Physiological Plant Response

Plant crops are an important component of the carbon cycle, and rely on carbon dioxide in the atmosphere for photosynthesis to produce biomass. Physiologically, a plant's ability to produce biomass is also dependent on temperature-based functionality. In the face of climate changes, these two factors will be altered, with potentially devastating effects for corn and wheat crops and global food security as a result.

For all organisms, there is an optimum range of temperatures at which performance is best, and this varies widely among plants (see Table 1 below). If the temperature is too low, reactions are too slow. If the temperature is too high, the organism undergoes a desiccation stage that interferes with metabolic processes and enzymes can denature. The rate of photosynthesis is very important for optimum functionality. According to Portner and Farrell et. al, "thermal windows likely evolved to be as narrow as possible to minimize maintenance costs" (690). As a result, the slightest change in temperature for a crop can have a significant drop-off in productivity.

Source: Pimentel, "Climate Changes and Food Supply"

Although climate change is usually translated into 'global warming' - the temperature changes that impact crops season-to-season can be either an extreme cooling or warming event exacerbated by climate change. The overall warming trend seen in the last century is also a problem for more subtle changes in what crops can be grown where. As Table 1 shows above, the overall temperature matters for optimal crop growth over time, and the season length (determined by temperatures year-to-year) also matters for the success of the crop.
An example of longer-term temperature effects is the lengthening of a growing season. If the average global temperature rise is "slightly more than one-half degree Centigrade" the frost-free growing season of corn in the U.S. would lengthen by two weeks (Pimentel). "However, if temperatures continue to increase beyond a specific threshold, a crop's productive summer growing season could become shorter, thus reducing the yield" (Pimentel).

Many plants, including corn and wheat, have carbon dioxide as a limiting resource and show an initial increase in production with an increase in partial pressure of carbon dioxide. Corn (an example of a C4 plant) is predated in evolutionary history by wheat (an example of a C3 plant). An increase in partial pressure of carbon dioxide saturates C4 plants sooner than C3 plants, as shown in the graph below:

Source: Akita and Moss 1973 (BIOL 315, Lecture #3, Slide 13)

At approximately present-day carbon dioxide concentrations (350 umol/mol), "C4 plants have higher rates of photosynthesis than C3 species" (Enriched CO2 Effects). However, net photosynthesis in corn would not increase much above 400 umol/mol, while wheat responds to carbon dioxide levels up to 800 umol/mol (Akita and Moss 1973). A growth response in wheat is almost certain (if examining carbon as the only factor) with the increasing levels of carbon dioxide in the atmosphere.
Carbon dioxide also affects the ability of a plant to take up nitrogen, a crucial element for growth (see nitrogen assimilation graph below). The increase in partial pressure of carbon dioxide inhibits nitrogen uptake in C3 plants, but not C4 plants. It inhibits photorespiration (involved in nitrogen uptake) which is minimally relevant for C4 plants. Therefore, it can be observed that the carbon dioxide increase cannot lead to a linear production increase for plants like wheat because they will be limited in other ways.

                                                    Source: Bloom 2009 (BIOL 315, Lecture #3, Slide 18)

                                    

Precipitation is also a crucial factor for the success of crops. Projected average rainfall in North America is projected to decrease by 10% (Pimentel) with the onset of climate change. Some crops will be better able to withstand drought conditions without excessive irrigation, while others will struggle to be successful. Water supplies in North America are already strained, so crops that require massively increased water inputs will not remain economically viable on the world market. C4 plants are more water efficient than C4 plants, and have a wider optimum range for precipitation levels (as depicted in the original figure below). This means that corn and wheat both have extreme limiting values, and can have production limited by weather events, but that corn will be more tolerant to an overall shift towards drier conditions.

Source: Original Figure

Since industrial agriculture is characterized by artificial inputs to maximize yield, the limiting factors described above could be perceived as manageable by technological "fixes" in some cases. For example, artificial fertilizers can boost nutrient uptake that may otherwise be limited naturally. Field irrigation also can overcome a decrease in precipitation. However, like many facets of industrial agriculture, the benefits of technology that make monocultures possible only go so far. Inefficient irrigation and over-use of fertilizer are unsustainable and only further exacerbate climate change problems. They will become even more unfeasible as water shortages arise and fossil fuels become more rare. Ultimately, non-manipulated conditions are the only ones that we know could sustain food production on the long-term.