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- Acid tolerance: The weathering of
soils can result in acidification, limiting the types of plant
or microbes that will grow. Repeated fertilization with ammonia
can also lead to acidification. To overcome such problems the
soil must be limed, or plant species and microbes used that are
tolerant of acidity.
- Aluminum toxicity: High levels
of aluminum or manganese are common in acid soils and can be toxic
to both the plant and rhizobia.
- Bagasse: The fine organic material
removed from vacuum drums during the processing of sugar cane.
Because it is of high organic matter and sugar content bagasse
makes a good inoculant carrier,
but bagasse must first be sterilized to stop the growth of fungi
and other contaminant organisms.
- Bean rust: A disease of beans caused
by the fungus Uromyces appendiculatus.
- Breeding: The identification
and selective combination of desirable genes, with the goal of
enhanced yield, reduced susceptibility to plant disease, improved
seed quality or better nitrogen-fixing
ability.
- Breeding lines: Lines identified
as superior in one or more selected traits and having value as
parents in breeding programs.
- Carbohydrate utilization: Plants
metabolize carbohydrates during growth; use them in the synthesis
of cell components such as cellulose; or store them as starch
against future need. Carbohydrate utilization patterns refer to
the balance between these activities.
- Chlorosis: Because iron is
needed for the synthesis of chlorophyll, one of the symptoms of
iron deficiency in the plant is a yellowing or chlorosis of leaves.
- Co-evolution: The co-existence
of legume hosts and rhizobia
in a particular location or ecosystem
over considerable periods of time can lead to modification of
their interaction in nodulation
and nitrogen fixation.
Thus the pea landrace Afghanistan will nodulate with pea rhizobia
from Middle Eastern countries, but not with those from Europe.
This difference is regulated by genes in both host and bacteria.
- Competition: Because each
plant only forms a finite number of nodules,
the ability of indigenous rhizobia
to form nodules will limit the number produced by inoculant
strains. This can limit nitrogen
fixation, and in the American Midwest can mean that the plant
derives less than 50% of its nitrogen from symbiosis.
While this is generally seen as a competition
between inoculant and indigenous rhizobia, various factors play
a role. These include the great numerical superiority of the indigenous
rhizobia in the bulk soil, and the limited mobility of the inoculant
rhizobia. It is also assumed that indigenous rhizobia are also
better adapted to soil conditions.
- Cross-Inoculation Groups:
Groups of legumes, any one of
which will form nodules
when inoculated with rhizobia
isolated from another legume in the group.
- Crown gall disease: A disease occurring
on many species of plant and caused by the bacteria Agrobacterium
tumefaciens. In this disease, in which large galls develop,
the bacteria transfers part of its genetic information to the
host plant.
- Cultivars: Lines of a particular
plant species that have been bred or selected for particular traits.
- Diversity of the host:
See Host diversity.
- Domestication: The collection
from the wild state and the use in agriculture of plants having
desirable traits.
- Ecosystem: A community of organisms
and the environment in which they live.
- Effectiveness: The ability
of nodules once formed to
actively fix nitrogen
.
- Fertilizer N: Forms of fertilizer
that include a source of N. The fertilizer is usually described
in terms of its content of nitrogen, phosphorus and potassium,
with one labelled 0:36:10 containing 0 nitrogen, 36 lbs P2O5
and 10 lbs K2O per 100 lb fertilizer. Fertilizer N
is usually in the form of urea, ammonium sulfate, potassium nitrate
or anhydrous ammonia.
- Freeze-Drying: One of the
methods used to preserve bacteria. Cells are suspended in a sucrose/peptone
solution and dried under vacuum at -40oC. They are
then sealed in vials while still under vacuum, and in this condition
can survive for many years. Vials prepared in this way serve as
a reserve supply of important strains.
- Gram negative bacteria: A common
staining technique used with bacteria stains the organisms with
crystal violet, then rinses them in alcohol. Some retain the dye
and appear purple in color, while in others including the rhizobia,
the stain washes out. To see such organisms they must be restained
with some other dye. Bacteria that stain with crystal violet are
termed Gram positive; those in which the dye washes out are called
Gram negative.
- Hemoglobin: A red pigment
present in nitrogen-fixing
root nodules that maintains
oxygen flow to the bacteria. It is similar in overall composition
and function to the hemoglobin in our blood.
