Plant Classification Systems and Physiological processes

In addition to characterizing plants by their taxonomic plant family, crop plants are also classified as either cool season or warm season, referring to the range of temperatures that are optimum for their growth. Examples of cool-season agronomic crops include wheat, oats, barley, rye, canola, and many forage grasses are called cool-season grasses, such as perennial ryegrass, timothy, orchardgrass, tall fescue, smooth bromegrass, and the bluegrasses. Warm season agronomic crops include corn or maize, sorghum, sugarcane, millet, peanut, cotton, soybeans, and switchgrass.

In addition, plants are classified by the type of photosynthetic pathway that they have.

Plant Photosynthesis, Transpiration, and Response to Changing Climatic Conditions

Plants require light, water, and carbon dioxide (CO₂) in their chloroplasts, where they create sugars for energy through photosynthesis. The chemical equation for photosynthesis is:

6 CO₂+ 6 H₂O → C₆H₁₂O₆+ 6 O₂

Carbon dioxide (CO₂) enters plants through stomata, which are openings on the surface of the leaf that are controlled by two guard cells. The guard cells open in response to environmental cues, such as light and the presence of water in the plant.

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Figure 6.2.2: Stomate on a Tomato leaf
Credit: Wikimedia Commons, Tomato leaf stomate, Public Domain

Water (H₂O) enters the plant from the soil through the roots bringing with it important plant nutrients in solution.

Transpiration or the evaporation of water from plant contributes to a "negative water potential." The negative water potential creates a driving force that moves water against the force of gravity, from the roots, through plant tissues in xylem cells to leaves, where it exits through the leaf stomata. Since the concentration of water is typically higher inside the plant than outside the plant, water moves along a diffusion gradient out through the stomata. Transpiration is also an important process for cooling the plant. When water evaporates or liquid water molecules are converted to a gas, energy is required to break the strong hydrogen bonds between water molecules, this absorption of energy cools the plant. This is similar to when your body perspires, the liquid water molecules absorb energy and evaporate, leaving your skin cooler.

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Figure 6.2.3: Picture of water molecules leaving stomata - side view
Credit: USDA National Institute of Food and Agriculture, found at Plant & Soil Sciences eLibrary

Carbon dioxide (CO₂) also diffuses into the plant through the stomata, because the concentration of carbon dioxide is higher outside of the plant than inside the plant, where carbon dioxide concentration is lower due to plant photosynthesis fixing the carbon dioxide into sugars. To conduct photosynthesis, plants must open their leaf stomata to allow carbon dioxide to enter, which also creates the openings for water to exit the plant. If water becomes limited such as in drought conditions, plants generally reduce the degree of stomatal opening (also called "stomatal conductance") or close their stomata completely; limiting carbon dioxide availability in the plant.

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Figure 6.2.4: Schematic of gas exchange across plant stomata
Credit: ASU School of Life Sciences, Snacking on Sunlight

Check Your Understanding

After completing the above reading about transpiration and factors that contribute to water entering and leaving the plant, answer the following questions:

How does transpiration influence the temperature of the plant?

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In many regions, climate change projections are for warmer air temperatures which will likely increase evapotranspiration (the loss of water as a gas from soil and plants). This will likely contribute to soil drying. If soil water is limited, plants tend to reduce their stomatal opening or close stomates to conserve water. How will reduced transpiration impact the plant's physiological ability to cool or avoid overheating?

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