It was originally thought that the effect of a particular nutrient
on plant yield would follow an asymptotic
curve. This was based on the logic that a nutrient
would keep affecting a plant in a beneficial manner until such a point
as deficiency of another substance became the limiting
factor. This however was found to be a simplification of the reality
in the field which didn’t take into account the effects of interactions
between ions, or toxic effects of oversupply. When
these factors are taken into account a skewed hyperbolic
relationship is often found.
Examples of such relationships include the reduction in yield brought
about by a deficiency of potassium despite the presence a high nitrogen
supply. Similar effect can be caused by a lack of water.
Generally of all of the macro-nutrients nitrogen and phosphorous affect
yield most significantly. They exert their influence in two main ways,
by affecting cell growth, and by affecting photosynthesis.
Their effect on cell expansion is mediated using the same method.
The deficiency in the nutrient leads to a reduction in hydraulic conductance.
This means that there is a lack of water in the leaves and so cell
expansion is vastly reduced. This reduces LAI and hence yield.
The reduction in photosynthesis is caused by separate
mechanisms by nitrogen and phosphorous. With nitrogen it is caused
by a lack of chloroplast components, and/ or components
of CO2 fixation. Phosphorous deficiency leads to
an accumulation of starch in the chloroplasts as
phosphorous is involved in its transport across the chloroplast
membrane. This backing up of starch inhibits the
action of the chloroplasts.
Nitrogen also has effects on sinks of nutrients such as flowering,
seeds, and tubers. The effects of flowering and seed
development (increasing flowering, and increasing seed number) are
mediated by its effect on phytohormones (CYT and
ABA) and their ratios.
Nitrogen plays a vital role in tuber formation. When
it is in high concentration it suppresses tuber filling , while at
the same time encourages other tubers to begin formation.
Low nitrogen has the opposite effect. However, if low nitrogen levels
are present and tuber filling begins in can be halted
by a short sharp burst of nitrogen.
Phosphorous deficiency in the flag leaf of cereals
can have significant effects on grain filling as the leaf can provide
up to 90% of plant phosphorous.
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