Magnesium Usage By turfgrasses By Richard J. Hull, University of Rhode Island
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Magnesium Usage
By turfgrasses
By Richard J. Hull, University of Rhode Island
urf is maintained for its uniform pre- magnesium requiring ions. In addition, the
T sentation of green relaxing hues.
Whether intended for athletic compe-
tition, recreation or aesthetics, the rich
genetic material, which chlorophyll synthe-
sizing enzymes depend on for expression, is
held together by magnesium. In short, mag-
green color of turfgrasses is a prominent fea- nesium is an essential macronutrient upon
ture of its appeal and impact on the land- which not only chlorophyll synthesis but all
scape. The green color of grass leaves is plant life depends.
imparted by the presence in them of a fat- The essential roles played by magnesium
soluble pigment, chlorophyll. This green in plant growth and function are well rec-
pigment brings us to the subject of the role ognized. Nevertheless, little research has
of magnesium in turf management. Each been reported on its importance for turf-
chlorophyll molecule is constructed around grass culture or on how best to make it
a magnesium atom. Also, several steps in the available to turf. In their review of turfgrass
biosynthesis of chlorophyll are catalyzed by nutrition, Turner and Hummel (1992) cite
Figure 1. Structure of chlorophyll a showing the bonds formed between four nitrogens covalently binding with a
single magnesium atom.only seven reports on magnesium in turf- would be disastrous to metabolic function.
grass culture since the middle 1960s. Even This cytosolic magnesium concentration is
the professional turf journals rarely con- stabilized by a large pool of reserve Mg+2
sider the requirements for maintained within the vacuole where it can
Magnesium and and management of magne- reach concentrations of over 100 uM. Con-
sium in turf. One exception sidering that the vacuole can constitute
calcium are chemically to this trend is an article in 85% of the volume of a mature cell, this
similar. Both bind to Golf Course Management represents an enormous reserve supply of
by Rodney Garrett (1996) magnesium for the cell. This probably
cation exchange sites on which described the contri- explains why magnesium deficiencies do
soil colloids and are butions made by magne- not occur immediately following the with-
sium toward solving some drawal of available Mg+2 from the roots.
retained within the soil long-standing problems on This also indicates that Mg+2 plays an
against leaching when two golf courses. He men- important role in balancing the ionic
tions several important con- charge between vacuole and cytosol and in
rain or irrigation water siderations in managing maintaining favorable water relations
percolates through the magnesium on turf, but not between the root and soil.
in any depth. I will try to Magnesium shares with calcium the
soil profile. provide an overview of mag- cation exchange sites within the cell walls
nesium's essential functions provided by pectate chains (Hull 1997b).
and discuss its contributions in the context Again because of its stronger ionic bonding,
of turf management for TurfGrass Ca+2 dominates the exchange sites within
TRENDS readers. cell walls but Mg+2 is present to a signifi-
cant extent (5-10 percent of the total cell
magnesium). This also helps maintain a
Magnesium in turfgrasses ready supply of magnesium for use within
The magnesium content of turfgrass cells when needed.
