Week 2 (Unit 3)

Centers of Diversity, Crop Adaptation

Vavilov, centers of origin, spread of crops
Biomes and Ecoregions
Environmental factors that affect plant growth
Crop adaptation: C3 vs. C4 plants, photoperiod, vernalization
Assignments
References

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Vavilov, Centers of Origin, Spread of Crops

Nicolai Vavilov (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.

In 1926 he published "Studies on the Origin of Cultivated Plants" which described his theories on the origins of crops. Vavilov concluded that each crop has a characteristic primary center of diversity which is also its center of origin. Eight areas were recognized and suggested as centers from which all of our major crops were domesticated. Later, he modified his theory to include "secondary centers of diversity" for some crops.

Vavilov’s Centers of Origin:

1 China
2 India
2a Indochina
3 Central Asia (N. India, Afghanistan, Turkmenistan)
4 The Near East
5 Mediterranean Sea, coastal and adjacent regions
6 Ethiopia
7 Southern Mexico and Middle America
8 Northeastern South America, Bolivia, Ecudor, Peru
8a Isle of Chile

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).

• Near East + Africa
• China + S. E. Asia
• Mesoamerica + S. America

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.

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Biomes and Ecoregions

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.

See also http://www.ucmp.berkeley.edu/glossary/gloss5/biome/index.html

Distribution of Biomes
(figures from R. Crang)

Geographic Distribution, Precipitation / Temperature Plot

Tundra Polar desert

• low rainfall and precipitation
• slow decomposition
• long cold winters
• permafrost - only the top 4 inches (10 cm) of soil thaws in the summer
• short growing season (less than 60 days), but very intensive due to long days
• treeless, low shrubs, grasses, sedges, mosses, and lichens
• not much potential for agricultural domestication
• slow recovery from disturbance

Evergreen coniferous forest (boreal forest, taiga)

• south of tundra - forms a huge band across North America, Europe and Asia
• broad ecotone
• cool summer, extremely cold winter
• 15-40 inches (38-102 cm) of precipitation
• acid podzol soil
• growing season 50-100 days
• soils completely thaw during growing season
• dominated by conifers
• low diversity - usually one or two tree species dominate
• susceptible to acid rain
• major source of wood

Temperate deciduous forest

• Eastern N. America, Europe, China
• well defined seasons (cold and warm)
• 5-6 month growing season
• 30-60 inches (75-152 cm) of precipitation, well-distributed throughout the year
• fertile soils
• dominated by broadleaf deciduous hardwood trees
• potential for domestication: fruits and nuts - apple, pear, peach, cherry, plum, grape, walnut, pecan, hazelnut, chestnut

Grasslands (pampas, veld, prairie, steppe)

• interior of continents and rainshadows
• continental climate with hot summer, cold winter
• rainfall of 10-30 inches (25 to 75 cm)
• frequent drought, fire, and intense grazing
• tallgrass prairie with fertile soil
• shortgrass prairie with thinner soil
• mixture of grasses and forbs, few trees
• potential for domestication - low due to difficulty in managing sod:
  • prairie: sunflower
  • steppe: proso millet, foxtail millet, hemp, Triticum tauschii (donor
    of D genome of bread wheat)

Deserts

• latitude between 20-30 degrees N and S of equator and in rainshadows
• evaporation exceeds rainfall; 1-10 inches (25 cm) of precipitation per year
• soils are often poor in nutrients because there is not enough moisture to decompose organic matter
• hot days and cold nights
• some grasses, shrubs, cacti, other
• many plant adaptations to conserve water such as deep roots, succulence, loss of leaves, thorns; desert annuals grown only during a rainy period
• potential for domestication: date palm

Dry woodland and shrubland (chaparral or mediterranean)

• western coastal regions between 30-40 degrees N and S
• hot dry summer and cool, rainy winter
• thin, highly weathered soils
• aromatic herbs, shrubs, scrubby trees plants adapted to fire
• Mediterranean woodland: center of domestication for wheat, barley, pea, rapeseed

