JARS v37n1 - Micronutrients and Plant Nutrition

Micronutrients and Plant Nutrition
Fred R. Davis
Physical Chemist, Kent, Ohio

Plants require several mineral elements for normal growth. Some elements, such as nitrogen, phosphorus, potassium, calcium and sulfur are needed in relatively large amounts and these are called the "major" elements; while the elements, such as iron, boron, magnesium, manganese, zinc, molybdenum and copper are needed in very small amounts and are called "micronutrients".

Some people may be skeptical about micronutrient supplements in plant nutrition, saying "their effect is rather unimportant and not measurable". Research scientists long have had evidence, both laboratory and field testing, to show that micronutrients are beneficial to plant growth and reproduction. My own experience in growing rhododendrons has borne out the need for micronutrients to correct the chlorotic (yellowing) condition that sometimes exists. The purpose of this article is to discuss the functions and deficiency symptoms of several of the micronutrients.

Magnesium and Its Functions
Some consider this element to be one of the major elements, but I will discuss its role as a micronutrient. Magnesium plays two very essential roles in the processes of photosynthesis and carbohydrate metabolism. Magnesium is the metal atom in the chlorophyll molecule without which photosynthesis would not occur. Magnesium also serves as an activator in carbohydrate metabolism. Magnesium serves as an activator for those enzymes involved in the synthesis of the nucleic acids (RNA, DNA ¹ ) from nucleotide polyphosphates. Magnesium also may be involved in protein synthesis serving as a binding agent in the microsomal particles.

Magnesium Deficiency Symptoms
Magnesium deficiencies show up as extensive chlorosis between the veins of the leaves. Chlorosis is followed by the appearance of anthocyanin pigments in the leaves such as the oranges, reds and purples. Whenever a severe deficiency exists, necrotic spotting (small patches of dead tissues) may be observed. This deficiency is best corrected by using a soluble magnesium salt, e.g. magnesium sulfate.

Iron and Its Functions
Iron serves a number of important functions in the overall metabolism of the plant. Chemists say that iron is frequently taken up in the trivalent state (Fe ⁺⁺⁺ ), but is generally accepted in the divalent state (Fe ⁺⁺ ) as the metabolically active form of iron in the plant. Its chemical role both in the synthesis and degradation of chlorophyll is still uncertain. Several researchers feel that iron functions in the synthesis of chloroplastic protein and thus may interfere with chlorophyll synthesis.

Iron has been identified as a component in the metalloflavoproteins which are active as enzymes in certain biological oxidations. Iron has been found in the iron-porphyrin proteins; the cytochromes belong to this enzyme class.

Iron Deficiency Symptoms
Iron deficiencies of plant leaves appear as extensive chlorosis in the foliage. The new foliage is generally most affected, although I have seen both old and new foliage affected to about the same extent on rhododendron. Iron-induced chlorosis will show up in the interveinal structure of the leaf and the surface of the leaf usually shows a grid network of green veins between the chlorotic areas.

There appears to be some correlation between iron deficiency and chlorophyll content, but there is other evidence that chlorotic leaves may contain as much or even more iron than their green counterparts. It has been proposed that the lack of iron may inhibit the formation of chlorophyll through inhibition of protein synthesis.

Iron deficiencies in many plants can be corrected by the use of chelated iron compounds. Some of the early work with ornamentals was done using the iron salts of ethylenediamine tetraacetic acid. My own experience with these types of compounds for rhododendron has shown a favorable response to correcting chlorosis and producing increased bud set. It is important to realize here that these metal chelates can be toxic to the plant if used indiscriminately. Toxic doses produce withered and curled leaves with browning at the edges. The toxicity is probably related to electrolyte disturbance or unbalance and enzyme inhibition.

Manganese and Its Function
Manganese seems to be an essential ion in the respiration and nitrogen metabolism where it functions as an enzyme activator. In some cases, especially with reactions in respiration, manganese can be replaced by other divalent cations, such as Mg ⁺⁺ , Zn ⁺⁺ and Fe ⁺⁺ .

Manganese functions in nitrate reduction where it acts as an activator for the enzymes nitrite reductase and hydroxylamine reductase. The preference of ammonia over nitrate as a nitrogen source by manganese deficient cells supports the above mentioned role of manganese. Manganese is also thought to be involved in the destruction or oxidation of indole-3-acetic acid.

Manganese Deficiency Symptoms
Manganese deficiency is characterized by chlorotic and necrotic areas in the portions of the leaf between the veins. This symptom appears on the young leaves of rhododendron, rather than the older leaves. I find that symptoms of manganese deficiency often are difficult to distinguish from symptoms of iron deficiency. In some areas of the U.S., grapes suffer from a manganese deficiency. This deficiency is best corrected by using one of the soluble manganese chelates.