leaves, averaged over seven species, was
reported to be 2.1 g/kg (ppm) by Wadding-
ton and Zimmerman (1972). This is lower Availability in the Soil
than the concentrations of calcium and Environment
phosphorus in turfgrasses by roughly com- Magnesium and calcium are chemically
parable to their content of sulfur. Suffi- similar elements. In the soil solution, both
ciency levels of magnesium generally aver- elements are present as divalent cations
age somewhat greater than 2 g/kg, which (Mg+2 and Ca+2). That means they bind to
suggests that under normal nutritional cation exchange sites on soil colloids and
management, turfgrasses may be chronical- are thereby retained within the soil against
ly deficient in magnesium. This could be a leaching when rain or irrigation water per-
serious matter as we shall discover later. colates through the soil profile. The bind-
Unlike calcium, magnesium is present ing strength of cations to exchange sites
in plant tissues in a metabolic pool of depends upon the charge strength of the
between 2-10 mM (Leigh and Wyn Jones cation and its degree of hydration. Calcium
1986) while that of free Ca+2 is closer to and magnesium have essentially the same
0.1-0.2 uM (Hull 1997b) or 10,000 times charge (+2) but magnesium is more highly
less than Mg+2. This is because Mg+2 is inti- hydrated than calcium: diameter of the
mately involved in many metabolic hydrated ion is 0.64 nm for magnesium
processes and its concentration within the and 0.56 nm for calcium (Mengel and
cytoplasm must be maintained within a Kirkby 1982). The non-hydrated calcium is
fairly tight range. The presence of Ca+2 in slightly larger than the non-hydrated mag-
the cytosol would effectively compete with nesium (0.21 vs. 0.16 nm diameter respec-
Mg+2 for enzyme binding sites and this tively). This larger amount of water sur-rounding the magnesium ion tends to insu- potassium (K+), ammonium (NH4+), Ca+2
late its charge making it less effective in and low pH (H+). Consequently, magne-
binding to cation exchange sites. Conse- sium deficiency induced by competing
quently, Mg+2 tends to be displaced from cations is a fairly common occurrence
exchange sites by Ca+2 when the two ions (Marschner 1995).
are present in equivalent amounts. In turf management, there is a long tra-
If pH adjustment is achieved by using dition of applying adequate potassium to
ground limestone (calcium carbonate = improve the texture of grass blades and
CaC0 3 ), Mg+2 will be displaced from increase disease resistance. Because most
exchange sites and lost during clipping turfgrasses grow best at a soil pH range of
removal if the Mg+2 is absorbed by the turf 6.5-7.0, liming materials rich in calcium are
or leached from the root zone if it is not often applied in the more humid regions. In
absorbed by roots. In any event, the amount areas where rainfall is limited, the soil pH is
of Mg+2 available to turf may become com- often near neutral because the soils are nat-
promised if large quantities of calcium are urally enriched with calcium and potassi-
applied with little regard for the magne- um. In either case, the soil's ionic environ-
sium supply. ment may not favor the uptake of adequate
A favorable magnesium status in turf is magnesium by turf even when its presence
even more at risk because of the unfavor- in the soil is reasonably high. This is an
able competition between Mg+2 absorption important consideration in turf manage-
by grass roots and other commonly applied ment which will be discussed later.
cations. The rate of Mg+2 uptake can be
strongly depressed by soil cations such as
Figure 2. Chemical structure of Mg-Adenosine Triphosphate (Mg-ATP) binding via a
magnesium ion to an enzyme protein in the process of transferring the terminal phos-
phate of ATP to a metabolite.| T U R F G R A S S N U T R I T I O N
Magnesium Functions Protein synthesis. One of the most devas-
tating effects of low magnesium on plant
Magnesium performs at least four major growth is its inhibition of protein synthesis.
functions in plant metabolism which will Inability to synthesize new proteins
be considered briefly. (enzymes) limits the plant's capacity to
Component of chlorophyll The chloro- respond to environmental stresses such as
phyll molecule is coordinated around a drought, heat and attack by disease organ-
magnesium atom to which it is covalently isms. While a connection between low mag-
(nonionically) bound (Figure 1). Depend- nesium and increased disease or stress
ing on the nutritional level, between 6 and injury has not been reported on turfgrasses,
25 percent of plant magnesium is bound to I am not aware that anyone has conductd
chlorophyll (Marshner research in this area.
When growth of 1995). Because the incorpo- Two essential processes in protein syn-
Mg-limited plants is ration of Mg+2 into Mg-pro- thesis depend on the availability of Mg+2.
toporphyrin IX (the step in The nuclear enzyme RNA polymerase,
stimulated by high which Mg+2 is introduced which catalyses the synthesis of RNA from
nitrogen or favorable into chlorophyll synthesis DNA that comprises the genes on which all
and is catalyzed by magne- plant genetic information is encoded,
conditions, a senescence sium chelatase) has a strong requires free Mg+2. Upon withholding mag-
stimulus is received by affinity for free Mg+2; it nesium from a culture of plant cells, the
enjoys a high priority in the synthesis of RNA immediately stops but
the oldest leaves. use of this nutrient (Held resumes as soon as magnesium is restored.