- Host diversity: Plants from a particular
crop, pasture or prairie species are not all the same. This may
be because of cross pollination between plants, or because of
genetic mutations. Such differences can mean that some plants
in a species will be resistant to particular diseases or are better
in their ability to fix
nitrogen. Because of such differences and their potential
importance, the US maintains a strong germplasm collection program.
(See Germplasm
Resources Information Network - GRIN.)
- Hydroponic culture: Growth
of plants in an aerated liquid medium that supplies all nutrients
needed for growth.
- Indigenous: Organisms that
are native to a specific environment.
- Infectiveness: The ability
of a particular rhizobial
strain to induce nodule
formation on a particular host.
- Inoculant: A commercial preparation,
often but not always based on peat,
that is used to introduce rhizobia
into soils. Inoculants may be seed applied or introduced directly
to the soil.
- Inoculant-quality rhizobia:
Strains of rhizobia
which combine superior nodulation
and nitrogen-fixing
ability with the other traits needed in a strain suitable for
use in inoculant preparations.
- Iron (Fe) Deficiency Chorosis: Iron
is one of the more abundant minerals on earth, but may often be
present in forms that are not readily available to plants. In
alkaline soils, particularly those that are rich in limestone
and low lying, plants cannot access the iron that is present,
and may become deficient in this element and chlorotic.
- Legume: The collective common
name for a large family of dicotyledonous plants (peas, beans,
clovers, soybean, etc.) that have irregularly shaped flowers,
produce pods and fruit of a particular shape, and form nitrogen-fixing
root or stem nodules in
symbiosis with rhizobia.
- Marker-assisted selection:
A molecular tool used by plant breeders that permits them to identify
plants having particular desirable traits without the need to
grow the plant out under field conditions.
- Medium (Media): A solid or liquid
preparation containing all of the different compounds (sugars,
vitamins, minerals) that microorganisms need for their development.
Such media may be liquid or solidified with an extract from seaweed
called agar.
- MesoAmerica: The region of
Mexico and Central America that is one of several possible centers
of domestication for Phaseolus vulgaris. Bean varieties
from this area tend to be small seeded compared to those from
the Andean region of South America.
- Micronutrients (Microelements):
Elements such as zinc, boron, iron and molybdenum that are essential
for plant and microbial growth, but are only needed in very small
amount. In the case of molybdenum, this may only be ounces per
ha.
- Microsymbiont: The bacteria
associated with legumes in the
formation of nitrogen
fixing nodules.
- Mutation: An inheritable change
in the DNA of an organism. Most commonly this will result in the
loss of some specific ability or abilities, for example the ability
to form nodules on a particular host.
- Nitrate-tolerant symbiosis:
The ability in some host strain
combinations to continue to nodulate
and fix nitrogen
at levels of soil nitrogen that would normally be inhibitory to
these activities. In the case of soybean this has been reported
most commonly in genotypes from Korea; it may also be due to mutations
affecting the regulatory process.
- Nitrogen fixation:
The process by which nitrogen gas from the atmosphere is converted
into ammonia. The transformation is energy requiring and can be
achieved industrially through the Haber Bosch process or by specific
microorganisms -- all bacteria.
- Nitrogen transport:
Soon after flowering, plants begin to mobilize fixed
nitrogen from the leaves to the developing pod. Species and
varieties can differ in the efficiency and timing of this process.
In some farming systems it may even be desirable that some N remain
in the stover for the fertilization of subsequent crops.
- Nodulation: The process by
which rhizobia induce the
formation of galls or nodules on the roots (or occasionally stems)
of their specific hosts.
- Parts per million: A rule of
thumb is that one hectare of land area, taken to the depth of
the plow layer weighs two million kilograms. ie. 30 ppm equals
60 kilograms per hectare.
- Peat-based inoculants: Because
it is readily available, and can absorb large quantities of inoculant
culture, peat has long been used as a carrier for inoculant bacteria.
Not all peats are equally effective, and some are acid and must
be limed before use.
- Persistence: The ability of
strains of rhizobia
to survive in the soil, even in the absence of an appropriate
host plant.
- Photosynthesis: The process
by which plants, algae and some other organisms convert the sun's
energy into the organic compounds they need for growth.
- Polymerase chain reaction (PCR):
A molecular technique in which the DNA of plant or microbe is
used to generate banding patterns that can be used in determining
the relationship between organisms.