Copper and Its Function
Copper acts as a component of phenolases and ascorbic acid oxidose (enzymes), and its role as a part of these enzymes probably represents the most important function of copper in plants. Research suggests that copper may function in photosynthesis. For example, it was found that the chloroplasts of clover contain most of the copper of the plant. Most plants are very sensitive to the concentration of copper irons, so one must be careful in its use.

Copper Deficiency Symptoms
Copper deficiency usually results in shriveling or malformation of the leaves along with tip burn. The most easily recognized symptoms of copper deficiency are those in a disease of fruit trees called "exanthema". A copper deficiency in almonds may result in roughening of the bark, gummosis and shriveling of the kernels. Copper deficiencies are corrected by using soluble copper chelates, e.g., copper ethylenediamine tetraacetate.

Zinc and Its Function
Zinc plays a role in protein synthesis as evidenced by the accumulation of soluble nitrogen compounds such as amino acids and amides. Zinc participates in the biosynthesis of the plant auxin indole-3-acetic acid. This was proved by observing that the content of tryptophan parallels the content of auxin in the plant, both when zinc is deficient and when it is supplied to deficient plants. It has been concluded that zinc reduces auxin content through its involvement in the synthesis of tryptophan, a precursor of the auxin. Zinc is also involved in the metabolism of plants as an activator of several enzymes. Carbonic anhydrase was the first zinc containing enzyme to be discovered. This enzyme is involved in the catalytic decomposition of carbonic acid to carbon dioxide and water. An accumulation of inorganic phosphorous in zinc deficient tomato plants indicates that zinc could act as an activator for some phosphate transferring enzyme, such as hexose kinase.

Zinc Deficiency Symptoms
Zinc deficiency is sometimes referred to as "rosette" or "little leaf disease". It is most evident in older leaves as chlorosis, necrosis or mottling of the leaves. The interveinal areas turn pale green to yellow; the leaf margins become irregular. The absence of zinc also may have a retarding effect on growth and the development of fruit. The use of soluble zinc chelates as in soil applications or foliar applications will correct the deficiency.

Boron and Its Function
Researchers have concluded that boron is involved in carbohydrate transport within the plant. They believe that the borate forms a complex with the sugar molecule. They propose that sugar is transported more readily across cell membranes as a borate complex.

The common features of boron deficiency in plants are the death of the stem and root tips and the abscission of flowers. Symptoms of boron deficiency are symptoms of sugar deficiency. The role of boron in sugar translocation has been supported using C ¹⁴ labeled sucrose.

Boron and Its Deficiencies
The first visible sign of boron deficiency is the death of the shoot tip. The leaves may have a thick coppery texture with curling. Generally, flowers do not form and root growth is stunted. In fleshy organs, there is a disintegration of internal tissues resulting in cork formation in apples and water core in turnips. Boron deficiencies can be corrected by using sodium tetraborate or boric acid.

Molybdenum and Its Functions
Molybdenum is involved in the nitrogen fixation and nitrate assimilation. Some investigators have found that molybdenum deficiency leads to a decrease in the concentration of ascorbic acid in the plant. There is some evidence that molybdenum is involved in the phosphorous metabolism of the plant, but the mechanism has not been explained.

Molybdenum Deficiency Symptoms
A condition due to molybdenum deficiency known as "whiptail" occurs in some plants of the cabbage family. The leaves first show an interveinal mottling and leaf margins become brown. The leaf tissues wither, leaving only the midrib and a few small pieces of leaf blade, giving the appearance of a whip or tail. Molybdenum deficiencies can be corrected by using a soluble molybdenum salt, e.g. sodium molybdate.

I conclude with a few words about nutrient translocation. First, let me say that the dynamics of plant nutrition is not completely understood. The mechanism for translocation of the nutrient molecule or iron involve diffusion and the development of osmotic pressure gradients. If one considers diffusion to be the predominate transport process, then the nutrient molecule is trapped at a reactive site. The energy for this process is derived from metabolism. All translocation through the phloem generally involves osmotic pressure gradients developed by concentration field gradients across cell and tissue membranes. It is difficult to predict the response of the plant to the nutrient, because of the lack of a satisfactory translocation mechanism. The pH, geometry and size, charge and solubility of the nutrient molecule all play an important role.

¹ RNA = Ribonucleic Acid  *DNA = Desoxyribonucleic Acid