SQL

SQL is a standard language for accessing and manipulating databases.

Abbreviation of structured query language, and pronounced either see-kwellor as separate letters. SQL is a standardized query language for requesting information from a database. The original version called SEQUEL (structuredEnglish query language) was designed by an IBM research center in 1974 and 1975. SQL was first introduced as a commercial database system in 1979 by Oracle Corporation.

Historically, SQL has been the favorite query language for database management systems running on minicomputers and mainframes. Increasingly, however, SQL is being supported by PC database systems because it supports distributed databases (databases that are spread out over several computer systems). This enables several users on a local-area network to access the same database simultaneously.

Although there are different dialects of SQL, it is nevertheless the closest thing to a standard query language that currently exists. In 1986, ANSI approved a rudimentary version of SQL as the official standard, but most versions of SQL since then have included many extensions to the ANSI standard. In 1991, ANSI updated the standard. The new standard is known as SAG SQL.

What is SQL?

• SQL stands for Structured Query Language
• SQL lets you access and manipulate databases
• SQL is an ANSI (American National Standards Institute) standard

What Can SQL do?

• SQL can execute queries against a database
• SQL can retrieve data from a database
• SQL can insert records in a database
• SQL can update records in a database
• SQL can delete records from a database
• SQL can create new databases
• SQL can create new tables in a database
• SQL can create stored procedures in a database
• SQL can create views in a database
• SQL can set permissions on tables, procedures, and views

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SQL is a Standard - BUT....

Although SQL is an ANSI (American National Standards Institute) standard, there are different versions of the SQL language.

However, to be compliant with the ANSI standard, they all support at least the major commands (such as SELECT, UPDATE, DELETE, INSERT, WHERE) in a similar manner.

RDBMS

RDBMS stands for Relational Database Management System.

RDBMS is the basis for SQL, and for all modern database systems such as MS SQL Server, IBM DB2, Oracle, MySQL, and Microsoft Access.

The data in RDBMS is stored in database objects called tables.

A table is a collection of related data entries and it consists of columns and rows.

SQL Commands

Database

A database consists of one or more tables. A table is identified by its name. A table is made up of columns and rows. Columns contain the column name and data type. Rows contain the records or data for the columns.

Basic SQL

Each record has a unique identifier or primary key. SQL, which stands for Structured Query Language, is used to communicate with a database. Through SQL one can create and delete tables. Here are some commands:

• CREATE TABLE - creates a new database table
• ALTER TABLE - alters a database table
• DROP TABLE - deletes a database table
• CREATE INDEX - creates an index (search key)
• DROP INDEX - deletes an index

SQL also has syntax to update, insert, and delete records.

• SELECT - get data from a database table
• UPDATE - change data in a database table
• DELETE - remove data from a database table
• INSERT INTO - insert new data in a database table

SELECT

The SELECT is used to query the database and retrieve selected data that match the specific criteria that you specify:

SELECT column1 [, column2, ...]
FROM tablename
WHERE condition

The conditional clause can include these operators

• = Equal
• > Greater than
• < Less than
• >= Greater than or equal
• <= Less than or equal
• <> Not equal to
• LIKE pattern matching operator

SELECT * FROM tablename 

returns all the data from the table.

Use single quotes around text values (most database systems will also accept double quotes). Numerical values should not be enclosed in quotes.
LIKE matches a pattern. The wildcard % is used to denote 0 or more characters.

• 'A%' : matches all strings that start with A
• '%a' : matches all strings that end with a
• '%a%' : matches all strings that contain an a

CREATE TABLE

The CREATE TABLE statement is used to create a new table. The format is:

CREATE TABLE tablename
(column1 data type,
column2 data type,
column3 data type);

• char(size): Fixed length character string.
• varchar(size): Variable-length character string. Max size is specified in parenthesis.
• number(size): Number value with a max number of columns specified in parenthesis
• date: Date value
• number(size,d): A number with a maximum number of digits of "size" and a maximum number of "d" digits to the right of the decimal

INSERT VALUES

Once a table has been created data can be inserted using INSERT INTO command.

INSERT INTO tablename
(col1, ... , coln)
VALUES (val1, ... , valn)

UPDATE

To change the data values in a pre existing table, the UPDATE command can be used.