- Preference in nodulation: Cultivars
in some plant species select for particular rhizobia
from among the numerous strains
present in their environment. They are said to have a preference
in nodulation for these
strains.
- Recurrent selection: A form
of plant breeding in which the best plants from one evaluation
are used as parents and intercrossed to generate plants for the
next round of evaluations.
- Rhizobia: The common name for
several genera of bacteria which have the ability to infect the
root of legumes and to produce root nodules.
Each species of rhizobia can infect some but not all legumes.
- Roadside revegetation: In a
number of states, roadsides damaged during roadwork must be revegetated.
Often, this revegetation makes use of a mix of native prairie
plants. In Minnesota, there is a chain of "wildflower highways"
that span the state. (see Minnesota
Department of Transportation -- MnDOT.)
- Seed Size: Seed size in Phaseolus
vulgaris is one of several seed characteristics influencing
consumer acceptance. Thus Guatemalans prefer small-seeded black
beans while Colombians favor large-seeded red beans. In general
beans from the Mesoamerican
center of domestication
are small-seeded; those having an origin in the Andes of Ecuador
and Peru are large-seeded.
- Seed sterilization:
A method used to remove contaminant organisms from the surface
of seeds. Laundry bleach is commonly used, although, where the
seed must be scarified as well to break dormancy, concentrated
sulfuric acid is also effective.
- Self Incompatible: In some plant species,
embryos initiated by fertilization of the ovary with pollen from
the same plant do not develop. All seed then results from cross
pollination between plants.
- Senescence: Nodules
formed on the root of a host have a finite life span, usually
50-60 days. Plants can have several "crops" of nodules
in a single growing season, and in leguminous
trees current nodules may be at some distance from the stem. The
breakdown of nodules over time is called senescence.
- Serological testing: Bacteria
injected into an animal such as a rabbit, induce the production
of substances termed antibodies. These then react with the bacteria
causing them to be precipitated. Since antibodies only react with
the organism that caused them to be formed, or one very similar
to it, serological reactions are also of value in identifying
particular bacteria, for example the rhizobia.
- Siderophores: Substances produced
and expelled from the cell of some species of bacteria under conditions
of iron deficiency. They complex to iron in the soil solution
and are then reabsorbed and processed, providing the organisms
with an efficient mechanism for obtaining a scarce resource.
- Solarization: Weevils are
a major problem in stored beans and force farmers to sell their
produce soon after harvest, lowering prices. In solarization,
the beans are covered with plastic and exposed to the heating
effects of the sun, killing the weevils.
- Specificity in nodulation:
Specificity in nodulation
can affect benefits to inoculation
for closely related species. As example rhizobia
nodulating white clover will also nodulate subterranean clover,
but will not fix nitrogen
with this host.
- Starter doses: Where soil N levels
are low, small doses of nitrogen fertilizer are sometimes applied
at the beginning of the season to ensure vigorous early plant
growth and nodule formation. The "starter dose" used
should be less than that causing inhibition of nodulation
and nitrogen fixation.
- Sticker: Substance used in inoculation
to ensure that the rhizobia
adhere to the seed during planting. They range from milk or sugar
solutions used in simple seed inoculation, to stronger adhesives
(40% gum arabic, 5% methyl ethyl cellulose) used in pelleting
seed.
- Strain(s): An isolate of a particular
organism, thought to be different from other known organisms of
that species.
- Strain diversity: Bacteria
have a single "chromosome" plus additional smaller pieces
of DNA termed plasmids, which can be transferred between organisms.
This, plus genetic mutation and DNA rearrangements within the
chromosome, means that organisms from the same species can show
significant variation.
- Sustainable agriculture: A
system of agricultural production that supplies the necessities
of the present without compromising or polluting the resources
of the future.
- Symbiosis: A relationship between
two organisms such that each benefits. With legumes
and rhizobia, the legume gains
by having the microbe supply it an additional source of nitrogen
it can use for growth: the microbe gains a source of energy for
growth and a habitat (the nodule) within which it is protected
from outside stress.
- Zinc chelate: Zinc deficiency in plants
can be controlled either by applying zinc sulfate to the soil
or by spraying the foliage with relatively small amounts of an
organic compound to which zinc has been chelated or bound.
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