|Figure 9.3 Transverse section of Marram
Grass leaf , showing adaptations to prevent
loss; outer thick cuticle, curling by means
of hinge cells to protect inner epidermis, stomata
sunken into surface to maintain high humidity
are those inorganic substances necessary for the
plant to grow and develop normally. They can be conveniently divided
into two groups. The major nutrients (macronutrients)
are required in
relatively large quantities whereas the micronutrients (trace elements)
are needed in relatively small quantities, usually measured in parts per
million, and within a narrow concentration range to avoid Deficiency or
toxicity. The list of essential nutrients is given in Table 9.1.
such as sodium and chlorine, appear to have
a role in the plants but not as a universal requirement for growth and
development. Sodium is made use of in many plants, notably those of
estuarine origins, but whilst it does not appear to be essential there is
an advantage in using agricultural salt on some crops such as beet or
carrots. Aluminium plays an important part in the colour of Hydrangea
flowers and silicon occurs in many grasses to give them a
cutting edge or sharp ridges on their leaves.
Functions and Deficiency symptoms of minerals
in the plant
|Table 9.1 Nutrient requirements
Many essential minerals have very specific functions in the plant cell
processes. When in short supply (deficient
) the plant shows certain characteristic symptoms, but these symptoms tend to indicate an extreme
Deficiency. To ensure optimal mineral supplies, growing media analysis
or plant tissue analysis can be used to forecast low nutrient
levels, which can then be addressed.
is a constituent of proteins
, nucleic acids and chlorophyll
and, as such, is a major requirement for plant growth. Its compounds
comprise about 50 per cent of the dry matter of protoplasm, the living
substance of plant cells.
Deficiency causes slow, spindly growth in all plants and yellowing of the
leaves (chlorosis) due to lack of chlorophyll. Stems may be red or purple
due to the formation of other pigments. The high mobility of nitrogen in
the plant to the younger, active leaves leading to the old leaves showing
the symptoms first.
is important in the production of nucleic acid and the
formation of adenosine triphosphate (see ATP). Large amounts are
therefore concentrated in the meristem. Organic phosphates, so vital for
the plant’s respiration, are also required in active organs such as roots
and fruit, while the seed must store adequate levels for germination.
Phosphorus supplies at the seedling stage are critical; the growing root
has a high requirement and the plant’s ability to establish itself depends
on the roots being able to tap into supplies in the soil before the reserves
in the seed are used up.
Deficiency symptoms are not very distinctive. Poor establishment of
seedlings results from a general reduction in growth of stem and root
systems. Sometimes a general darkening of the leaves in dicotyledonous
plants leads to brown leaf patches, while a reddish tinge is seen in
monocotyledons. In cucumbers grown in deficient peat composts or
NFT, characteristic stunting and development of small young leaves
leads to brown spotting on older leaves.
. Although present in relatively large amounts in plant
cells, this mineral does not have any clear function in the formation
of important cell products. It exists as a cation and acts as an osmotic
regulator, for example in guard cells, and is involved in
resistance to chilling injury, drought and disease.
Deficiency results in brown, scorched patches on leaf tips and margins
(see Figure 21.4), especially on older leaves, due to the high mobility
of potassium towards growing points. Leaves may develop a bronzed
appearance and roll inwards and downwards.
is a constituent of chlorophyll. It is also involved in the
activation of some enzymes and in the movement of phosphorus in the
Deficiency symptoms appear initially in older leaves because
magnesium is mobile in the plant. A characteristic interveinal chlorosis
appears (see Figure 21.5), which subsequently become reddened and
eventually necrotic (dead) areas develop.
is a major constituent of plant cell walls as calcium pectate,
which binds the cells together. It also influences the activity of
meristems especially in root tips. Calcium is not mobile in the plant
so the Deficiency symptoms tend to appear in the younger tissues first.
It causes weakened cell walls, resulting in inward curling, pale young
leaves, and sometimes death of the growing point. Specific disorders
include ‘topple’ in tulips, when the flower head cannot be supported by
the top of the stem, ‘blossom end rot’ in tomato fruit, and ‘bitter pit’ in
is a vital component of many proteins that includes many
important enzymes. It is also involved in the synthesis of chlorophyll.
Consequently a Deficiency produces a chlorosis that, due to the relative
immobility of sulphur in the plant, shows in younger leaves first.
are involved in the synthesis of chlorophyll;
although they do not form part of the molecule they are components of
some enzymes required in its synthesis. Deficiencies of both minerals
result in leaf chlorosis. The immobility of iron causes the younger leaves
to show interveinal chlorosis first. In extreme cases, the growing area
affects various processes, such as the translocation of sugars and
the synthesis of gibberellic acid in some seeds (see dormancy).
Deficiency causes a breakdown and disorganization of tissues, leading to
early death of the growing point. Characteristic disorders include ‘brown
heart’ of turnips, and ‘hollow stem’ in brassicas. The leaves may become
misshapen, and stems may break. Flowering is often suppressed, while
malformed fruit are produced, e.g. ‘corky core’ in apples, and ‘cracked
fruit’ of peaches.
is a component of a number of enzymes. Deficiency in many
species results in dark green leaves, which become twisted and may
, also involved in enzymes, produces characteristic Deficiency
symptoms associated with the poor development of leaves, e.g. ‘ little
leaf ’ in citrus and peach, and ‘rosette leaf’ in apples.
|Figure 9.4 Cross-section of Zea mais root showing its structure
in the absorption and
transport of water and minerals
Minerals are absorbed to form the soil solution (see Plant nutrition
plants take up only water-soluble material so all supplies of nutrients
including fertilizers and manures must be in the form of ions
particles). The movement of the elements in the form of ions
in the direction of root cells containing a higher mineral concentration than the soil, i.e. against a concentration gradient.
The passage in the
water medium across the root cortex is by simple diffusion, but transport
across the endodermis requires a supply of energy from the root cortex.
The process is therefore related to temperature and oxygen supply (see
Nutrients are taken up predominantly by the extensive network of fine
roots that grow in the top layers of the soil (see Figure 9.1). Damage to
the roots near the soil surface by cultivations should be avoided because
it can significantly reduce the plant’s ability to extract nutrients. It is
recommended that care should be taken to ensure that trees and shrubs
are planted so their roots are not buried too deeply and many advocate
that the horizontally growing roots should be set virtually at the surface
to give the best conditions for establishment.
The surface thickening that occurs in the ageing root does not
significantly reduce the absorption ability of most minerals, e.g.
potassium and phosphate, but calcium is found to be principally taken
up by the young roots.