UPDATE tablename
SET colX = valX [, colY = valY, ...]
WHERE condition

DELETE

The DELETE command can be used to remove a record(s) from a table.

DELETE FROM tablename
WHERE condition

To delete all the records from a table without deleting the table do

DELETE * FROM tablename

DROP

To remove an entire table from the database use the DROP command.

DROP TABLE tablename

ORDER BY

ORDER BY clause can order column name in either ascending (ASC) or descending (DESC) order.

ORDER BY col_name ASC

AND / OR

AND and OR can join two or more conditions in a WHERE clause. AND will return data when all the conditions are true. OR will return data when any one of the conditions is true.

IN

IN operator is used when you know the exact value you want to return for at least one of the columns

SELECT * FROM table_name WHERE col_name IN (val1, val2, ...)

BETWEEN / AND

The BETWEEN ... AND operator selects a range of data between two values. These values can be numbers, text, or dates.

SELECT * FROM table_name WHERE col_name BETWEEN val1 AND val2

JOIN

There are times when we need to collate data from two or more tables. That is called a join. Tables in a database are related to each other through their keys. We can associate data in various tables without repeating them. For example we could have a table called Customers which could have information about customers like their name, address, phone numbers. We could have another table called Products that has information regarding the products like part number, product name, manufacturer, number in stock, unit price. A third table called Orders could have information regarding what product was ordered, by whom, the date the order was placed, and quantity. Here are the tables:

Customers

Cust_ID	FirstName	LastName	Address	Phone
01	Mickey	Mouse	123 Gouda St.	456-7890
02	Donald	Duck	325 Eider Ln.	786-2365

Products

Part_No	Name	Manufacturer	In_Stock	Price
20-45	Hammer	Stanley	57	3.50
21-68	ScrewDriver	DeVries	84	2.75

Orders

Order_No	Part_No	Cust_ID	Date	Quantity
2005-27	21-68	02	31 Oct 2005	2
2005-34	20-45	01	02 Nov 2005	3

We can obtain information on who has ordered what:

SELECT Customers.FirstName, Customers.LastName, Products.Name
FROM Customers, Products, Orders
WHERE Customers.Cust_ID = Orders.Cust_ID AND Products.Part_No = Orders.Part_No
We can select data from two tables with INNER JOIN. The INNER JOIN returns all rows from both tables where there is a match. If there are rows in Customers that do not have matches in Orders, those rows will not be listed.
SELECT Customers.FirstName, Customers.LastName, Orders.Date
FROM Customers
INNER JOIN Orders
ON Customers.Cust_ID = Orders.Cust_ID
The LEFT JOIN returns all the rows from the first table (Customers), even if there are no matches in the second table (Orders). If there are rows in Customers that do not have matches in Orders, those rows also will be listed.
SELECT Customers.FirstName, Customers.LastName, Orders.Date
FROM Customers
LEFT JOIN Orders
ON Customers.Cust_ID = Orders.Cust_ID
The RIGHT JOIN returns all the rows from the second table (Orders), even if there are no matches in the first table (Customers). If there had been any rows in Orders that did not have matches Customers, those rows also would have been listed.
SELECT Customers.FirstName, Customers.LastName, Orders.Date
FROM Customers
RIGHT JOIN Orders
ON Customers.Cust_ID = Orders.Cust_ID

ALTER TABLE

With ALTER TABLE you can add or delete columns in an existing table. When you add a column you must specify a data type.
ALTER TABLE table_name
ADD col_name datatype

ALTER TABLE table_name
DROP COLUMN col_name

UNION

The UNION command is used to select data from two tables very similar to the JOIN command. But the UNION command can be used only with columns having the same datatype. With UNION only distinct values are selected, i.e. if there are common data in the two tables only one instance of that data is returned.

SELECT Name FROM Customers_USA
UNION
SELECT Name FROM Customers_Asia

This will select all the customers from USA and Asia but if there is a name that occurs in both the tables it will return only one such name. To get all the names use UNION ALL instead.

SQL Functions

There are several built-in functins in SQL. The basic function types are:

• Aggregate Functions: These are functions that operate against a collection of values, but return a single value.
• Scalar Functions: These functions operate against a single value, and return a single value.

