Pigweed (Amaranthus spp.) Taxa

-Pigweed Outline
-Pigweed Bibliography (1991)
-Pigweed Bibliography (1996)
a. redroot pigweed (A. retroflexus)
b. prostrate pigweed (A. blitoides)
c. smooth pigweed (A. hybridus)
d. green pigweed (A. palmerii)
e. spiny pigweed (A. spinosus)
f. tall waterhemp (A. tuberculatus)
g. tumble pigweed (A. albus)

4.16.96
Agronomy 517: Weed Biology and Ecology
Spring Semester, 1996
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A. General
1. Found in temperate and tropical regions of America and Africa.

2. 12 genera are native to the United States
a. Well represented in the southeast.
b. A few are ornamentals.
c. Overall, these species are of little economic importance.

3. The genus Amaranthus contains several noxious weeds.

B. Weedy amaranths - exact origin unknown; North America, South America or Europe.
1. A. retroflexus
2. A. blitoides
3. A. hybridus
4. A. palmerii
5. A. spinosus
6. A. tuberculatus
7. A. albus

C. Grain amaranths
1. A. cruentus
a. Tentatively identified on the basis of seeds alone at a level dated 4000 B.C. Positive identification was dated at six different times between 2400 B.C. and the Spanish Conquest.
b. Guatemalan cultivated species.

2. A. hypochondriacus = Mexican cultivated species.
3. A. caudatus

4. History
a. Records show that amaranth was consumed as a vegetable and cereal grain in Central America.
b. Mayan civilization was among the first to use amaranth as a crop
c. Before the Spanish Conquest, the Aztecs produced 15-20 thousand tons of amaranth grain per year.
d. In 1519, Cortes invaded Mexico and banned the production of amaranth due to religious beliefs.
e. In 1975, amaranth was named one of the 36 most potential food crops.
f. Intensive research continues on the grain species.

II. Taxonomy (Plant Morphology and Anatomy)

A. Identification
1. Leaf structure
a. Amaranth leaves are alternate, entire and petiolate.
b. CO2 in the leaf is assimilated by the C4 pathway.

2. Root structure
a. Little research performed on root systems in amaranths.
b. Information on root morphology and spacial distribution in the soil profile are important topics for crop management and yield improvement.

3. Distinguishing between species
a. Difficult to distinguish between species while in the seedling stage.
b. Some herbicide labels group all species under a generic term "pigweed". Not good because control is not the same for all pigweed species.

B. Mating systems - flower and inflorescences
1. Found to be monecious or dioecious
a. Monecious flower arrangement
i. First flower of each glomerulus is staminate.
ii. Following flowers are pistillate.
iii. Each glomerulus has only one staminate flower.
* Typical to grain amaranth species.
iv. Flowers of upper glomeruli in the inflorescence are all staminate and the lower ones pistillate.
b. Dioecious flower arrangement
i. Contains separate male and female flower parts on separate plants.
ii. Basic unit is the same as the maniacs species.

2. Flowers are small and inconspicuous.

3. When fertilization is prevented, pistillate flowers are successfully produced in the glomerulus.

4. In all grain species, each flower is subtended by a sharp-pointed bract.

5. Flowering time
a. Flowering is induced by differences in photoperiod.
b. No vernalization or low temperature requirements for flowering have been reported.
c. Inflorescence length and branching is highly affected by day length.

C. Seeds
1. Structural characteristics
a. Amaranths were the first reported dicot containing both glutinous an non-glutinous starch
i. A feature found in most cereal grains.
ii. Different starch types for grain amaranth and wild types.
iii. Controlled by a single major gene.

b. Color
i. Seed coat of amaranth can be black, brown, yellow, and cream- white.
ii. Light colored seed most commonly used for grain types.
iii. Chemical basis of the color is unclear
iv. May be associated with the presence of anti-nutritive factors in the seed coat.
v. Some studies show that genetic inheritance may be involved in controlling seed color.

2. Size
a. Length = 1mm
b. Weight = 0.2-1.1mg
* grain amaranths are at upper range of seed weight.
c. Different up to 20% from seed weight averages were found in Amaranthus dubius even between individuals of the same population.
d. Elevation differences for A. retroflexus have occurred = lighter seeds at higher altitudes.
e. Interpopulation phenotypic variation in seed size can be reduced by growing plants from different populations in the same environment, where seeds are produced in similar conditions (A. retroflexus and A. hybrius).

D. Life cycle
1. Annual or perennial (amaranthaceae) = we are most concerned about the annual species.
a. Amaranthus = monecious and dioecious (see Section I-B, Mating Systems) annuals.
b. Common in barnyards, cultivated fields, waste ground and yards.

