Maroco et al. (1999) grew maize plants for 30 days in plexiglass chambers maintained at either ambient or triple-ambient concentrations of atmospheric CO2 to determine the effects of elevated CO2 on the growth of this important agricultural C4 species. This exercise revealed that elevated CO2 (1100 ppm) increased maize photosynthetic rates by about 15% relative to those measured in plants grown at 350 ppm CO2, in spite of the fact that both rubisco and PEP-carboxylase were down-regulated. This increase in carbon fixation likely contributed to the 20% greater biomass accumulation observed in the CO2-enriched plants. In addition, leaves of CO2-enriched plants contained approximately 10% fewer stomates per unit leaf area than leaves of control plants; and atmospheric CO2 enrichment reduced stomatal conductance by as much as 71% in elevated-CO2-grown plants. As a result of these several different phenomena, the higher atmospheric CO2 concentration greatly increased the intrinsic water-use efficiency of the CO2-enriched plants.

In a study designed to examine the effects of elevated CO2 under real-world field conditions, Leakey et al. (2004) grew maize out-of-doors at the SoyFACE facility in the heart of the United States Corn Belt, while exposing different sections of the field to atmospheric CO2 concentrations of either 354 or 549 ppm. The crop was grown, in the words of the researchers, using cultural practices deemed "typical for this region of Illinois," during a year that turned out to have experienced summer rainfall that was "very close to the 50-year average for this site, indicating that the year was not atypical or a drought year." Then, on five different days during the growing season (11 and 22 July, 9 and 21 August, and 5 September), they measured diurnal patterns of photosynthesis, stomatal conductance and microclimatic conditions.

Contrary to what many people had long assumed would be the case for a C4 crop such as corn growing under even the best of natural conditions, Leakey et al. found that "growth at elevated CO2 significantly increased leaf photosynthetic CO2 uptake rate by up to 41%." The greatest whole-day increase was 21% (11 July) followed by 11% (22 July), during a period of low rainfall. Thereafter, however, during a period of greater rainfall, there were no significant differences between the photosynthetic rates of the plants in the two CO2 treatments, so that over the entire growing season, the CO2-induced increase in leaf photosynthetic rate averaged 10%.

Additionally, on all but the first day of measurements, stomatal conductance was significantly lower (-23% on average) under elevated CO2 compared to ambient CO2, which led to reduced transpiration rates in the CO2-enriched plants on those days as well; and since "low soil water availability and high evaporative demand can both generate water stress and inhibit leaf net CO2 assimilation in C4 plants," they state that the lower stomatal conductance and transpiration rate they observed under elevated CO2 "may have counteracted the development of water stress under elevated CO2 and prevented the inhibition of leaf net CO2 assimilation observed under ambient CO2."

The ultimate implication of their research, in the words of Leakey et al., was that "contrary to expectations, this US Corn Belt summer climate appeared to cause sufficient water stress under ambient CO2 to allow the ameliorating effects of elevated CO2 to significantly enhance leaf net CO2 assimilation." Hence, they concluded that "this response of Z. mays to elevated CO2 indicates the potential for greater future crop biomass and harvestable yield across the US Corn Belt."

Also germane to this subject and supportive of the above conclusion are the effects of elevated CO2 on weeds associated with corn. Conway and Toenniessen (2003), for example, speak of maize in Africa being attacked by the parasitic weed Striga hermonthica, which sucks vital nutrients from its roots, as well as from the roots of many other C4 crops of the semi-arid tropics, including sorghum, sugar cane and millet, plus the C3 crop rice, particularly throughout much of Africa, where Striga is one of the region's most economically important parasitic weeds. Here, too, materials archived on our website describe how atmospheric CO2 enrichment greatly reduces the damage done by this devastating weed [see, for example, our reviews of the papers of Watling and Press (1997) and Watling and Press (2000)].

Baczek-Kwinta and Koscielniak (2003) studied yet another phenomenon that is impacted by atmospheric CO2 enrichment and that can affect the productivity of maize. Noting the tropical origin of maize and that the crop "is extremely sensitive to chill (temperatures 0-15°C)," they report that it is nevertheless often grown in cooler temperate zones because of its high yield potential. In such circumstances, however, maize can experience a variety of maladies associated with exposure to periods of low air temperature. Hence, to see if elevated CO2 either exacerbates or ameliorates this problem, they grew two hybrid genotypes - KOC 9431 (chill-resistant) and K103xK85 (chill-sensitive) - from seed in air of either ambient (350 ppm) or elevated (700 ppm) CO2 concentration (AC or EC, respectively), after which they exposed the plants to air of 7°C for eleven days, whereupon they let them recover for one day in ambient air of 20°C, all the while measuring several physiological and biochemical parameters pertaining to the plants' third fully-expanded leaves.

The two researchers' protocol revealed that "EC inhibited chill-induced depression of net photosynthetic rate (PN), especially in leaves of chill-resistant genotype KOC 9431," which phenomenon "was distinct not only during chilling, but also during the recovery of plants at 20°C." In fact, they found that "seedlings subjected to EC showed 4-fold higher PN when compared to AC plants." They also determined that "EC diminished the rate of superoxide radical formation in leaves in comparison to the AC control." In addition, they found that leaf membrane injury "was significantly lower in samples of plants subjected to EC than AC." Last of all, they report that enrichment of the air with CO2 successfully inhibited the decrease in the maximal quantum efficiency of photosystem 2, both after chilling and during the one-day recovery period. And in light of all of these positive effects of elevated CO2, they concluded that "the increase in atmospheric CO2 concentration seems to be one of the protective factors for maize grown in cold temperate regions."