To use a built-in function the syntax is: 

SELECT function (col_name) FROM table_name 

GROUP BY

The GROUP BY was added to SQL so that aggregate functions could return a result grouped by column values.

SELECT col_name, function (col_name) FROM table_name GROUP BY col_name
HAVING keyword was introduced because the WHERE keyword could not be used. HAVING states a condition.

SELECT clo_name, function (col_name) FROM table_name
GROUP BY col_name
HAVING function (col_name) condition value

CREATE VIEW

A view is a virtual table that is a result of SQL SELECT statement. A view contains fields from one or more real tables in the database. This virtual table can then be queried as if it were a real table.

CREATE VIEW view_name AS
SELECT col_name(s)
FROM table_name
WHERE condition

A view could be used from inside a query, a stored procedure, or from inside another view. You can add functions and joins to a view and present the data you want to the user.

Shailesh kr shukla
directoratace@gmail.com

Earth's Moon: Formation, Composition and Orbit

The moon is the easiest celestial object to find in the night sky — when it's there. Moon phases and the moon's orbit are a mystery to many. Because it takes 27.3 days both to rotate on its axis and to orbit Earth, the Moon always shows us the same face. We see the Moon because of reflected sunlight. How much of it we see depends on its position in relation to Earth and the Sun.
Though a satellite of Earth, the Moon is bigger than Pluto. Some scientists think of it as a planet (four other moons in our solar system are even bigger), though that viewpoint has never caught on officially. There are various theories about how the Moon was created, but recent evidence indicates it formed when a huge collision tore a chunk of the Earth away.


The moon is Earth's only natural satellite. The moon is a cold, dry orb whose surface is studded with craters and strewn with rocks and dust (called regolith). The moon has no atmosphere. Recent lunar missions indicate that there might be some frozen ice at the poles. 
The same side of the moon always faces the Earth. The far side of the moon was first observed by humans in 1959 when the unmanned Soviet Luna 3 mission orbited the moon and photographed it. Neil Armstrong and Buzz Aldrin (on NASA's Apollo 11 mission, which also included Michael Collins) were the first people to walk on the moon, on July 20, 1969. 

Physical characteristics of Earth's moon

1.Formation

The leading explanation for how the moon formed was that a giant impact knocked off the raw ingredients for the moon off the primitive molten Earth and into orbit. Scientists have suggested the impactor was roughly 10 percent the mass of Earth, about the size of Mars.

2.Internal structure

The moon very likely has a very small core just 1 to 2 percent of the moon's mass and roughly 420 miles (680 km) wide. It likely consists mostly of iron, but may also contain large amounts of sulfur and other elements.

3.Surface composition

Like the four inner planets, the moon is rocky. It's pockmarked with craters formed by asteroid impacts millions of years ago. Because there is no weather, the craters have not eroded.

4.

Atmosphere of the moon

The moon has a very thin atmosphere, so a layer of dust — or a footprint — can sit undisturbed for centuries. And without much of an atmosphere, heat is not held near the planet, so temperatures vary wildly. Daytime temperatures on the sunny side of the moon reach 273 degrees F; on the dark side it gets as cold as -243.

Moon Facts

There are many interesting facts about the moon and trivia that may or may not be important to you. Some interesting facts include:

• When Alan Sheppard was on the moon, he hit a golf ball and drove it 2,400 feet, nearly one half a mile.
  
• In a survey conducted in 1988, 13% of those surveyed believed that the moon is made of cheese.
  
• The multi layer space suits worn by the astronauts to the moon weighed 180 pounds on earth, but thirty pounds on the moon due to the lower gravity.
  
• How close can you get without completely running out of gas? Apollo 11 had only 20 seconds of fuel left when they landed on the moon.
  
• Apollo 15 was the first mission to use a lunar rover. The top speed that was ever recorded in this 4-wheeled land vehicle was 10.56 miles per hour.
  
• It is possible to have a month without a full moon. This occurs in February, but either January or March will have two moons.
  
• In China, the dark shadows that are on the moon are called "the toad in the moon".
  
• The Apollo missions brought back 2196 rock samples weighing 382 kg in total

Facts About the Moon

• The moon is not a planet, but a satellite of the Earth.
  
