|
Visual Diagnosis
Careful observations of the growth of plants can furnish direct evidence of their nutritional conditions.
Metabolic disruptions resulting from nutrient deficiencies provide links between the function of an element
and the appearance of a specific visible abnormality. Symptoms of disorders, therefore, provide a
guide to identify nutritional deficiencies in plants. Careful experimental work and observations are
needed to characterize symptoms. For example, nitrogen is needed for protein synthesis and for chlorophyll
synthesis, and symptoms appear as a result of the disruption of these processes. Symptoms of
nitrogen deficiency appear as pale-green or yellow leaves starting from the bottom and extending
upward or sometimes covering the entire plant. Magnesium deficiency also affects protein synthesis
and chlorophyll synthesis, but the symptoms may not resemble those of nitrogen deficiency, which
affects the same processes. Experience is necessary to distinguish the symptoms of nitrogen deficiency
from symptoms of magnesium deficiency or in the identification of the deficiency of any nutrient.
|
|
Symptoms on foliage have been classified into five types (12): (a) chlorosis, which may be uniform
or interveinal (Figure 1.1); (b) necrosis, which may be at leaf tips or margins, or be interveinal
(Figure 1.2); (c) lack of new growth, which may result in death of terminal or axillary buds and
leaves, dieback, or rosetting (Figure 1.3); (d) accumulation of anthocyanin, which results in an overall
red color (Figure 1.4); and (e) stunting with normal green color or an off-green or yellow color
(Figure 1.5). Symptoms of deficiency can be quite specific according to nutrient, especially if the
diagnosis is made early in the development of the symptoms. Symptoms may become similar
among deficiencies as the intensities of the symptoms progress.
Generalities of development of deficiency symptoms can be made among species. Many references
are available with descriptions, plates, or keys that enable identification of nutrient deficiencies (12–20). |
As mentioned above, for example, nitrogen deficiency appears across plant species as
chlorosis of lower or of all leaves on plants. Advanced stages of nitrogen deficiency can lead to
leaf death and leaf drop. Nitrogen-deficient plants generally are stunted and spindly in addition to
showing the discoloration that is imparted by chlorosis. Potassium-deficient plants have marginal and
tip necrosis of lower leaves. On the other hand, for elements that are immobile (not transported in
phloem) or slowly mobile in plants, the deficiency symptoms will appear on the young leaves first.
The symptoms might appear as chlorosis, as with sulfur, iron, manganese, zinc, or copper deficiency,
or the symptoms might be necrosis of entire plant tips, as occurs with boron or calcium deficiency.
Brooms or rosetting may occur in cases where deficiencies (e.g., copper or zinc) have caused death
of the terminal bud and lateral buds have grown or where internode elongation has been restricted by
nutrient (e.g., zinc) deficiencies. Accumulation of anthocyanin, exhibited by reddening of leaves,
may indicate phosphorus deficiency, although nitrogen deficiency can lead to a similar development.
 |
|
 |
| FIGURE 1.1 Interveinal chlorosis of iron-deficient borage (Borago officinalis L.). |
|
FIGURE 1.2 Deficiency symptoms showing necrosis of leaf margins, as in this case of potassium deficiency
on cucumber (Cucumis sativus L.) leaf. |
 |
|
 |
FIGURE 1.3 Deficiency symptoms showing necrosis on young leaves of (a) calcium-deficient lettuce (Lactuca
sativa L.) and necrosis on young and old leaves of (b) calcium-deficient cucumber (Cucumis sativus L.). With
cucumber the necrosis has extended to all leaves that have not expanded to the potential size of full maturity. |
|
FIGURE 1.4 Stunting and development of red color and loss of green color of phosphorus-deficient tomato
(Lycopersicon esculentum Mill.) |
Some people try to distinguish the two deficiencies by noting whether the symptoms of reddening
develop between the veins (phosphorus deficiency) or along the veins (nitrogen deficiency). Stunting
is a good indication of nutrient deficiency, but often stunting cannot be recognized unless a wellnourished
plant is available as a standard of comparison. A stunted plant may have normal color and
not be recognized as being deficient until abnormal coloration develops with advanced stages of deficiency.
In some cases, symptoms may not develop during the growth cycle of crops, but yields may
be suppressed relative to plants that have optimum nutrition. Hidden hunger is a term applied to cases
where yield suppression occurred but symptoms did not develop.
 |
FIGURE 1.5 Cabbage (Brassica oleracea var. capitata L.) plants showing symptoms of stunting. Left: stunting
and dark green color diagnosed as being caused by salinity in nutrient solution. Middle: stunting and mottling
of foliage due to condition diagnosed as magnesium deficiency. Right: stunting and discoloration of
foliage due to condition diagnosed as phosphorus deficiency. |
Deficiency symptoms can occur at any stage of growth of a plant. The most typical symptoms
are those that appear early in the cycle of deficiency. Early diagnosis of deficiencies may also allow
time for remedial action to take place. Generally, however, if symptoms have appeared, irreparable
damage has occurred, with quantity or quality of yields being suppressed or diminished with annual
crops or with slowing or damaging of growth and development of perennial crops. Also, symptoms
that resemble nutrient deficiency can develop on plants as a result of conditions that are not related
to nutrient deficiencies, for example, drought, wet soils, cold soils, insect or disease infestations,
herbicide damage, wind, mechanical damage, salinity, or elemental toxicities. Deficiency symptoms
are only one of several diagnostic criteria that can be used to assess the nutritional status of plants.
Plant analysis, biological tests, soil analysis, and application of fertilizers containing the nutrient in
question are additional tools used in diagnosis of the status of plant nutrition.
|