But what about the effects of climate change, both past and possibly future, on corn production? For nine areas of contrasting environment within the Pampas region of Argentina, Magrin et al. (2005) evaluated changes in climate over the 20th century along with changes in the yields of the region's chief crops. Then, after determining upward low-frequency trends in yield due to technological improvements in crop genetics and management techniques, plus the aerial fertilization effect of the historical increase in the air's CO2 concentration, annual yield anomalies and concomitant climatic anomalies were calculated and used to develop relations describing the effects of changes in precipitation, temperature and solar radiation on crop yields, so that the effects of long-term changes in these climatic parameters on Argentina agriculture could be determined.

Noting that "technological improvements account for most of the observed changes in crop yields during the second part of the 20th century," which totaled 110% for maize, Magrin et al. report that due to changes in climate between the periods 1950-70 and 1970-99, maize yields increased by 18%. As a result, it can be appreciated that late 20th-century climate change, which is claimed by climate alarmists to have been unprecedented over the past two millennia and is often described by them as one of the greatest threats ever to be faced by humanity, has definitely not been a problem for corn cultivation in Argentina. In fact, it has actually helped it.

Much the same has been found to be true in Alberta, Canada, where Shen et al. (2005) derived and analyzed long-term (1901-2002) temporal trends in the agroclimate of the region. They report, for example, that "an earlier last spring frost, a later first fall frost, and a longer frost-free period are obvious all over the province." They also found that May-August precipitation in Alberta increased 14% from 1901 to 2002, and that annual precipitation exhibited a similar increasing trend, with most of the increase coming in the form of low-intensity events. In addition, the researchers note that "the area with sufficient corn heat units for corn production, calculated according to the 1973-2002 normal, has extended to the north by about 200-300 km, when compared with the 1913-32 normal, and by about 50-100 km, when compared with the 1943-72 normal."

In light of these findings, Shen et al. conclude that "the changes of the agroclimatic parameters imply that Alberta agriculture has benefited from the last century's climate change," emphasizing that "the potential exists to grow crops and raise livestock in more regions of Alberta than was possible in the past." They also note that the increase in the length of the frost-free period "can greatly reduce the frost risks to crops and bring economic benefits to Alberta agricultural producers," and that the northward extension of the corn heat unit boundary that is sufficient for corn production "implies that Alberta farmers now have a larger variety of crops to choose from than were available previously." Hence, they say "there is no hesitation for us to conclude that the warming climate and increased precipitation benefit agriculture in Alberta."

With respect to the future, Bootsma et al. (2005) derived relationships between agroclimatic indices and average yields of major grain crops, including corn, from field trials conducted in eastern Canada, after which they used them to estimate potential impacts of projected climate change scenarios on anticipated average yields for the period 2040 to 2069. Based on a range of available heat units projected by multiple General Circulation Model (GCM) experiments, they determined that average yields achievable in field trials could increase by 40 to 115% for corn, "not including the direct effect of increased atmospheric CO2 concentrations." Adding expected CO2 increases to the mix, along with gains in yield anticipated to be achieved through breeding and improved technology, these numbers rose to 114 to 186%.

In light of their fantastic findings, Bootsma et al. predict there will be a "switch to high-energy and high-protein-content crops (corn and soybeans) that are better adapted to the warmer climate." Consequently, if the GCM-based climate-change predictions prove correct, Canada will be immensely blessed by the incredible boost the changed conditions will bring to the country's agricultural productivity, and especially that of corn.

In summary, it seems fairly clear that as the air's CO2 content continues to rise, and even if the climate of the world changes in the ways suggested by GCM calculations, maize plants will likely display greater rates of photosynthesis and biomass production, as well as reduced transpirational water losses and increased water-use efficiencies; and to top it all off, more areas of the world will likely become suitable for growing this important crop.

References
Baczek-Kwinta, R. and Koscielniak, J. 2003. Anti-oxidative effect of elevated CO2 concentration in the air on maize hybrids subjected to severe chill. Photosynthetica 41: 161-165.

Bootsma, A., Gameda, S. and McKenney, D.W. 2005. Potential impacts of climate change on corn, soybeans and barley yields in Atlantic Canada. Canadian Journal of Plant Science 85: 345-357.

Conway, G. and Toenniessen, G. 2003. Science for African food security. Science 299: 1187-1188.

Leakey, A.D.B., Bernacchi, C.J., Dohleman, F.G., Ort, D.R. and Long, S.P. 2004. Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres? An analysis of diurnal courses of CO2 uptake under free-air concentration enrichment (FACE). Global Change Biology 10: 951-962.

Magrin, G.O., Travasso, M.I. and Rodriguez, G.R. 2005. Changes in climate and crop production during the 20th century in Argentina. Climatic Change 72: 229-249.

Maroco, J.P., Edwards, G.E. and Ku, M.S.B. 1999. Photosynthetic acclimation of maize to growth under elevated levels of carbon dioxide. Planta 210: 115-125.

Shen, S.S.P., Yin, H., Cannon, K., Howard, A., Chetner, S. and Karl, T.R. 2005. Temporal and spatial changes of the agroclimate in Alberta, Canada, from 1901 to 2002. Journal of Applied Meteorology 44: 1090-1105.

Watling, J.R. and Press, M.C. 1997. How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated CO2? Plant, Cell and Environment 20: 1292-1300.

Watling, J.R. and Press, M.C. 2000. Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2. Global Change Biology 6: 919-930.