• The surface area of the moon is 14,658,000 square miles or 9.4 billion acres
  
• Only 59% of the moon's surface is visible from earth.
  
• The moon rotates at 10 miles per hour compared to the earth's rotation of 1000 miles per hour.
  
• When a month has two full moons, the second full moon is called a blue moon. Another definition of a blue moon is the third full moon in any season (quarter of year) containing 4 total full moons.
  
• From Earth, we always see the same side of the moon; the other side is always hidden.
  
• The dark spots we see on the moon that create the image of the man in the moon are actually craters filled with basalt, which is a very dense material.
  
• The moon is the only extraterrestrial body that has ever been visited by humans. It is also the only body that has had samples taken from it.
  
• The first space craft to send back pictures from the moon was Luna 3 (built by the Soviet Union) in October 1959.
  
• The moon has no global magnetic field.
  
• The moon's diameter is about 1/4 the diameter of the Earth. About 49 moons would fit inside the Earth.

What is a Super Full Moon?

The distance of the moon from the Earth varies throughout the month and year. The average distance is about 238,000 miles (382,900 kilometers). The moon's position furthest away from Earth is called “apogee” while its closest approach to Earth is referred to as “perigee”. These events do not regularly coincide with the phases of the moon. However, it can happen that the moon is at perigee during the phase of full moon. This event is referred to as Super Full Moon.

A super full moon occurs when the moon’s closest approach to the Earth (lunar perigee) coincides with the phase of full moon. When this happens the moon may seem bigger and brighter. However, for the ordinary star-gazer there will be no significant difference.

10 Need-to-Know Things About Earth's Moon:

1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and the moon would the size of a green pea.
2. The moon is Earth's satellite and orbits the Earth at a distance of about 384 thousand km (239 thousand miles) or 0.00257 AU.

   Twelve human beings have walked on the moon.

3. The moon makes a complete orbit around Earth in 27 Earth days and rotates or spins at that same rate, or in that same amount of time. This causes the moon to keep the same side or face towards Earth during the course of its orbit.
4. The moon is a rocky, solid-surface body, with much of its surface cratered and pitted from impacts.
5. The moon has a very thin and tenuous (weak) atmosphere, called an exosphere.
6. The moon has no moons.
7. The moon has no rings.
8. More than 100 spacecraft been launched to explore the moon. It is the only celestial a body beyond Earth that has been visited by human beings (The Apollo Program).
9. The moon's weak atmosphere and its lack of liquid water cannot support life as we know it.
10. Surface features that create the face known as the "Man in the moon" are impact basins on the moon that are filled with dark basalt rocks.

shailesh kr shukla

directoratace@gmail.com

Standard Units (SI Units)

The International System of Units (abbreviated SI) is the metric system used in science, industry, and medicine.

Brief history of the SI

The creation of the decimal Metric System at the time of the French Revolution and the subsequent deposition of two platinum standards representing the meter and the kilogram, on 22 June 1799, in the Archives de la République in Paris can be seen as the first step in the development of the present International System of Units.

In 1832, Gauss strongly promoted the application of this Metric System, together with the second defined in astronomy, as a coherent system of units for the physical sciences. Gauss was the first to make absolute measurements of the earth’s magnetic force in terms of a decimal system based on the three mechanical units millimeter, gram and second for, respectively, the quantities length, mass and time. In later years, Gauss and Weber extended these measurements to include electrical phenomena

These applications in the field of electricity and magnetism were further developed in the 1860s under the active leadership of Maxwell and Thomson through the British Association for the Advancement of Science (BAAS). They formulated the requirement for a coherent system of units with base units and derived units. In 1874 the BAAS introduced the CGS system, a three-dimensional coherent unit system based on the three mechanical units centimeter, gram and second, using prefixes ranging from micro to mega to express decimal submultiples and multiples. The following development of physics as an experimental science was largely based on this system.

The sizes of the coherent CGS units in the fields of electricity and magnetism, proved to be inconvenient so, in the 1880s, the BAAS and the International Electrical Congress, predecessor of the International Electrotechnical Commission (IEC), approved a mutually coherent set of practical units. Among them were the ohm for electrical resistance, the volt for electromotive force, and the ampere for electric current.

