42 R. C. Sicher and J. A. Bunce
Hatfield et al. ( 2011 ) summarized the effects of CO 2 enrichment on maize and
concluded that seed yields would only increase 3–4 % on average in response to
doubling CO 2 levels. The combined effects of CO 2 enrichment and elevated temper-
atures on maize yields have not been characterized adequately in field experiments.
However, Prasad et al. ( 2008 ) demonstrated that elevated CO 2 treatments increased
internal tissue temperatures of grain sorghum and this exacerbated the negative ef-
fects of elevated air temperatures on seed yields. Due to a lack of experimental
data, estimating the combined effects of CO 2 and temperature on maize yields has
relied, in part, on crop modeling approaches. Hatfield et al ( 2011 ) concluded that
temperatures in the North American Corn Belt would increase to 0.8 °C in the next
30 years when atmospheric CO 2 concentrations could reach 440 μmol mol−1. These
authors suggested that these conditions would result in a minimum 2–3 % decrease
in maize grain yields under water-sufficient conditions. Easterling et al. ( 2007 ) con-
cluded that a 1–2 °C increase in global mean temperatures would increase maize
yields by a few percent in the mid latitudes, that maize grown in the tropics would
have major yield losses due to temperatures 3–5 °C above today’s values and that
the elevated atmospheric CO 2 concentrations would have negligible benefits for
maize production.
2.12 Summary
CO 2 enrichment is capable of mitigating the effects of moderate heat stress on
plants, such as soybean, that have the C 3 pathway of photosynthesis. Evidence
for this was based on changes of net photosynthetic rate, primary metabolism,
plant growth, and yield. However, the mitigation of heat stress by CO 2 enrich-
ment diminishes in soybean and other species as temperatures elevate further and
heat stress becomes more acute. Very high air temperatures, i.e., those that exceed
40.0–42.5 °C, frequently cause irreversible damage to plant tissues and may cause
death or reproductive failure. Unlike soybean, the reversal of moderate heat stress
by CO 2 enrichment is almost immeasurable for maize and other plants that pos-
sess the C 4 photosynthetic pathway. This is because maize has high internal CO 2
concentrations that almost completely saturate rates of photosynthesis in ambient
air. Second, elevated CO 2 concentrations induce stomatal closure of many plant
species and this decreases evapotranspiration rates from leaves. The resultant im-
proved water status would certainly benefit maize and soybean in the field during
prolonged exposures to heat stress. Note that acute air temperatures create a de-
mand for lower leaf temperatures and this requires stomatal opening and increased
evapotranspiration rates. Thus, very high temperatures negate the effects of CO 2
enrichment on stomatal aperture. Third, plant growth in elevated CO 2 is capable of
accelerating or delaying the onset of senescence of several annual crops. Elevated
growth temperatures accelerate plant development and this shortens the growing
season and negatively affects crop production. Therefore, delaying the onset of
senescence via CO 2 enrichment should mitigate the effects of a shortened growing