1997). Consequently, other This is one of the most rapid plant respons-
metabolic functions which depend on Mg+2 es to magnesium deficiency and may initi-
may suffer while chlorophyll synthesis uti- ate many of the subsequent symptoms that
lizes available magnesium. When the are observed.
amount of leaf magnesium contained in Ribosomes are the subcellular structures
chlorophyll exceeds 20 percent of the total, on which proteins are synthesized follow-
plant growth is normally being depressed ing the amino acid sequence encoded by
and the plant probably is or soon will be the mRNA synthesized in the nucleus.
exhibiting magnesium deficiency symp- These cytoplasmic particles consist of RNA
toms (Marschner 1995). and proteins but require magnesium to sta-
When growth of Mg-limited plants is bilize their structure. Ribosomes bind with
being stimulated by high nitrogen or other mRNA and read its code while directing
favorable conditions, a senescence stimulus the assembly of polypeptides (proteins)
is received by the oldest leaves. This pro- using activated amino acids which are
motes two enzymes which catalyze the bound to specific tRNA molecules and are
breakdown of chlorophyll, in particular a free within the cytoplasm.
Mg-dechelatase which liberates magnesium If magnesium is withdrawn from a
from the porphyrin rings of chlorophyll. preparation of ribosomes, they disassemble
The Mg+2 freed from chlorophyll can be into smaller units which are incapable of
translocated to shoot growing points where protein synthesis.
it will be utilized to synthesize chlorophyll Only when ribosome structure is stabi-
in new leaves. lized via Mg+2, which bridges negative
This accelerated senescence of mature charges on the ribosome subunits, does pro-
leaves by low magnesium not only pro- teins synthesis occur. Thus Mg+2 is required
duces the classical symptoms of magnesium both for the synthesis of messenger RNA
deficiency (interveinal chlorosis) of the (mRNA) transcribed from genetic DNA in
lower leaves but also reduces the photosyn- the nucleus and for stabilizing the protein
thetically active leaf surface thereby limit- synthesizing units (ribosomes) on which
ing plant growth. the genetic code is translated into proteins.Phosphorylation. In plant and animal Sometimes, however, an enzyme can be
metabolism, one of magnesium's most crit- activated by ionically binding directly with
ical functions is its capacity to bind ATP a Mg+2. A good example of Mg+2 activation
(adenosine triphosphate) to the enzyme of an enzyme is RubisCo (ribulose bisphos-
proteins which catalyze the formation of phate carboxylase/oxygenase).This enzyme
phosthate esters (Figure 2). These phos- catalyzes the primary reaction whereby
phorylated metabolites are much more atmospheric CO z is photosynthetically
reactive and will enter into chemical reac- fixed into an organic molecule. Pretty much
tions much more readily than their non- all carbon enters the biosphere via the
phosphorylated form (Hull 1997a). The action of this one enzyme.
phosphorylation of enzyme proteins also is However, before this enzyme can func-
now recognized as an important way by tion to assimilate C0 2 , it must be activated
which the activity of such enzymes can be by binding with a Mg+2. This Mg-RubisCo
regulated; either activated or inhibited. In complex requires a couple additional acti-
most cases, the source of phosphorus uti- vation steps but only when fully activated
lized in forming phosphate esters is ATP, can it catalyze the fixation of C 0 2 .
more specifically Mg-ATP where a Mg+2 is The binding of a Mg+2 to the enzyme
ionically bound to the ATP's terminal two increases its affinity (ability to bind) for
phosphates. It is through this Mg-bridge CO z and also increases the rate of its cat-
that Mg-ATP binds with the enzyme pro- alytic action. Because the action of this
tein before a phosphate ester is formed (Fig- enzyme is so important to all of biology, the
ure 2). Thus, wherever a phosphorylated involvement of magnesium in its activation
metabolite enters a biochemical reaction, its stands out as an important chemical role of
synthesis probably required the binding of this nutrient. There are other enzymes,
Mg+2 with an ATP molecule. some even involved in photosynthesis, that
Enzyme activation. As mentioned above, also require activation by Mg+2 binding
Mg-ATP can sometimes activate an enzyme before they can function at high efficiency.