After the establishment of the Meter Convention on May, 20 1875 the CIPM concentrated on the construction of new prototypes taking the meter and kilogram as the base units of length and mass. In 1889 the 1st CGPM sanctioned the international prototypes for the meter and the kilogram. Together with the astronomical second as unit of time, these units constituted a three-dimensional mechanical unit system similar to the CGS system, but with the base units meter, kilogram and second. 
  

• No matter what field of science you enter, you will need to take measurements, understand them, communicate them to others, and be able to repeat them. In other words, we all have to speak the same basic language.
• The ability to obtain accurate measurements and communicate those measurements is a key requirement for progress.
• These are the seven basic units in the SI system: the kilogram (kg) (mass), the second (s) (time), the Kelvin (K) (temperature),  the ampere (A) (electric current), the mole (mol) (amount of a substance), the candela (cd) (luminous intensity), and the meter (m), (distance).

The Seven SI Units

This figure displays the fundamental SI units and the combinations leading to more complex measurement units.

1. 

   SI Units

   SI = Système International d'unités

   1. Quantity, Name, Symbol
   2. SI Prefixes
   3. SI Derived Units
   4. cgs Units

   Quantity, Name, Symbol

   length

   metre (meter): m

   (the correct English spelling of the unit is "metre", but the variant "meter" is frequently used in the United States)

    

   mass

   kilogram: kg

    

   time

   second: s

    

   electric current

   ampere: A

    

   thermodynamic temperature

   kelvin: K

    

   amount of substance

   mole: mol

    

   luminous intensity

   candela: cd

   
   

   SI Derived Units

   (Please note: all units to the right of the slash are actually in the denominator!)

   Frequency

   hertz: Hz = 1/s

    

   Force

   newton: N = m kg/s²

    

   Pressure, stress

   pascal: Pa = N/m² = kg/m s²

    

   Energy, work, quantity of heat

   joule: J = N m = m² kg/s²

    

   Power, radiant flux

   watt: W = J/s = m² kg/s³

    

   Quantity of electricity, electric charge

   coulomb: C = s A

    

   Electric potential

   volt: V = W/A = m² kg/s³ A

    

   Capacitance

   farad: F = C/V = s⁴ A²/m² kg

    

   Electric resistance

   ohm: Omega = V/A = m² kg/s³ A²

    

   Conductance

   siemens: S = A/V = s³ A²/m² kg

    

   Magnetic flux

   weber: Wb = V s = m² kg/s² A

    

   Magnetic flux density, magnetic induction

   tesla: T = Wb/m² = kg/s² A

    

   Inductance

   henry: H = Wb/A = m² kg/s² A²

    

   Luminous flux

   lumen: lm = cd sr

    

   Illuminance

   lux: lx = lm/m² = cd sr/m²

    

   Activity (ionizing radiations)

   becquerel: Bq = 1/s

    

   Absorbed dose

   gray: Gy = J/kg = m²/s²

    

   Dynamic viscosity

   pascal second: Pa s = kg/m s

    

   Moment of force

   metre newton: N m = m² kg/s²

    

   Surface tension

   newton per metre: N/m = kg/s²

    

   Heat flux density, irradiance

   watt per square metre: W/m² = kg/s³

    

   Heat capacity, entropy

   joule per kelvin: J/K = m² kg/s² K

    

   Specific heat capacity, specific entropy

   joule per kilogram kelvin: J/kg K = m²/s² K

    

   Specific energy

   joule per kilogram: J/kg = m²/s²

    

   Thermal conductivity

   watt per metre kelvin: W/m K = m kg/s³ K

    

   Energy density

   joule per cubic metre: J/m³ = kg/m s²

    

   Electric field strength

   volt per metre: V/m = m kg/s³ A

    

   Electric charge density

   coulomb per cubic metre: C/m³ = s A/m³

    

   Electric displacement, electric flux density

   coulomb per square metre: C/m² = s A/m²

    

   Permittivity

   farad per metre: F/m = s⁴ A²/m³ kg

    

   Permeability

   henry per metre: H/m = m kg/s² A²

    

   Molar energy

   joule per mole: J/mol = m² kg/s² mol

    

   Molar entropy, molar heat capacity

   joule per mole kelvin: J/mol K = m² kg/s² K mol

    