Tropical savanna

• seasonally dry, tropical grasslands with scattered trees, thorny shrubs, acacias, eucalypts
• areas in dry tropics and subtropics in which grasses are conspicuous
• often on either side of rain forests
• dry parched season (4-8 mo drought) and rainy season
• typically 34 to 60 inches (85-150 cm) of rainfall
• fire is an important management tool
• Potential for domestication:
  • seed annuals or tuberous perennials: tuber remains dormant during dry season
  • maize, rice, sorghum, cassava, sweet potato, bean, peanut, yams

Tropical deciduous forest

• forest with trees that lose leaves for part of year
• pronounced wet and dry season
• relatively open; diversity less than evergreen forest
• threatened by grazing and easily eroded soils
• potential for domestication: sugarcane, banana & plantain, orange, mango, cacao

Tropical evergreen forest

• Warm, moist tropical lowlands
• high temperature with little seasonal variation
• daily rainfall, 80 to 180 inches (200 to 450 cm) per year
• soils highly weathered, high aluminum and iron
• rapid decomposition
• high diversity and tall trees
• dark interior and many layers
• major threat from logging and clearing

Tropical highland

• Andes was an important center for domestication: potato
• East African Highlands: arabica coffee

Ecoregions of the world

http://www.nationalgeographic.com/wildworld/

National Geographic presents 867 ecoregions, distinguished by shared ecological features, climate, plant and animal communities. Commonalities in ecological features or environmental constraints would represent good opportunities for successful species or varietal introductions.

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 megaenvironment concept)
2) For strategic introduction of germplasm (e.g., the joint CIAT/IITA program for introduction of Latin American cassava germplasm into Africa)

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.

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Environmental Factors that Affect Plant Growth

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.

• CO₂
• O₂
• light (proper quality, intensity and duration) http://www.hydrofarm.com/articles/factors_plantgrowth.php
• adequate water

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html
• nutrients (16 essential nutrients)

http://msucares.com/crops/soils/mgfertility.html
http://www.ctahr.hawaii.edu/oc/freepubs/pdf/pnm3.pdf
http://www.soils.wisc.edu/~barak/soilscience326/essentl.htm
• proper temperature
• http://cmg.colostate.edu/gardennotes/143.pdf
• proper pH
  http://hort.ifas.ufl.edu/gt/soilph/soilph.htm

Three important processes in plants that are essential for plant growth and development are:

• photosynthesis
• respiration
• transpiration

For a general overview of these processes, see
http://ag.arizona.edu/pubs/garden/mg/botany/physiology.html

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Crop Adaptation

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.

Respiration is the opposite of photosynthesis -- the stored energy in the sugar is released in the presence of oxygen, and this reaction releases the CO₂ and H₂O originally joined together by the sun's energy.

Photorespiration

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.

	RUBISCO
O2 + Rubp	-------->	3-PGA + 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.

C3 Photosynthesis

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.

	RUBISCO
CO2 + Rubp	-------->	2 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 Photosynthesis

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.

Source: http://www2.mcdaniel.edu/Biology/botf99/photodark/c4.htm

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 photosynthesis

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 and bromeliads.

The Table below summarizes the features and effects of photosynthetic pathway on plant adapatation to different environments, growth and productivity.

Photoperiod

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 P_R form absorbs red light during the day and is converted to P_FR . The P_FR form can absorb far red light, and will spontaneously convert back to the P_R 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

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.

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Assignments

Quiz

Take the quiz on this Unit on the Blackboard.

References

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:
C. Darwin (UK), A. de Candolle (CH), N. Vavilov (RU), J. Harlan (US)
http://www.agronomy.ucdavis.edu/gepts/pb143/lec02/pb143l02.htm

Gepts, Paul, 2003. Where did Agriculture Start?
http://www.agronomy.ucdavis.edu/gepts/pb143/lec10/pb143l10.htm

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:
http://highered.mcgraw-hill.com/sites/0072528427/student_view0/chapter4/chapter_outline.html

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:
http://highered.mcgraw-hill.com/sites/0072528427/student_view0/chapter26/chapter_outline.html

Levin, D.A. 2000. Biomes.
http://www.sbs.utexas.edu/levin/bio311d/biomes/biomes.html

VIR. 2002. Biography of Nikolai I. Vavilov.
http://www.vir.nw.ru/history/vavilov.htm

Woodward, S.L. 1997. Major Biomes of the World.
http://www.runet.edu/~swoodwar/CLASSES/GEOG235/biomes/main.html

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