2. Can emerge throughout the spring and summer if conditions are favoraable (eg. light, water, and orther stimuli necessary for growth).
III. Vegetative Development

A. Light and temperature
1. C4 plant. Usually found in warm regions and thrive in high sunlight

2. As like other C4 plants, optimum temperatures range from 30-40 degrees C.

3. Biomass allocation
a. Root/shoot ratio in A. hybridus is not affected by temperature or light.
- High temps favor stem growth while leaf growth is unaffected.
b. High temps increase plant weight and leaf area but the LAR is reduced.
B. Mineral requirements and pH toxicity
1. Nitrogen (N) - macronutrient
a. Nitrophilous plants with a strong positive growth response to increased nitrogen.
b. Interspecific differences in growth response to amaranths to N application have been found.
c. Research in Minnesota has shown that N is a main limiting factor in (grain) amaranth yield.

2. Sodium (Na) - micronutrient
a. Shown to be essential in C4 plants.
b. In A. tricolor, Na deficiency was shown to reduce growth and chlorophyll content.
c. High levels can be toxic. eg. Growth is gradually reduced by increasing salinity.

3. Aluminum (Al) and Manganese (Mn)
a. Al is antagonistic to soil potassium, which is required by amaranth in moderate amounts.
b. Low pH increases Al and Mn solubility, which could become toxic. Important limiting factors when liming is not possible.

4. pH effects
a. Acid soils can severely reduce growth in amaranth.
b. Some specie can tolerate various pH (resulting in potential Al and MN problems) levels.

IV. Dormancy and Germination

IV. Dormancy and Germination

A. Seed dormancy
1. Timing is important for successful establishment and for the ability of mature plants to set seed before the end of the season.

2. Differences in genetic and physiological factors result in different dormancy levels and the responsiveness to germination stimuli.

3. At dispersal seeds are usually dormant (Primary Dormancy)
a. Early germination (e.g. after dispersal) can occur with high light and temperatures.

b. Afterripening
i. Light requirement diminishes
ii. Germination occurs over a broader temp range

4. Parent plant effects
a. Dormancy differs on lateral inflorescences compared to main shoot inflorescences.
b. Seeds collected from senescing plants were more dormant than those from green plants.

B. Longevity
1. Seeds buried in the soil have less longevity than under dry storage conditions
a. In the soil seeds can be consumed by herbivores or pathogens.
b. May senesce and decay.

2. Weedy amaranth species usually have prolonged seed dormancy.
a. Seeds of A. blitoides, A. gracizans, and A. retroflexus germinated after six years of burial.
b. Long-term studies show germination from 10-40 years after burial (undisturbed sites).

3. Factors which reduce longevity/persistence
a. Tillage = increased aeration and exposure to light
b. Amount of seed which can survive after 5 years of burial, 1-5 seedlings emerged per M2.

C. Germination
1. Seed coat effects
a. Dormancy imposed by seed coats
i. NOT due to lack of water uptake.
ii. Probably due to mechanical constraints or limited O2 diffusion.

b. Scarification enhances germination.

2. Light effects
a. Most species produce light-responsive seeds and germination is under phytochrome control.
b. Inhibition or promotion of germination is, however, temperature dependent.
i. Prolonged illumination usually inhibits germination at low temps.
ii. Germination increases as temps increase.

3. Temperature effects
a. Germination can occur between 10 and 45 degrees C.
b. Best emergence in field occurs in range of 24-34 degrees C.

4. Depth of emergence
a. Small seeds and limited food reserves limits emergence depths.
b. Best emergence occurs between 10-15mm.

V. Alleopathy

A. Effects of allelopathic chemicals on amaranth germination has only been examined in a few cases.
B. Rye extracts have been shown to strongly inhibit amaranth seedling growth.

VI. Inter- and Intraspecific Competition

A. Common lambsquarters (Chenopodium album)
1. Germination differences
a. Both germinate better at higher rather than lower temps.
b. At lower temps (18 degrees C day/13 night), redroot pigweed was 37 fold less than at the higher temps (29 day/ 24 night) compared to 2 fold for common lambsquarters.

2. Growth
a. Redroot pigweed had much greater stem length (108cm) compared to common lambsquarters (43cm) art high temps. Both were the same at low temps.
b. Leaf area in redroot pigweed developed faster.

3. Dominance in field
a. Common lambsquarters will dominate if land is prepared in April-May with extended period of cool, wet conditions.
b. Late plowing or warm temps in early May - dominant redroot pigweed infestation would be expected.

B. Crops
1. Amaranth lags slightly behind common cocklebur as being the most competitive weed in field crops.
2. Competes for light, nutrients, and other essential resources.
3. Polymorphism allows the plant to adjust to different competition environments (eg. can grow very tall if competition for light is high).

C. Other pigweed plants
1. Compete with one another for the available resources.

2. "Phenotypic Breakdown" can occur with amaranthus when the temperatures are high. (With respect to any kind of competition, e.g.. intra- or interspecific).
a. Can occur at temps only slightly above optimum.
b. Chlorotic leaves, twisted stalks, and suppressed lamina growth can occur.

VII. Economic Effects

A. Disadvantages
1. Reduces crop yield.
2. Reduces resources available in soil.
B. Advantages
1. Seeds = Better essential amino acid than that of cereals and legumes.
2. Plant = Green parts are rich in protein, minerals, and vitamins.
= Comparable to spinach.
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