   Exposure (ionizing radiations)

   coulomb per kilogram: C/kg = s A/kg

    

   Absorbed dose rate

   gray per second: Gy/s = m²/s³

    The Importance of Standard Units in Everyday Life

   The system of measurements is very important in everyday life . In the past , various systems of measurements were used. For example , length was measured in units like foot , yard , chain and mile . Weight was measured in units like pound , ounce , kati , tahil . 
    Today , many countries in the world use the same units of measurements . We say that they use standard units . The standard units used are the S.I. units .
    Under this system of units , mass is measured in kilograms (kg) and length is measured in metres (m).
    The use of standard units makes it easier for people from different countries to communicate with each other . Furthermore , the use of a standard units means a measurement in that unit has the same value anywhere in the world .

   
   

   Shailesh kr shukla

   directoratace@gmail.com

   
   

   
   

    

Rhizosphere: environment and mycorrhizal fungi

Rhizosphere

Rhizoplane The rhizosphere is a microecological zone in direct proximity of plant roots. It is functionally defined as the particulate matter and microorganisms that cling to roots after being gently shaken in water. The theoretical extent of the rhizosphere is dependent on the zone of influence of the plant roots and associated microorganisms. The rhizosphere is a metabolically busier, faster moving, more competitive environment than the surrounding soil.

The rhizoplane is the root epidermis and outer cortex where soil particles, bacteria and fungal hyphae adhere (Singer, 2006; Sylvia, 2005). The functional definition is the remaining microorganisms and soil particles after the roots have been shaken vigorously in water. There are more microbes in the rhizoplane than in the more loosely assoicated rhizosphere. This is determined by counting the number of colony forming units (CFUs) which are determined by spreading extracted soil microorganisms across an agar and counting the number of independent clusters of microorganisms. Microbes are most abundant where the integrety of the root is compromised. For this reason, rhizoplane microorganisms tend to be found on older rather than younger roots. Bacteria and fungi that live within the cells of the root are not considered a part of the rhizoplane, but instead called endophytes (Sylvia, 2005).

Physical Environment

Water Potential

The plant roots which the rhizosphere is associated with can effect the physical environment of the rhizosphere. As plants transpire water with more force during the day than during the night, they change the soil water potential immediately near their roots and so the rhizosphere undergoes fluctuations that the bulk soil avoids.

Soil Texture

Movement of organic matter away from the root as well as bacteria colonizing new locations occurs more readily in sandy soils than clayey soils. Sand has larger pores between each granule allowing microorganisms and exudates can travel. Therefore, the larger the granule size, the further the rhizosphere and microorganisms associated with it will extend into the surrounding soil. (Sylvia, 2005)

Plant roots compact the soil on the short term as they grow, but once they die and decay, can actually leave soil more porous.

pH

Several factors can lower the pH in the rhizosphere. Respiration leads to carbon dioxide (and eventually to bicarbonate/carbonic acid) generation. In addition to respiration of the roots themselves, the rhizosphere is very rich in carbon results in other organisms from prokaryotes to fungi to small animals living and respiring in the rhizosphere more than in the bulk soil.

The availible habitat that microbes are limited in part by pH of the soil. Fungi are found in more acidic soils than alkaline, and bacteria have a very broad pH spectrum where they can survive. The influincing effects of pH in the rhizosphere is critical in supporting a biologically diverse microbial community.

Plant-Derived Compounds

Plant-derived compounds are responsible for providing the additional carbon that allows the rhizosphere to host a large variety of organisms. These compounds fall into five categories: exudates, secretions, mucilages, mucigel, and lysates.

Exudates include surplus sugars, amino acids, and aeromatics that diffuse out of cells to the intercellular space and surrounding soil. Due to their diffusive nature, exudates are limited to compounds of low molecular weights. Secretions are byproducts of metabolic activity. Because they are actively released from the cell, secretions can be of both low and high molecular weight. When an epidermal root cell dies and is broken open, lysates from within the cell become available to the surrounding microbial community.

