MICRONUTRIENTS - THE ROLE OF THE ROOT - THE ROLE OF ROOTS IN MICRONUTRIENT UPTAKE

 

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We have had an introduction into how these micronutrients behave in the soil. Before they can take an active part in plant metabolism they must be taken into the roots of the plant. Here again we'll see that these nutrients are individuals demonstrating their own peculiar characteristics. Let's make a close examination of a root.

 

Roots are the Sector of Plant Nutrition

The main functions of roots are absorption, anchorage, and storage. Water and macronutrient absorption have received by far the most research effort in the past and, while there is abundant knowledge in the field, there is much yet to learn. The importance of micronutrient absorption to high yields has stimulated much recent research.

There is a preferential sieving or screening action carried on by the root hairs. This activity or ability of the root to be selective in nutrient uptake is at least partially related to the activity of calcium in the epidermal cells – at the outside margins of the root hairs. It is also known that high plant potassium levels must be maintained in order to keep micronutrient ratios nearly ideal. The exact mechanism of the potassium regulation is unknown at this time.

 

Mechanisms of Root Nutrient Uptake

Scientists generally agree there are three methods by which nutrients reach root surfaces:

• mass flow
• root interception
• diffusion

As roots take in nutrients, they exude waste elements – positive hydrogen ions and negatively charged anions. As the hydrogen separates from the root it leaves negative charges on the root surface. This action is predominant. Where the surface of the root is negatively charged, the nutrient elements manganese, zinc, copper and iron, which are positively charged, are electrically attracted to the root. Release of anions from the root leaves a positive charge and attracts the boron and molybdenum oxides.

 

Mass Flow

Mass flow is the movement of soil water and nutrients in solution to the root. A difference in water pressure between the inside and the outside of the plant is created by loss of water from the leaves. This pressure difference is passed down to the roots. Water then flows towards the roots.

Mass flow is important in getting water-soluble micro and macro-elements into plants.

 

Root Interception

Root interception is the growth of the root into contact with ions that are held by electro-static charge to clay and organic matter. It is important in the uptake of those nutrients that have low activity.

Copper and iron for example, are less active at normal root pH, are commonly found in less water-soluble forms and are usually held by electrical charges. In their case, root interception is important: as plant roots grow, they come into contact with these ions.

 

Diffusion

Diffusion is the movement of ions from a higher concentration of soil solution to a lower one. It is most significant with those ions high in ionic energy. For example, uptake of manganese, zinc, and to a lesser degree, iron, has shown to be related to movement by diffusion in the soil.

Approximate % Supplied by
Nutrient	Root Interception	Mass Flow	Diffusion	Significance for Application
Manganese (Mn)	15	5	80	Band
Zinc (Zn)	20	20	60	Band or Broadcast
Iron (Fe)	50	10	40	Band or Broadcast
Copper (Cu)	70	20	10	Broadcast
Boron (B)		65		Broadcast
Molybdenum (Mo)		?		Broadcast

What are the practical applications of this knowledge of root interception, mass flow and diffusion? In general, we should recommend that with the more active ions (Mn, Zn, Fe) part of the fertilizer carrier of these materials be banded in order to keep them from combining with other soil ions into less available forms.

 

Foliar Application

Plants also have the ability to absorb nutrients through their leaf and stem tissues. most micronutrient deficiencies can be temporarily overcome by foliar treatment with proper concentrations of the elements. However, spray applications do not correct the basic problems of soil deficiency.

 

Nutrient Uptake

The ways that nutrients arrive at root surfaces have been explained. At those surfaces, each nutrient is screened through a barrier system that accepts certain ions at metered rates.

The uptake process is only partially understood, but it is known to occur at a plant cell membrane or filter. The nutrient ion enters this membrane at a rift, or hole, that is aligned with a carrier system to transport the ions into the cell. The carriers are energized organic acids, each acid specifically made to carry its own particular nutrient ions.

Power to energize these ion carriers comes from respiration – The burning of plant sugars. Phosphorus is involved in the needed energy transfers. The waste products of the energy reactions, carbon dioxide and water, exit from the cell at the rifts as hydrogen ions and carbonic acid which attracted the nutrient ions in the first place by creating positive and negative charges at root surfaces.

We've gone from micronutrient mineral crystal to solution to charged particle to the plant root. Now let's look at micronutrients after they are in the plant and begin by studying a "miracle worker", the fellow who makes plant metabolism go – the enzyme.

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