Week 2 (Unit 3)
Centers of Diversity, Crop Adaptation
(1887-1943) was a Russian scientist who headed the Lenin All-Union
Academy of Agricultural Sciences (later named the Vavilov All-Union
Institute of Plant Industry in his honor) in St. Petersburg (Leningrad)
from 1920 to 1940. He established 400 research institutes that employed
up to 20,000 people. He planned to collect all of the useful germplasm
that had potential in the Soviet Union, to classify it, and to use
it in a national plant breeding effort. He and his colleagues conducted
extensive germplasm explorations and collections in many parts of
the world. The Vavilov Institute remains an important resource for
germplasm maintenance, access, and utilization.
Vavilov’s Centers of Origin:
In recent years, the diversity of DNA fractions and other approaches have been used to study diversity of crop species. In general, these studies have not confirmed Vavilov's theory that the centers of origin are the areas of greatest diversity. They have identified centers of diversity, but these are often not the centers of origin. For some crops there is little connection between the source of the wild ancestors, areas of domestication, and the areas of evolutionary diversification. Species may have originated in one geographic area, but were domesticated in a different region. Some crops do not even have centers of diversity.
In 1971, Jack Harlan described his own views on the origins of agriculture. He proposed three independent systems, each with a center and a "noncenter" (larger, diffuse areas where domestication is thought to have occured).
Evidence since that time suggests that these centers are also more diffuse than he had envisioned. After the initial phases of evolution, the species spread out over large, ill-defined areas. This is probably due to crops traveling with man and evolving along the way. Regional and/or multiple areas of origin may be better models than the idea of a unique, localized origin for many crops.
Nonetheless, we can make some statements about the probable geographic origin of many crops:
See http://www.agronomy.ucdavis.edu/gepts/pb143/lec10/pb143l10.htm for a thorough presentation on the geographic origins of crops.
Biome: A major regional terrestrial community, or grouping, with its own type of climate, vegetation, and animal life. Biomes are not sharply separated, but merge gradually into one another over what is called an ecotone. A biome embraces the idea of community, interactions among vegetation, animal populations, and soil types within a regional climate.
Climatic conditions, such as rainfall and temperature, are the major determinants of the type of biome that develops in an area. Deserts develop in areas with low annual precipitation. Slightly higher rainfall will favor a grassland, and high yearly precipitation will favor the development of a forest. Altitude is another important factor determining the distribution of biomes. Cool temperatures at high altitudes may favor a type of plant community normally encountered at higher latitudes.
Harlan's new theory (1992): Certain biomes or vegetative types may have been more conducive to crop domestication than others. The Mediterranean woodlands and tropical savannas were ideal for domestication because they both have long dry seasons, which generate annuals.
Description of major biomes
Biomes are classified in various ways. We will discuss eight biomes in this section, but as you can see from the map below, additional types can easily be distinguished.
Geographic Distribution, Precipitation / Temperature Plot
Tundra Polar desert
Evergreen coniferous forest (boreal forest, taiga)
Temperate deciduous forest
Grasslands (pampas, veld, prairie, steppe)
Dry Woodland and Shrubland
Dry woodland and shrubland (chaparral or mediterranean)
Tropical Deciduous Forest
Tropical deciduous forest
Tropical Evergreen Forest
Tropical evergreen forest
Ecoregions of the world
The concept of agroecological zones has been used by the International Centers for two purposes:
1) To define target breeding environments (e.g., the CIMMYT maize program
Agroecological production zones for the Pacific Northwest were defined
by Douglas et al. (1990) based on annual precipitation, soil depth, and
growing degree days. The zones were proposed to facilitate communication
and adoption of appropriate farming technology throughout the region.
For plant breeders in the region, they also serve as a basis for ‘design’
of varieties better adapted and able to ‘exploit’ environmental
variations; i.e., breeding for agroecological adaptation.
In order to discuss the ways that plants have evolved to be better adapted to their environments, it is necessary to have a basic understanding of plant growth requirements. This is a very broad topic, so we will only outline the important factors and provide links for further information.
Three important processes in plants that are essential for plant growth and development are:
For a general overview of these processes, see
In this section we will discuss several mechanisms plants have developed that make them adapted to particular environments.
Types of Photosynthesis
Photosynthesis is the joining together of CO2 (carbon dioxide) with H2O (water) to make CH2O (sugar) and O2 (oxygen), using the sun's energy. The sugar contains the stored energy and serves as the raw material from which other compounds are made.
Basic photosynthetic pathway:
6CO2 + 12 H2O --> C6H12O6 + 6O2 + 6H2O
Energy to carry out the reaction comes from light absorbed by chlorophyll, stored as ATP and NADH.
There are three types of photosynthesis: C3, C4, and CAM.
The type of photosynthesis utilized by a species influences its adaptation
to different environments.
Under high light and high heat, the enzyme (RUBISCO) that grabs carbon dioxide for photosynthesis may grab oxygen instead, causing respiration to occur instead of photosynthesis, thus reducing the production of sugars from photosynthesis.
During photorespiration, O2 + Rubp (ribulosebisphosphate, a 5-carbon compound) are catalyzed by RUBISCO to produce one molecule of 3-PGA (3-phosphoglycerate, a 3-carbon organic acid) and one molecule of phosphoglycolate.
Photorespiration can reduce photosynthetic efficiency by 30%. Furthermore, the phosphoglycolate is toxic and must be broken down by the plant. Higher levels of CO2, or lower levels of O2, will increase photosynthesis by decreasing photorespiration.
Most plants use C3 photosynthesis. It is called the C3 pathway because CO2 is first incorporated into a 3-carbon compound. CO2 and ribulosebisphosphate are combined by RUBISCO, resulting in the production of two molecules of the 3-carbon organic acid 3-PGA.