Mucilages are cells sloughed off the root cap as the root grows. Abrasive forces of the root against soil particulate matter is responsible for the removal of cells. These cells consist of cellulose, pectin, starch, and lignin. Mucigel is a slime coating the surface of a root that increases the connectivity between plant roots and the surrounding soil. It is more common on the main body of the root and root hairs than the tip. During dry spells, mucigels are responsible for allowing plants to continue to uptake water and nutrients. (Sylvia, 2005)

Microbial Communities

Rhizosphere Bacteria source

Bacteria

Bacteria are the most numerous organisms in the soil, averaging between 10^6 to 10^9 organisms per gram of rhizosphere soil. Due to their small mass, they only account for a small amount of the biomass of soil. Nonsporulating rods, pseudomona, and acetinomycetes are the most common bacteria in the soil. (Sylvia, 2005)

Fungi

Both pathogenic and symbiotic fungi associate with the rhizosphere. They average between 10^5 and 10^6 organisms per gram of rhizosphere soil. Zygomycetesand hyphomycetes establish the most readily in the rhizosphere because they metabolize simple sugars. (Sylvia, 2005)

In times of flooding, when too much moisture accumulates around the base of the tree, a bacterial pathogen Armillaria may infect and cause root rot.

Biotic Interactions in the Rhizosphere

Up to 15% of the root surface area is covered with rhizosphere-specific microorganisms - providing many sites for biological interactions. A range of interactions are present in the rhizosphere: from beneficial symbiotic relationships to detrimental pathogenic interactions. (Sylvia, 2005)

General Impacts on Plants of Rhizosphere Microorganisms

Microorganisms in the rhizosphere complete both chemical and physical modifications to the soil profile in and around the rhizosphere that affect plants. They can be beneficial to the plant (by pathogen suppression) or detrimental (by competition for nutrients).

Chemical changes occur as a result of humification of organic matter. The resultant mineralization of various organic compounds (phosphorous, sulfur, and nitrogen, for example) provides plants with forms of nutrition that are readily available for uptake. The turnover of microbial populations also results in the release of nutrients. The fixation of atmospheric dinitrogen by both asymbiotic and symbiotic bacteria results in increases to the available nitrogen pool that can be accessed by plants in and near the rhizosphere. Symbiotic mycorrhizae cause an increase in the effective rooting area of plants, thereby providing added nutrient mining capabilities to the plant. Rhizosphere microbes can also release plant growth regulators.

Physical changes occur primarily through the production of extracellular polymeric substances such as polysaccharides and glomalin, which improve soil aggregation and soil texture. The presence of mucigel in the rhizoplane is crucial to the water relations of plants, providing a bridge that prevents dessication by maintaining the water column during water stress events. (Sylvia, 2005)

General Impacts on Rhizosphere Microorganisms of Plants

In addition to soil properties and the types of microorganisms present in a soil, plants play an important role in the community diversity of rhizosphere microorganisms. Plant roots cause chemical and physical changes to the soil they inhabit and these changes will affect the microbial diversity in and around the rhizosphere. Root exudates will select for/against certain populations of microorganisms. Many plants exhibit genetic resistance/tolerance to rhizosphere microorganisms; the variety of plant will determine, in part, the community makeup of the microorganisms in the rhizospere. The ability of a plant to form symbiotic relationships with soil microbes will also determine rhizosphere microbial populations. The age and health of the plants present will also play a role in the microbial community dynamics of the rhizosphere. Plant roots increase the tilth of a soil and subsequently affect the physical properties of the soil.

Plants can also compete with rhizosphere microorganisms for resources like water and nutrients. (Sylvia, 2005)

Symbiotic or Mutualistic Relationships

In a relationship, the plant usually provides a source of carbon and the bacteria or fungus fills some other more specialized function. In many cases the association is not absolutely necessary for the survival of both members, but provides significant benefit.

The recently sequenced genome of Laccaria Bicolor (an ectomycorrhizal fungus) shows a excess of enzymes used in ammonia uptake, and a lack of enzymes that would be needed to degrade plant wall material. Thus, it is assumed the fungus is providing nitrogen (and perhaps other nutrients), and a guarantee not to attack to the tree in return for glucose. (Martin, 2008). As described below, fungi are also often an important source of phosphorous.

