In addition to their lifecycles, crop plants are characterized and classified in multiple ways that are relevant for crop production and management. Common plant features include similar morphology, growth and reproduction; and environmental and climatic adaptions. This module will help you understand more about how crops are adapted to different environments and diversified to interrupt pest lifecycles.
Plants that have similar flowers, reproductive structures, other characteristics, and are evolutionarily related, are grouped into plant families (See Figure 2). Species in the same plant family tend to have similar growth characteristics, nutrient needs, and often the same pests (pathogens, herbivores). Planting crops from different plant families on a farm and the landscape; and rotating crops of different plant families over time can interrupt the crop pest life cycles, particularly insect pests, and pathogens, and reduce yield losses due to pests. Increasing plant family diversity can also provide other agrobiodiversity benefits including, diverse seasonal growth and adaptation to weather stresses such as frosts, and drought; different soil nutrient needs, as well as producing diverse foods that provide for human nutritional needs.
Read this summary of the major world food crop plant families and the value of knowing what family plants are in, The Organic Way - Plant Families [2], then consider these questions.
The Fabaceae/Leguminosae, commonly called the Legume plant family, is important for soil nitrogen management in agriculture and for soil, human and animal nutrition. Legume plants can form a mutualistic, symbiotic association with Rhizobium bacteria which inhabit legume roots in small growths or nodules in the roots (seed images in the video listed below). The rhizobia bacteria have enzymes that can take up nitrogen from the atmosphere and they share the “fixed nitrogen” with their legume host plant. Nitrogen is an important nutrient for the plants and animals, it is a critical element in amino acids and proteins, genetic material and many other important plant and animal compounds. Legume grains crops, also called pulses are high in protein, such as many species of beans, lentils, peas, and peanuts. Most of their plant nitrogen is harvested in grain, although there is some in crop residues that can increase soil nitrogen content. Perennial legume crops are typically grown as forage crops for their high protein for animals. Because they allocate a large portion of their growth to vegetative plant parts and storage organs, perennial legumes also return a significant quantity of nitrogen to the soil, enhancing soil fertility for non-legumes crops grown in association or in rotation with legumes.
Watch the following NRCS video about legumes and legume research.
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.
Learn more about the differences in cool and warm season plants and the types of vegetable crops in these categories by reading Season Classification of Vegetables [3].
In addition, plants are classified by the type of photosynthetic pathway that they have.
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.
For a brief and helpful review of photosynthesis and plant anatomy such as the plant leaf structures, see Plant Physiology - Internal Functions and Growth [5].
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.
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.
Read more about how water moves through the plant and factors that contribute to water moving into the roots and out of the plant, as well as carbon dioxide movement in Transpiration - Water Movement through Plants [9].
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 [10].
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 [11].
In addition to the climate and soil resources for crop production, many socioeconomic factors influence which crops farmers chose to cultivate, including production costs, domestic and international market demand; and government policies that subsidize agricultural producers, and reduce trade barriers or export costs. As discussed in Module 3, the protein, energy, fat, vitamins, and micro-nutrients of crops for human nutrition are one predictor of the market value of a crop. However some food crops are highly valued and cultivated for their cultural and culinary qualities, such as flavor (ex. chilies, vanilla, coffee, wine grapes); and their high economic value often reflects high production and processing costs, as well as market demand for their unique culinary and cultural properties.
Some crops are cultivated for non-human food uses such as livestock feed, biofuel, fiber, industrial oil and starch, and medicinal uses. Crop processing often creates by-products that can be used for other purposes, adding market value. For example, when oil is extracted from oilseeds such as soybean, the soybean meal by-product is high in protein and sold for livestock feed or added to human food products. And for crops that are cultivated on many acres often with support from government policies, the consistent, abundant supply of these commodity crops has contributed to the development of multiple processing technologies, uses, and markets. To better understand factors that contribute to the production of commodity crops, we will now examine two case studies of corn and sugarcane.
In the following two agricultural crop case studies, you will have the opportunity to apply your understanding of crop plant life cycles, classification systems, and crop adaption to climatic conditions to understand how plant ecological features and human socioeconomic factors influence which crops are some of the major crops produced in the world.
Corn or maize is a summer annual C4 crop in the Poaceae, or grass family that has high nutrient demands. Unless soil conservation practices are used, corn fields do not have live roots protecting the soil from erosion and providing other soil quality benefits after harvest in the fall, winter and spring. The US is the largest corn producer in the world. Soils and climate, particularly in the Midwest, permit high corn yields; and significant investment in agricultural research has produced high-yielding corn hybrids and production technologies, such as fertilizers, pest control practices, farming equipment, and irrigation. Research has also developed diverse uses for the large quantities of corn produced in the US, and the US is also a major exporter of corn.
Read this overview of US corn production and uses from the US Department of Agriculture, Economic Research Service, Corn and Other Feed Grains [12].
The US consumes the most sweeteners of any country in the world. In the US, high-fructose syrup is made from corn, which has displaced some sugarcane production for sugar for the US market. Sugarcane production, however, has continued to increase in Brazil, the biggest sugarcane producer in the world. Sugarcane is a C4 perennial crop in the grass family and it's not grown just for sugar as a food sweetener.
Watch this United Nations video below, about the factors contributing to increased sugarcane production and some of the consequences. Then answer the questions below.
If the video does not play, please see Brazil: The ethanol revolution (United Nations) [13].
Links
[1] https://www.usbg.gov/sites/default/files/images/--how_plants_work-_student_discovery_journal-are_plants_like_us.pdf
[2] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod6/OrganicWay.pdf
[3] https://extension.psu.edu/seasonal-classification-of-vegetables
[4] https://commons.wikimedia.org/wiki/File:Tomato_leaf_stomate_1-color.jpg
[5] http://content.ces.ncsu.edu/extension-gardener-handbook/3-botany#section_heading_6937
[6] http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&topicorder=5
[7] http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&topicorder=5
[8] https://askabiologist.asu.edu/cam-plants
[9] https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1092853841&topicorder=1&maxto=8&minto=1
[10] https://www.nature.com/scitable/knowledge/library/effects-of-rising-atmospheric-concentrations-of-carbon-13254108
[11] https://nca2018.globalchange.gov/chapter/10/
[12] https://www.ers.usda.gov/topics/crops/corn-and-other-feedgrains/
[13] https://www.youtube.com/watch?v=ZitcSUdqmhM