The communication
between nutrient movement in the plant, and plant growth rate is a major factor
which influences the type and location of deficiency symptoms that develop. For
mobile nutrients such as nitrogen and potassium, deficiency symptoms develop
predominantly in the older and more mature leaves. This is a result of these
nutrients being preferentially mobilized during times of nutrient stress from
the older leaves to the newer leaves near the growing regions of the plant. Mobile
nutrients which are newly acquired by the roots are also preferentially
translocated to that of new leaves and the growing regions. This makes the old
and mature leaves being depleted of mobile nutrients during times of stress
while the new leaves are maintained at a more favorable nutrient status.
The typical
localization of deficiency symptoms of very weakly mobile nutrients such as
calcium, boron, and iron is the opposite to that of the mobile nutrients; these
deficiency symptoms are displayed in the growing regions and new leaves while
the old leaves remain in a favorable nutrient status. Plants growing very
slowly for reasons other than nutrition (such as low light) a normally limiting
supply of a nutrient could, under these conditions, be sufficient for the plant
to slowly develop, maybe even with absence of symptoms. This type of
development is likely to take place in the case of weakly mobile nutrients
because of the excess nutrients in the older leaves, which will eventually be
mobilized to supply newly developing tissues. In contrast, a plant with a
similar supply that is growing rapidly will develop severe deficiencies in the
actively growing tissue such as leaf edges and the growing region of the plant.
When nutrients
are moderately mobile such as sulfur and magnesium, which are the limiting
nutrients of the system, deficiency symptoms are normally seen over the entire
plant. However the growth rate and rate of nutrient availability can make a
considerable difference on the locations at which the symptoms are developing.
If the nutrient supply is marginal compared to the growth rate, symptoms will
appear on the older tissue, but if the nutrient supply is very low compared to
the growth rate, or the nutrient is totally depleted, the the younger tissue
will become first of all deficient.
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