The microbial partner can also help the plant to survive in an otherwise inhospitable environment. For example, specific endophytes have been shown to confer heat resistance to grasses that grow in a hydrothermal area, or salt tolerance to costal grasses. (Rodreguez, 2008)

Mycorrhizal Fungi

Over 80 percent of all land plants have a mutualistic relationship with one or more mycorrhizal fungi. Mycorrhizal fungi extend the effective root length by 100 fold or more. It would therefore seem appropiate that mycorrhiza means fungus-root in Greek. Not only do they increase the surface area over which nutrients and water can be taken up by the plant, but fungal hyphae can reach into smaller pores than roots can. The most valuable asset mycorrhizae provide is accessing immobile nutrients, such as phospherous. In return, the plant passes a significant amount of its carbon reserves to the fungi. (Sylvia, 2005)

Ectomycorrhizae

Ectomycorrhizae Reference to Source of Image

Ectomycorrhizae interact with the plant root by producing a net-like structure called a Hartig net that weaves between the root cortical cells. A sheath or mantle of fungal tissue covering part or all of the root is responsible for most of the increase in surface area caused by mycorrhiaze. Some fungi, such as Boletusbetulicola, have only a narrow range of plants they can associate with. Other fungi have a much larger range of host plants. One such fungus, Pisolithus tinctorius, associates with 46 tree species and eight genera. (Sylvia, 2005)

Arbuscular Mycorrhizae

Image of Arbuscular MycorrhizaeReference to Source of Image

Arbuscular mycorrhiza is also referred to as endomycorrhiza because it has a branched arbuscule that grows within the root cortical cell. Direct connection between the plant and fungal cytoplasm allows the transfer of nutrients from the fungi to the plant and carbon from the plant to the fungi to be more efficient. Examples of mycorrhizal fungi include Glomus tenue and Scutellospora. (Sylvia, 2005)

Nitrogen Fixing Bacteria

Nodes Source

Due to the high energetic cost of fixing dinitrogen, a significant part of nitrogen fixation occurs near the plant roots, where there is an influx of sugar to power the process. Some nitrogen fixation occurs in the rizosphere by free-living bacteria (the ability is found only in prokaryotes). When these otherwise free living bacteria form a close association with a plant they’re called “associative symbiotes”. An example of a free living nitrogen fixer is Bradyrhizobium.

There are several drawbacks to this lifestyle: 1. Nitrogen fixing bacteria are sensitive to fluxuations in oxygen. Many are aerobes who use oxygen as a terminal electron acceptor, but on the other hand, oxygen can poison the nitrogenase enzyme. Hence free-living bacteria need to take energetically costly steps such as having extremely fast respiration or producing large amounts slime to protect nitrogenase. Another disadvantage Associative symbiotes have do deal with is that they are in direct competition with the rest of the rizosphere for the available carbon. Experiments indicate that carbon is a limiting factor in rhizosphere nitrogen fixation.

An alternate form of nitrogen-fixing lifestyle is to actually live in the plant root cells themselves. Bacteria who do this are called ”symbiotes” or “mutualistic symbiotes”, and an example of one such bacteria is Rhizobia. These bacteria form “nodules” on plants, which are huge masses of bacteria living inside plant cells which have been modified for the purpose. The process of forming a nodule requires recognition between bacteria and plant (usually very specific), and gene expression changes in both players. The plant provides carbon to the bacteria and the bacteria provide fixed nitrogen to the plant. The masses of bacteria produce leghemoglobin to protect nitrogenase from oxygen, and do not have to compete with other rhizosphere microbes for carbon (Sylvia, 2005).

Inoculants

Inoculation of fields with the soil from other fields has been practiced for centuries as a method of inoculation. Modern seed companies are dipping seeds in beneficial inoculants before sale. However, the specific conditions surrounding the seed at germination and the specific soil characteristics determining the ability of the soil to support a population of desirable rhizosphere microbes will be the ultimate determinants in the successful colonization of the desired inoculant. Carriers of inoculants are also being explored, such as peat and other potting media. The primary organisms being used for these products are mycorrhizal fungi, dinitrogen-fixing bacteria, and beneficial rhizobacteria. Some work has explored the possibility of using biological control organisms as inoculants on seeds or seedlings. (Sylvia, 2005)

A partial list rhizosphere microorganisms that have been used as inoculatns:

Agrobacterium tumefaciens

Alcaligenes spp.

Bacillus subtilis

shailesh kr shukla

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