by binding a phosphate from the ATP Root:shoot partitioning. For the turf man-
directly to the enzyme protein through the ager, one of magnesium's most important
action of a separate protein kinase enzyme. functions is its involvement in the transport
Figure 3. Magnesium uptake by ryegrass as affected by soil pH and potassium concen-
tration (Based on Marschner 1995).of soluble carbohydrates from leaves to sites Magnesium and turfgrass
management
of utilization in roots, rhizomes and grow-
ing points. One common observation asso-
ciated with a magnesium deficiency is a In an earlier article, I argued that pro-
sharp decline in the root: shoot ratio (weight moting root growth was among the most
of roots/weight of shoots). important tasks confronting the turf man-
When magnesium is inadequate, the ager (Hull 1996). A strong root system will
root: shoot ratio will often decline to less make a turfgrass stand better able to mine
than half that of a magne- the soil for water and mineral nutrients, sta-
When magnesium is in sium sufficient plant. bilize the plants against wear, tolerate insect
This means that when turf- feeding and be more resistant to disease.
short supply; carbon grasses are experiencing Thus any condition that does not favor root
fixed through inadequate magnesium, growth is not working with the turf manag-
they will produce less than er. Inadequate magnesium may be just such
photosynthesis half the root mass and a condition.
accumulates in the depth that normally would The likelihood of low magnesium hav-
be present. ing a depressing effect on turf root growth
chloroplasts as starch It is not known at exactly can be increased by turf management prac-
and in the celb as what step in the transport tices. If soil pH adjustment is achieved
of photosynthetic products through the application of lime, the
sucrose, which in turn out of leaves that magne- increased Ca+2 in the soil solution along
can exert feed-back sium is required, but the with K+ displaced from cation exchange
H+ pumping ATPase sites by Ca+2 may inhibit root absorption of
inhibition on further responsible for phloem Mg+2. Magnesium uptake might be even
photosynthesis. loading is a likely candidate further reduced if additional potassium fer-
(Marshner 1995). tilizer is applied (Figure 3) and an ammoni-
This enzyme—which cre- um source of nitrogen is used. In acid soils,
ates the transmembrane pH gradient that the high H+ concentration can also displace
powers sucrose loading into the phloem of Mg+2 from absorption sites on root cells and
leaves—is known to require the presence of thereby retard magnesium uptake. Thus,
free Mg+2 (~2 mM). nutrient management of turf can have a
Since the utilization of ATP is involved strong impact on magnesium availability
in this process, it is not surprising that Mg+2 and content within the turfgrass plants.
would be an essential component. Clipping removal will gradually deplete
The impact of a magnesium deficiency the magnesium content of the turf-soil
on photosynthate transport is important to ecosystem. The frequent use of irrigation
the turf manager because it profoundly will also increase Mg+2 leaching from the
influences the ability of the turf to obtain soil or sand growth medium. This latter
water and nutrients and resist injury. effect will be promoted if sulfate, nitrate or
When magnesium is in short supply, car- chloride salts are used to supply other nutri-
bon fixed through photosynthesis accumu- ents. Because salts of magnesium produced
lates in the chloroplasts as starch and in the with these anions are highly soluble they
cells as sucrose which in turn can exert tend to promote Mg+2 leaching.
feed-back inhibition on further photosyn- Thus, even though there is little research
thesis. available to confirm these problems of low
These events are also associated with a magnesium on turf growth and perfor-
much reduced sugar content of roots indi- mance, it is prudent for the turf manager to
cating that photosynthate transport was take them seriously. Regular applications of
most inhibited by low magnesium magnesium along with periodic leaf tissue
(Marschner 1995). analyses will help the manager keep on top
of a turf's magnesium status. Such diligencemight very well offset problems of turf erals and organic matter contain numerous
decline and help turf to tolerate environ- cation exchange sites. In plant tissues, pec-
mental stresses including disease infections. tice substances in cell walls, proteins and
nucleic acids all contain exchange sites.
