Download Functions of manganese (Mn)

Functions of manganese (Mn)
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Involved in the production of amino acids and proteins.
An activator of several enzymes.
Plays an essential role in respiration and N metabolism.
Necessary for the reduction of nitrates and helps make them usable by plants.
Plays a role in photosynthesis and in the formation of chlorophyll.
Manganese (Mn)
Manganese deficiency occurs commonly in Florida and it is also known in many other
areas of the world. It is particularly evident in the spring after a cold winter. There has
been a delay in the recognition of Mn deficiency symptoms due to masking by severe Zn
or Fe deficiencies. Sometimes the deficiency can be confused with symptoms of Fe and
Zn deficiency or B toxicity. Manganese deficiency leads to a chlorosis in the interveinal
tissue of leaves, but the veins remain dark green. Young leaves commonly show a fine
pattern or network of green veins on a lighter green background but the pattern is not so
distinct as with Zn or Fe deficiencies because the leaf is greener. By the time the leaves
reach full size, the pattern becomes more distinct as a band of green along the midrib and
principal lateral veins, with light green areas between the veins.
In more severe cases, the color of the leaf becomes dull-green or yellowish-green along
the midrib and main lateral veins, and pale and dull in the interveinal areas. Whitish
opaque spots may develop in the interveins, which give the leaf a whitish or gray
appearance. The leaves are not reduced in size or changed in shape by Mn deficiency, but
affected leaves prematurely fall from the tree. No particular twig symptoms are related to
Mn deficiency. Growth is reduced in an acutely affected tree, giving it a weak
appearance.
Manganese deficiency may greatly reduce crop volume and the fruit color. The fruit may
become smaller and softer than normal and the rind pale in color. Manganese deficiency
is frequently associated with Zn deficiency. A combination of the two deficiency
symptoms on leaves is called "marl frenching" or "marl chlorosis," and is characterized
by dark green veins with dull whitish green areas between the veins. In such
combinations, the Mn deficiency is acute and the Zn deficiency is relatively mild.
In Florida, Mn deficiency occurs on both acidic and alkaline soils. It is probably due to
leaching in the acid soils and to insolubility in the alkaline soils. It can be associated with
deficiencies of Zn, Fe, and Cu on both acid and alkaline soil and with Mg deficiency on
acidic sandy soils. For deficient trees on alkaline soils, foliar Mn sprays are
recommended. On acid soils, Mn can be included in fertilizer applied to the soil. Foliar
spray application quickly clears up the deficiency pattern on young leaves, but older
leaves respond less rapidly and less completely. When Mn is sprayed on Mn-deficient
orange trees, fruit yield, total soluble solids in the juice and pounds solids per box of fruit
increase. A foliar spray of a solution containing 2 to 3 lbs per acre of elemental Mn
applied to two-third to fully expanded spring or summer flush leaves is recommended. If
N is needed, adding 7 to 10 lbs per acre of low biuret urea will increase foliar Mn uptake.
Manganese (Mn) reactions in soil
Manganese is abundant in Oregon soils; however, most of it is unavailable for plant uptake.
Plants can absorb Mn only
when it occurs in solution as a divalent cation (Mn2+). Mn2+ is referred to as the reduced
form.
Mn also occurs in oxidized forms (Mn3+ and Mn4+), both of which are unavailable for plant
uptake.
Soil microorganisms oxidize plant-available Mn2+ to Mn3+, which makes it unavailable to
plants. This biological reaction
occurs slowly when soil pH is between 5 and 6.5. However, it proceeds more rapidly as pH
increases up to 7.5 (Russell,
1988). Thus, the form of Mn in a soil system depends largely on the functioning of soil
microorganisms, and their activity
depends on soil pH.
Conversely, Mn can be reduced (from Mn3+ to Mn2+), making it available to plants, either
chemically or by other soil
microorganisms that function more efficiently at low pH (Russell, 1988). Increased Mn
reduction to Mn2+ also can result
from the action of materials secreted by plant roots. These root exudates are solutes that aid
in nutrient acquisition, increase
root tolerance to high concentrations of aluminum, and/or act as a lubricant as roots grow
through soil. The organic acids
contained in root exudates, particularly malic acid, increase the solubility of Mn in the soil,
making it more available to
plants. Attachment of Mn2+ to organic compounds in root exudates (chelation) prevents the
Mn2+ from reoxidizing to the
http://extension.oregonstate.edu/catalog/html/em/em8905-e/ (3 of 10)5/27/2008 2:52:30 AM
Managing Manganese Deficiency in Nursery Production of Red Maple, EM 8905-E
unavailable form.
Mn is relatively immobile in soil. Therefore, Mn applied to the soil surface will remain at the
surface. To alter soil Mn
levels and prevent Mn deficiency, Mn must be distributed throughout the root zone so that all
roots can intercept and
absorb it.
Mn movement in plants
Plants have a vascular system for moving water, metabolites, and solutes from one part to
another.
The plant vascular system consists of two components, xylem and phloem. Xylem transports
water and dissolved nutrients
from roots upward to shoots, with virtually no downward movement. Phloem transports
water, metabolites, and solutes in
all directions throughout the plant.
Mn is absorbed by roots and moved upward to the leaves through the xylem; however, Mn
cannot be transported through
the phloem.
Therefore, Mn accumulated in leaves cannot be remobilized in any significant quantity
(Graham et al., 1988).
Similarly, Mn absorbed by foliage directly via foliar sprays will not move out of the foliage
and back into stem or root
tissue. While foliage can be made to look more green and healthy with supplemental foliarapplied Mn, root systems on
those plants will still be deficient. Likewise, Mn absorbed by one root cannot be redistributed
to another part of the root
system (Nable and Loneragan, 1984).
Mn function in plants
Mn plays four major roles in plant growth and development. It is involved in the plant's
ability to capture light energy for
use in photosynthesis. In nitrogen (N) metabolism, it plays a role in the conversion of nitrate
to ammonium, probably
through interaction with an enzyme known as nitrate reductase (Marschner, 1997).
Critical to production of nursery crops is the role of Mn as a precursor to the plant hormone
auxin. Mn activates the auxin
oxidase system (Russell, 1988). Mn deficiency reduces auxin levels and causes hormone
imbalance. A decrease in the ratio
of auxin to other plant hormones causes reduced lateral root development and root extension
(Landis, 1998).
Finally, Mn plays a vital role in carbohydrate production. Carbohydrates are molecules
containing carbon, hydrogen, and
oxygen that are used by plants for energy storage. An abundant supply of carbohydrates is
produced in leaves through
photosynthesis. These carbohydrates can be used locally to fuel cellular processes within the
leaf, or they can be shuttled to
other parts of the plant to be used as an energy source. During the winter when plants are
dormant, carbohydrates are stored
in stem and root tissue.
Stored carbohydrates in the root system are important for root regeneration the following
year. Mn deficiency reduces the
plant's ability to produce carbohydrates, thus reducing a harvested plant's ability to regenerate
roots and grow vigorously
when replanted the following year.