This post is reprinted from the IPNI publication "
Better Crops". It was originally published in 2010. Due to the interest and questions I receive about magnesium nutrition in plants, I am reproducing it here. The original pdf version of the article can be downloaded
here.
By Ismail Cakmak and Atilla M. Yazici
Dr. Cakmak (cakmak@sabanciuniv.edu) and Dr. Yazici are with the Faculty
of Engineering and Natural Sciences at Sabanci University. Istanbul, Turkey.
 |
| Magnesium deficiency symptoms on bean leaves |
Magnesium
nutrition of plants is frequently overlooked and shortages will adversely
impact plant growth. Many essential plant functions require adequate Mg
supplies, the most visible being its role in root formation, chlorophyll, and
photosynthesis. Many
less visible reactions are also dependent on an adequate supply of Mg. This
review briefly summarizes some of the essential roles of Mg for plants.
Magnesium has a number of key
functions in plants. Particular metabolic processes and reactions that are influenced
by Mg include: 1) photophosphorylation (such as ATP formation in chloroplasts),
2) photosynthetic carbon dioxide (CO2) fixation, 3) protein synthesis, 4)
chlorophyll formation, 5) phloem loading, 6) partitioning and utilization of photoassimilates,
7) generation of reactive oxygen species, and 8) photooxidation in leaf
tissues. Consequently, many critical physiological and biochemical processes in
plants are adversely affected by Mg deficiency, leading to impairments in
growth and yield. In most cases, the involvement of Mg in metabolic processes
relies on Mg activating numerous enzymes. An important Mg-activated enzyme is
the ribulose- 1,5-bisphosphate (RuBP) carboxylase, which is a key enzyme in the
photosynthesis process and the most abundant enzyme on earth.
Leaf yellowing in the form of
interveinal chlorosis on older leaves is one of the typical symptoms of Mg
deficiency stress (Figure 1). It is reported that up to 35% of the total Mg in plants
is bound in chloroplasts (Figure 2).
 |
| Chloroplasts are the organelles that host thylakoids, the Mg-containing compartments where light energy is converted to chemical energy through the process of photosynthesis |
However, the appearance of Mg
deficiency symptoms is highly dependent on light intensity. High light
intensity increases the development of interveinal chlorosis, together with
some reddish spots on the leaf blade (Figure
3). Therefore, the well-documented differences between plant species in
the expression of visual Mg deficiency symptoms and also in critical deficiency
concentrations of Mg in the leaf tissue may be related to the light intensity
in a particular growth environment.
 |
| Symptoms of leaf chlorosis in Mg-deficient bean plants grown at high light intensity. The green portion of the leaves was partially shaded with filter paper. With an adequate Mg supply, high light did not cause any leaf chlorosis (Cakmak & Kirkby, 2008). |
The leaf damage that occurs in Mg-deficient plants
exposed to high light intensity
has been ascribed to enhanced generation
of damaging highly reactive oxygen species in chloroplasts at
the expense of inhibited photosynthetic CO2 fixation. Plants growing under conditions of high light intensity appear to have a higher requirement for Mg than the plants
grown under lower light intensity.
Magnesium Deficiency Is a Growing Problem
Despite the well-known role of Mg for various
critical functions, there is surprisingly little research
activity on the role of Mg nutrition in crop production
and quality. Hence, Mg is often considered a “forgotten element”. However, Mg deficiency is increasingly becoming an important limiting factor in intensive crop production systems,
especially in soils fertilized only with N,
P, and K. In particular, Mg depletion
in soils is a growing concern for high-productivity agriculture.
Due to its potential
for leaching in highly weathered soils and the interaction with Al, Mg deficiency is a critical concern
in acid soils. One of the well-documented plant adaptation mechanisms to acid soils is the release
of organic acid anions
from roots. Organic
acid anions released from roots
will
chelate toxic Al ions and form Al-organic acid complexes that are no longer phytotoxic. It is well-documented that Mg is required for effective release of organic acid anions from roots to modify an Al-toxic rhizosphere (Yang et
al., 2007). Like Mg, Ca is also important
in alleviating Al toxicity
in acid soils. However, Mg can be protective against Al toxicity
when added in micromolar
levels, while Ca exerts its protective role in millimolar concentrations (Silva et al., 2001).
This result indicates Mg has very specific benefits in
protecting against Al toxicity.
 |
| Growth of common bean (left) and wheat (right) plants with low and adequate magnesium nutrition |
Early
Reaction to Mg Deficiency
In view of diverse functions of Mg in plants, a question arises
as to which function or structure is affected first under Mg deficiency. The
most common answer was chlorophyll level, or photosynthesis, or protein
synthesis. There are a few studies published previously by Cakmak et al. (1994)
in common bean, Hermans et al. (2004) in sugar beet, and Hermans and Verbruggen
(2005) in Arabidopsis that provide a clear and convincing answer to that
question, as discussed below in this short review paper.
Hermans et al. (2004) grew sugarbeets with either a low or
an adequate Mg supply and analyzed 1) plant growth, 2) photosynthetic CO2 fixation,
3) chlorophyll concentrations, 4) photosynthetic electron transport and 5) leaf
concentration of sucrose. The results obtained were clear: before any
noticeable or significant change occurred in the first four measurements, there
was a large accumulation of sucrose in the fully expanded leaves of the
Mg-deficient plants. Magnesium-deficient leaves accumulated up to 4-fold more
sucrose when compared to the Mg-adequate leaves, indicating a severe inhibition
in sucrose transport out of the Mg-deficient leaves.
