significant increase in the atmosphere's CO2 concentration that will surely occur over the next several decades, irrespective of how rigorously the nations of the world might attempt to curtail their CO2 emissions. These important crop characteristic assessments must be made, in spite of everything else we might rightly -- or wrongly -- do concomitantly.

Sherwood, Keith and Craig Idso

Reference
Bunce, J.A. 2008. Contrasting responses of seed yield to elevated carbon dioxide under field conditions within Phaseolus vulgaris. Agriculture, Ecosystems and Environment 128: 219-224.