PLANT NUTRITION 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 products.

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.

Macronutrients
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:

Micronutrients
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?

  • 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 magnesium.

  • 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 Recall:

  • 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 nod genes.
  • 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.