C3 and C4 Photosynthesis
The majority of plants and crop plants are C3 plants, referring to the fact that the first carbon compound produced during photosynthesis contains three carbon atoms. Under high temperature and light, however, oxygen has a high affinity for the photosynthetic enzyme Rubisco. Oxygen can bind to Rubisco instead of carbon dioxide, and through a process called photorespiration, oxygen reduces C3 plant photosynthetic efficiency and water use efficiency. In environments with high temperature and light, that tend to have soil moisture limitations, some plants evolved C4 photosynthesis. A unique leaf anatomy and biochemistry enables C4 plants to bind carbon dioxide when it enters the leaf and produces a 4-carbon compound that transfers and concentrates carbon dioxide in specific cells around the Rubisco enzyme, significantly improving the plant’s photosynthetic and water use efficiency. As a result in high light and temperature environments, C4 plants tend to be more productive than C3 plants. Examples of C4 plants include corn, sorghum, sugarcane, millet, and switchgrass. However, the C4 anatomical and biochemical adaptations require additional plant energy and resources than C3 photosynthesis, and so in cooler environments, C3 plants are typically more photosynthetically efficient and productive.
Since carbon dioxide is the gas that plants need for photosynthesis, researchers have studied how the elevated CO2 concentrations impact C4 and C3 plant growth and crop yields. Although C3 plants are not as adapted to warm temperatures as C4 plants, photosynthesis of C3 plants is limited by carbon dioxide; and as one would expect research has shown that C3 plants have benefitted from increased carbon dioxide concentrations with increased growth and yields (Taub, 2010). By contrast, with their adaptations, C4 plants are not as limited by carbon dioxide, and under elevated carbon dioxide levels, the growth of C4 plants did not increase as much as C3 plants. In field studies with elevated carbon dioxide levels, yields of C4 plants were also not higher (Taub, 2010). In addition, if soil nitrogen was limited, C3 plant response to elevated CO2 concentration was reduced or crop plant nitrogen or protein content was reduced compared to plants grown in high soil N conditions (Taub, 2010). These results suggest that crops will likely require higher soil nutrient availability to benefit from elevated atmospheric carbon dioxide concentrations. For more optional reading information about C3 and C4 plant response to elevated carbon dioxide concentrations, see the following summary of research that is also listed in the additional reading list, Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants.
Other Drought Tolerant Crop Plant Traits
Some additional plant traits that help plants tolerate drought and heat stress include deep root systems (typical of perennials) and/or thick leaves with waxes that reduce water loss and the rate of transpiration. In addition, some plants roll their leaves to reduce the surface area for solar radiation reception and heating, and some reduce their stomatal conductance more (water loss) more than others.
Elevated temperatures projected with climate change can have multiple impacts on plant growing conditions. Climate change may lengthen growing seasons in some regions, although day lengths will not change. As planting dates are altered with longer growing seasons, crops may also be exposed to high temperature, moisture stress, and risk of frost. Elevated temperatures may also increase evaporation of water from the soil, reducing soil water availability. Higher temperatures are not necessarily ideal for yield, even if the temperatures are below a plants’ optimal temperature. At elevated temperatures, plants grow faster which tends to, one, reduce the amount of the time for photosynthesis and growth, resulting in smaller plants, and two, reduce the time for grain fill, reducing yield, particularly if nighttime temperatures are high (Hattfield et al., 2009). High temperatures can also reduce pollen viability, be lethal to pollen. The multiple effects of high temperatures on plant physiological process and soil moisture likely explain why research has found that grain development and yield are often reduced when temperatures are elevated (Hattfield et al., 2009).
Many factors that are projected to change with climate change could influence plant growth. These include carbon dioxide concentration, temperature, precipitation, and soil moisture, and ozone concentrations in the lower atmosphere.
Read the Introduction and Key Message 1 (Increasing Impacts on Agriculture) of the National Climate Assessment.