Dr. Richard J. Hull is professor of Plant Sci-
ence and Chairman of the Plant Sciences Covalent bond - A chemical bond in which
Department at the University of Rhode Island. two atoms share one or more pairs of elec-
His research has concentrated on nutrient use trons. Sometimes referred to as an organic
efficiency and photosynthate partitioning in turf. bond, it can occur between an organic mol-
ecule and a mineral element. Magnesium
References
bonding within a chlorophyll molecule is
Garrett, R. 1996. Elementary: Turfgrass manage- covalent. Should not be confused with ionic
ment case studies. Golf Course Management or hydrogen bonding.
64(8):81, 84 & 86.
Held, H-W. 1997. Plant Biochemistry and Molecular
Cytoplasm - That portion of a cell proto-
Biology. Oxford University Press, Oxford.
plast exclusive of the vacuole. The part of
the cell in which most metabolic reactions
Hull, R.J. 1996. Managing turf for maximum root take place. The fluid matrix in which all
growth. TurfGrass Trends 5(2): 1 -9. cytoplasmic inclusions (chloroplasts, mito-
Hull, R.J. 1997a. Phosphorus usage by turfgrasses: chondria, ribosomes, etc.) are suspended is
The energy nutrient often neglected by turf man- called the cytosol.
agers. TurfGrass Trends 6(5): 1-12.
Enzyme - A protein catalyst that alters the
Hull, R.J. 1997b. Calcium usage by turfgrasses: The rate of a chemical reaction without being
nutrient forgotten by turf managers. TurfGrass consumed in the process. An enzyme nor-
Trends 6(10):6-13. mally speeds a biochemical reaction rate
Leigh, R.A. and R.G. Wyn Jones. 1986. Cellular but its effectiveness depends upon its
compartmentation in plant nutrition: the selective degree of activation.
cytoplasm and the promiscuous vacuole, pages
249-279. IN B. Tinker and A. Lauchli (eds.). Metabolite - A biochemical that is part of a
Advances in Plant Nutrition 2. Praeger Scientific, metabolic pathway or the end-product of
New York. such a pathway if it is subject to further
reactions. Starch, lipids or proteins are not
Marschner, H. 1995. Mineral Nutrition of Higher
metabolites, but glucose and citric acid are.
Plants, 2nd Edition. Academic Press, London.
Mengel, K. and E.A. Kirkby. 1982. Principles of Phloem - Vascular tissue within leaves,
Plant Nutrition. International Potash Inst., Bern, stems and roots, which is responsible for the
Switzerland. long-distance translocation of sugars, amino
acids, and mineral nutrients from photosyn-
Turner, T.R. and N.W. Hummel, Jr. 1992. Nutritional
thetically active leaves to metabolically
requirements and fertilizers, pages 385-439. IN D.V.
active sinks. Phloem consists of sieve tubes
Waddington, R.N. Carrow and R.C. Shearman (eds.).
through which transport occurs and com-
Turfgrass. Agronomy Monograph No. 32, ASA, CSSA
panion and parenchyma cells which sup-
and SSSA, Madison, Wl.
port their function.
Waddington, D.V. and T.L. Zimmerman. 1972. Growth
and chemical composition of eight grasses grown Phosphorylation - The chemical reaction by
under high water table conditions. Communications which a phosphate is transferred from ATP
in Soil & Plant Analysis 3(4):329-337. (or similar compound) to another organic acid
or aldehyde forming a phosphate ester or
anhydride.
TERMS TO KNOW
Cation Exchange Capacity - Fixed negative
charges that are capable of ionically binding
positively charged cations. In soils, clay min-You can also read
