Plant Mineral Nutrition

Mineral nutrition

Plants (and animals for that matter), are composed of many elements. The most important are:

Early experimenters (such as Knop) believed S, P, N, K, Ca, Fe and Mg were all that was required for healthy growth of plants. Although this is now known to be untrue, these macronutrients are required in large amounts. Missing macronutrients produce specific deficiency symptoms. Hydrogen and oxygen are largely gained from water, and carbon form carbon dioxide. The other minerals are generally taken up as inorganic ions from the soil.

Plants with nitrogen deficiency have chlorosis, particularly of lower leaves.

Nitrogen is needed for protein and nucleic acid synthesis. It is mobile in the plant (i.e. it can be recovered from proteins in senescent leaves and recycled to some extent into new proteins in young leaves). Consequently, deficiency strikes the older leaves first, and leads to stunting and chlorosis (yellowing) primarily of the lower leaves.

Plants with sulfur deficiency have chlorosis, particularly of upper leaves.

Sulfur is needed for protein synthesis and, e.g. FeS prosthetic groups in mitochondria and chloroplasts. It is much less mobile than nitrogen, so deficiency manifests as chlorosis of the upper leaves.

Plants with magnesium deficiency have interveinal chlorosis, particularly of lower leaves.

Magnesium is needed for chlorophyll synthesis. Like nitrogen, it can be scavenged from chlorophyll in senescent leaves, and is mobile in the plant (remember that the leaves of deciduous trees go yellow in autumn: this is partly due to the recovery of magnesium from their chlorophyll). Deficiency shows as interveinal chlorosis of the lower leaves.

Plants with iron deficiency have interveinal chlorosis, particularly of upper leaves.

Iron is also needed for chlorophyll synthesis, and is an integral part of many respiratory chain proteins. It is immobile in plant, so deficiency generally manifests as interveinal chlorosis of the upper leaves. Ericaceous (calcifuge) plants (those that live on acidic soil), often have the symptoms of iron deficiency if grown on alkaline soils (those rich in calcium salts, i.e. limestone. This is not because of an excess of calcium, but rather the reduced solubility of iron salts at high pH. Calcicole plants (those preferring alkaline soil), have a related problem when trying to grow on acid soil: the increased solubility of toxic aluminium salts at low pH.

Plants with phosphorus deficiency have greening or purpling of their leaves.

Phosphorus is needed for nucleic acid synthesis, for the production of important cofactors such as ATP and NADH, and for the production of sugar-phosphates (and hence cellulose cell walls). Deficiency manifests as excessive greening or even purpling (anthocyanosis) of the leaves. Phosphorus deficiency causes chloroplast production to outstrip cell wall growth, hence the leaves become excessively green. Anthocyanosis (which can also be a feature of nitrogen and sulfur deficiency) occurs because Krebs intermediates build up because they cannot be used for nucleic acid or protein synthesis, hence they are shuttled to anthocyanin pigment synthesis instead.

Plants with calcium deficiency have laves with withered edges.

Calcium is needed for cell wall stiffening, and intracellular signalling via calmodulin. The edges of leaves with calcium deficiency wither and blacken.

Plants with potassium deficiency have stunting, and produce few flowers or fruit.

Potassium is needed for electrolyte balance and the activation of many enzymes. A lack of potassium leads to generalised stunting and sterility without other obvious symptoms (deficiency of most of these macronutrients will cause some degree of stunting, but generally with the other symptoms noted above).

Later workers (such as Hoagland) found that other minerals were needed in trace quantities, including some exotica such as:

These are now called 'trace elements' or micronutrients.

These nutrients, both macro and micro, all have a significant effect on crop yields. It is important to detect and identify mineral deficiencies in crops, but you can't usually rely on specific deficiency symptoms, because these are too complicated if more than one is missing. You can analyse the soil, but some minerals may be present but unavailable to the plant (as noted with the iron-deficiency in alkaline soils). The best way is to analyse the crop for each mineral in turn and compare it with the internal concentration that it needs. You can find this by growing it in culture with different concentrations of the mineral.

Most nutrients show an inverted U shaped curve when availability is plotted against growth, from deficiency, through sufficiency, to toxicity.

Although almost any part of the plant can absorb minerals they are usually taken up from the soil by the roots. A few plants absorb nutrients mostly through their leaves (carnivorous plants, bromeliads, etc; however, most have to rely on the soil. Soil consists of a mixture of inorganic rock fragments, organic material, air spaces and water. Mineral ions interact with this matrix and may adsorb onto soil particles.

Soil usually consists of negatively-charged particles, such as aluminosilicates and organic ('humic') acids. Positively charged ions (cations) such as Ca2+ and K+ adsorb readily to this matrix. These are not easily washed off but can be exchanged with H+ secreted by root ATPases.

Anions, (e.g. NO3−) are not adsorbed onto the soil. They are easily washed off and can be lost permanently by leaching into groundwater or rivers (hence the problem of over-fertilised fields causing eutrophication in rivers). They are easily absorbed by the plant, but they must still be moved against an electrochemical gradient.

The root has to make intimate contact with the soil particles. It must be highly branched and therefore must have lots of root hairs.

Plant root ATPases pump out H+ to displace cations in the soil (ion-exchange). The insides of plant cells are therefore electrically negative. Cations are usually taken up along their electrochemical gradient via ion channels.

Cations are taken up by ion channels.

Anions cannot be taken up like this. This would go against their electrochemical gradient, so they are taken up by symport with H+, expending the energy stored in the H+ gradient.

Anions are taken up by proton symports.

Test yourself

  1. Justify the statement: both anion and cations are taken in by active transport.
  2. What mineral deficiencies are these plants suffering from?

    Plant with interveinal chlorosis of older leaves.

    Plant with chlorosis of new growth.

Answers

Bibliography

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