An essential nutrient is one required by an organism for normal growth and
development, but which it cannot manufacture on its own. These vary widely across species.
Animals, for example, have many essential organic nutrients (fatty acids,
amino acids, vitamins, etc.) that they cannot manufacture themselves, and
so must ingest them as other organisms containing those finished
Because PLANTS manufacture all the organic nutrients they need, they have
no essential organic nutrients. However, plants do require very specific
INORGANIC nutrients in order to grow, develop and thrive.
These are elements (usually taken up in the form of compounds) required by plants in relatively large
quantities. They are often major components of the plant's body. In
plants, six of the main inorganic nutrients required are the six main
components of organic molecules:
Three additional macronutrients needed by plants are:
These are the elements (often taken up as compounds) needed in
relatively small quantities.
The main function of these micronutrients is to serve as coenzymes in
various enzymatic pathways.
Some of the functions of the various macro- and micronutrients, as well as physical symptoms of their deficiencies are listed in your text, in Table 29-2. Be sure to review it!
A shortage of any of these nutrients will often have characteristic
symptoms in the plant, with older plant parts showing the effects sooner
than younger parts. (Younger organs act as nutrient sinks, drawing more incoming
material to themselves than the older parts do; they are the last to suffer from nutrient deficiencies.)
WHERE DOES THE PLANT GET ITS NUTRIENTS?
One of the major sources, of course, is SOIL.
Organic versus Inorganic Farming Techniques
What is the difference between ORGANIC and INORGANIC farming?
- inorganic fertilizer is applied as the name implies: in the form
of inorganic compounds of nitrogen, phosphorus and potassium (N-P-K) as
well as trace elements, in lower quantities.
- These may leach out of soil rather quickly, though they are
instantly available to plants.
- organic fertilizer, as its name implies, consists of complex
organic material that's in the process of decomposing.
- Although it takes longer to become available to plants (depending
on the speed with which decomposers release inorganic forms to the
plants), it will stay in the soil much longer.
Decaying organic matter within soil is often called humus. The more
humus there is in the soil, the less likely it is to be compacted, the more
likely it is to retain water (humus has a spongelike capacity to hold
water), and the more gradual (and permanent) the release of nutrients.
Read the special "box" in your text on Composting (page 664). Very important!
Soil is also composed of inorganic components named by their particle
size. From largest to smallest granule, these are
sand --> clay --> silt
Because sand, clay, and silt are slightly negatively charged, they help to
retain postively charged nutrient ions such as potassium, calcium, and
In very acid soils, negatively charged ions become bound to the soil
particles, and are difficult for plants to take up. Example: very marshy
areas where the organic component of the soil is extremely high, making the
soil very acid.
(Some plants growing here have special adaptations for
obtaining nitrogen. What are they?)
What role do you suppose water/precipitation plays in affecting nutrient
content of soil? (Think: Rainforest)
Soils tend to leach anions, but retain cations, as the colloidal surface of clay and humus tend to have an excess of negative charges that hang onto the cations in interstitial water.
The Importance of Nitrogen
As we have already mentioned, nitrogen fixation by bacteria is vital to
the survival of all life on earth.
Recall that some plants (notably those in the Family Fabaceaea, the Pea Family) have
specialized root nodules that house symbiotic nitrogen-fixing bacteria.
This is one reason that legumes are so high in protein: no shortage of
nitrogen for building it!
Rhizobium and other nitrogen-fixing prokaryotes are able to reduce
elemental nitrogen to ammonium via the activity of an enzyme known as
NITROGENASE. The fixation of a single N2 molecule into two
molecules of ammonia requires 16 ATPs. This is an expensive process!
Let's follow the Amazing and Wonderful communication cycle
between the symbiotic nitrogen-fixing bacterial genus Rhizobium
(each legume species has its own specific symbiotic species of
Rhizobium) and the leguminous plant they call home...
This results in growth of root nodules like so:
The mechanism by which bacteria initially attach to the root hairs is not yet well understood, but may involve the activity of sugar-binding proteins known as lectins.
Root nodules are perfectly evolved to be hospitable to prokaryotic
symbionts. Legumes even produce a form of hemoglobin (leghemoglobin) that
retains oxygen and acts as a slow-release "buffer" source of
O2 for the metabolically very active nitrogen fixers as they
make and use all that ATP to fix nitrogen.
The host/symbiont relationship is highly species specific; each legume has its own species of Rhizobium that will generally not colonize other species of plants.
A Link to the Evolution of Mycorrhizal Associations
Mycorrhizal plants deprived of their fungal symbionts do not thrive, and
grow far more slowly than their mycorrhizal conspecifics.
Cytokinin (a hormone we'll discuss shortly) appears to activate the
Both symbiotic bacteria and mycorrhizal fungi cause an increase in
cytokinin production in roots.
Cytokinins are evidently part of this complex signalling process,
though the exact mechanism is not yet known.
The Nod proteins secreted by Rhizobium are chemically similar to
chitin. What other Famous Microorganism also contains chitin? And how
might this be related to the evolution of root nodule formation?
Amazing sidelight: The nodulin (nod) genes involved in the formation of root
nodules are the same ones activated in mycorrhizal associations, to form
the fungus/plant connections.
Mycorrhizae appear to date back at least 400 million years
Root nodules are probably no older than 160 million years
It's probable that the nod gene function was an exaptation just
"waiting" for the nitrogen fixation symbiosis to develop.
Review the Nitrogen Cycle...
as well as the pathway of all other elemental nutrients, as exemplified by the Phosphorus Cycle...
...and treat your plants well.