Cakmak (1994 a,b) studied the role of Mg nutrition in 1)
shoot and root growth, 2) concentration and distribution of carbohydrates among
root and shoot organs, and 3) phloem export of sucrose in bean plants. Results
showed pronounced inhibition of root growth before any noticeable change in
shoot growth and chlorophyll concentration. Consequently, the shoot: root ratio
for both bean and wheat plants increased in Mg-deficient plants (Figure 4).
This early negative effect of Mg deficiency on root growth before the
development of visible leaf chlorosis is a critical issue for growers because
of the importance of a good root system for plant production. Therefore, special
attention should be given to the Mg nutritional status of plants before the
development of any visible deficiency symptoms.
Accumulation of carbohydrates in fully-expanded leaves is
a common phenomenon with Mg deficient plants.
At the beginning of Mg deficiency and under severe Mg deficiency, Cakmak (1994
a, b) found that older leaves contained 3.5-fold and 9-fold more sucrose,
respectively, compared to the Mg-adequate plants.
Magnesium-deficient leaves also contained elevated amounts of starch and
reducing sugars. In bean plants grown with a low Mg supply for 12 days, only 1%
of the total plant carbohydrates were found in roots, whereas in the Mg-adequate
plants, this value was 16%. All these results clearly indicate a severe
inhibition in phloem export of sugars out of Mg-deficient leaves.
Phloem exudates were collected from bean plants with low
and adequate Mg supply to study the role of Mg nutrition on the movement of
sucrose out of the leaf. Magnesium deficiency resulted in severe and very early
inhibition of the phloem transport of sucrose (Figure 5). There was an inverse relationship between sucrose
concentration in leaf tissues and the sucrose export rate in phloem during the
12 days of Mg deficiency treatment. The inhibitory effect of Mg deficiency on
sucrose transport via phloem occurred before any adverse effect on shoot
growth. Re-supplying Mg to the deficient plants restored the phloem export of
sucrose within 12 hours.
 |
| Leaf sucrose concentration (A) and sucrose export rate (B) in bean plants grown with adequate magnesium (control) or deficient magnesium for 12 days (Cakmak et al., 1994b) |
These results strongly suggest that the effect of Mg on phloem
loading of sucrose is specific and not related to any secondary effect. The
mechanism by which Mg deficiency affects phloem loading of sucrose is still not
fully understood, but it appears to be related to the low concentrations of the
Mg-ATP complex at the phloem loading sites. It is widely believed that Mg-ATP
is required for a proper function of H+-ATPase, an enzyme that provides energy
for the phloem loading process and maintains sucrose transport into phloem
cells.
Practical Importance of Early Mg Deficiency
High carbohydrate accumulation coupled with inhibited phloem
export of sucrose from Mg-deficient leaves show the importance of maintaining
adequate Mg nutrition of plants during periods of intensive carbohydrate
transport from leaves to the growing cells. Sufficient Mg is required for
maximizing the carbohydrate transport into sink organs (such as roots and
seeds) to promote high yields. Maintenance of adequate Mg nutrition at the late
growth stages is also essential for minimizing generation of harmful reactive
oxygen species and photooxidative damage in chloroplasts. The application of
late-season Mg through fertilization or foliar sprays may be useful in some
circumstances. The impairment in root growth due to Mg deficiency may have also
serious impacts on uptake of mineral nutrients and water, especially under
marginal soil conditions.
Producing plant-based biomass as a renewable energy source
is a growing and promising alternative to fossil fuel. But the productivity of
these systems is directly dependent on 1) the capacity of plants to fix CO2 into
organic carbon (C) through photosynthesis, 2) translocation of the assimilated
C from source into sink organs, and 3) utilization of assimilated C in the sink
organs for growth. All of these steps are specifically controlled by Mg.
Therefore, attention must be directed to the Mg nutritional status of biofuel
plants in order to achieve high biomass production and partitioning of the
assimilated C in the desired plants organs (such as grains or roots).
Magnesium has long been noted for its essential role in chlorophyll
formation and photosynthesis. However, growing evidence shows that sink organs
(such as growing roots and developing seeds) are also severely affected by Mg
deficiency.
For too long, Mg has been a forgotten element for crop
production, but its vital role is increasingly being recognized in plant
nutrition.
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References
Cakmak, I., C. Hengeler, and H. Marschner. 1994a. J. Exp. Bot.
45:1245–1250.
Cakmak, I., C. Hengeler, and H. Marschner. 1994b. J. Exp. Bot.
45:1251–1257.
Cakmak, I. and E.A. Kirkby. 2008. Physiol. Plant. 133:692-704.
Hermans, C. and N. Verbruggen. 2005. J. Exp. Bot. 56:2153–2161.
Hermans, C., G.N. Johnson, R.J. Strasser, and N. Verbruggen.
2004. Planta 220:344–355.
Silva, I.R., T.J. Smyth, D.W. Israel, C.D. Raper, and T.W.
Rufty. 2001. Plant Cell Physiol. 42:538-545.
Yang, J.L., J.F. You, Y.Y. Li, P. Wu, and S.J. Zheng. 2007.
Plant and Cell Physiol. 48: 66–74.