Photosynthesis takes place throughout the leaf. Stomata are open during the day.
Photorespiration may occur in C3 plants during light fixation of CO2.
Adaptive Value: C3 plants are more efficient than C4 and CAM plants under cool and moist conditions and under normal light because they have less machinery (fewer enzymes and no specialized anatomy).
C4 plants can photosynthesize faster under high heat and light conditions than C3 plants because they use an extra biochemical pathway and special leaf anatomy to reduce photorespiration.
The leaves of C4 plants have Kranz anatomy. The xylem and phloem of these leaves are surrounded by thick walled parenchyma cells called bundle sheath cells where most of the photosynthesis takes place.
Stomata open in the morning. CO2 is first combined with phosphoenolpyruvic acid (PEP) in mesophyll cells by phosphoenolpyruvate carboxylase (PEPCase). This allows CO2 to be taken into the plant very quickly. The 4-carbon compound oxaloacetic acid is produced, and then converted to malic or aspartic acid. These are also 4-carbon compounds, hence the name C4 photosynthesis. The malic or aspartic acid is then moved through plasmodesmata (at the expense of ATP) into the bundle sheath cells.
In the bundle sheath cells, the malic or aspartic acid is broken into CO2 and PEP. The CO2 is "delivered" to the RUBISCO enzyme for photosynthesis. This system allows the plant to maintain a high concentration of CO2 in the bundle sheath cells for photosynthesis. The higher concentration of CO2 prevents photorespiration and allows the plant to close its stomata during the hot hours of the day.
Adaptive Value: C4 plants photosynthesize faster than C3 plants under high light intensity and high temperatures. C4 plants do not have a photorespiration pathway, increasing photosynthetic efficiency. Water use efficiency of C4 plants is high because PEP Carboxylase brings in CO2 faster, so the plant does not need to keep stomata open as much (less water lost by transpiration) to have sufficient CO2 for photosynthesis. The C4 pathway is more expensive energetically than normal photosynthesis, but not as expensive as photorespiration.
C4 plants include several thousand species in at least 19 plant families. Examples include corn, sorghum, and many of our summer annual plants.
CAM stands for Crassulacean Acid Metabolism. Stomata open at night (when evaporation rates are usually lower) and are usually closed during the day. The CO2 is converted to an acid and stored during the night. During the day, the acid is broken down and the CO2 is released to RUBISCO for photosynthesis.
Adaptive Value: Better water use efficiency than C3 plants under arid conditions because stomata are open at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.).
When conditions are extremely arid, CAM plants can CAM-idle. They leave
their stomata closed night and day. Oxygen given off in photosynthesis
is used for respiration and CO2 given off in respiration is
used for photosynthesis. CAM-idling allows the plant to survive dry spells,
and to recover very quickly when water is available again. CAM plants
include many succulents such as cactuses and agaves and also some orchids
The Table below summarizes the features and effects of photosynthetic
pathway on plant adapatation to different environments, growth and productivity.
See also http://eee.uci.edu/99w/07350/Doc/C3C4CAM.html
Many angiosperms will flower at about the same time every year, regardless of when they are planted. This is a response to daylength that promotes cross-pollination and ensures that plant development is well synchronized with the length of the growing season. Short-day plants flower only after exposure to short days, long-day plants flower only after exposure to long days, and day-neutral plants show no response to daylength.
Some plants can only be grown at certain latitudes due to photoperiodism. Spinach is a long-day plant, and will never flower if it is grown in the tropics. Maize is a short-day plant - if a tropical variety is grown at northern latitudes it will grow very tall and may never flower. Selection for the day-neutral characteristic has permitted many crops, such as maize and soybeans, to be grown over much wider geographic areas.
Phytochrome is the plant compound that is responsible for the photoperiod response. The PR form absorbs red light during the day and is converted to PFR . The PFR form can absorb far red light, and will spontaneously convert back to the PR form in the night. Consequently, it is the length of the night period that is really important in determining photoperiod response. Short-day plants should really be called long-night plants and long-day plants should be called short-night plants.
Vernalization in crops is the acceleration of flowering in response to a long period of cold temperature. Winter crops require a period of exposure to temperatures between 0 to 12 °C for a period of time from 10 to 60 days from germination to proceed into the reproductive phase. Vernalization requirements vary greatly among species and cultivars. Vernalization ensures that plants overwinter vegetatively and flower in the favorable conditions of spring. The majority of crops grown in northern Europe and Canada (wheat, barley, oilseed rape and sugarbeet) have been bred with a strong vernalization requirement to extend geographical range or prevent bolting.
Take the quiz on this Unit on the Blackboard.
Douglas, C.L., D.J. Wysocki, J.F. Zuzel, R.W. Rickman, and B.L. Klepper. 1990. Agronomic zones for the dryland Pacific Northwest. Pacific Northwest Extension Publication, Washington, Oregon, Idaho State University Cooperative Extension Service.
Gepts, Paul, 2004. Who's Who in the History of Crop Evolution Studies:
Gepts, Paul, 2003. Where did Agriculture Start?
Harlan, J.R. 1992. Views on Agricultural Origins. Chapt. 2 In Crops and Man, 2nd ed. American Soc. Agronomy, Madison, WI.
Levetin, E. and K. McMahon. 2005. Plant Physiology. Chapter 4 in Plants
and Society, 4th edition. McGraw-Hill, New York, NY. Additional on-line
notes and references:
Levetin, E. and K. McMahon. 2005. Ecology. Chapter 26 in Plants
and Society, 4th edition. McGraw-Hill, New York, NY. Additional on-line
notes and references:
Levin, D.A. 2000. Biomes.
VIR. 2002. Biography of Nikolai I. Vavilov.
Woodward, S.L. 1997. Major Biomes of the World.