Zinc seems to be distributed fairly well within the plant. The concentration varies with the amount of available zinc in the soil, the kind of plant, the part of the plant sampled, and the stage of growth. Differences in varieties and in environmental factors probably affect the zinc content. Generally the zinc content of normal plants is higher than that of zinc-deficient plants grown in a similar environment, although a considerable overlap seems to exist.

E. Archibald and F. B. Wann, of Utah State University, have discovered that the limiting value for the zinc content of the leaves of several fruit trees (below which the development of zinc deficiency may be expected) is 0.0123 percent. Normal leaves contained between 0.0123 and 0.0345 percent of zinc. A critical value for tung is reported to be 0.0010 percent of zinc. Other research workers have reported that the percentage composition of zinc in the leaves of normal plants is considerably lower than the above values as low as 0.0004 percent of zinc in normal leaves of some plants.

Frank Viets and his coworkers in Washington reported values ranging from 10.5 to 32.7 P.P.m. of zinc, with an average of 16.86 for selected parts of several crops that were growing on a normal area without zinc deficiency. In an area where crops showed a zinc deficiency, the values varied from 9.3 to 22.5 p.p.m., with an average of 15.44 p.p.m. When the crops were fertilized with zinc, the average content of the crops became 22.58 and 18.34 p.p.m.

Both the tops and roots of corn at several sampling dates did not show great or consistent differences in zinc concentration between normal and zinc-deficient plants. The total zinc uptake, however, was much greater for normal plants than for zinc-deficient plants, indicating greater growth for the normal plants.

P. R. Stout and G. Pearson, of California, discovered that plants very low in zinc have a higher concentration of zinc than larger plants that also are deficient in zinc. When zinc was added to the culture solution, the amount of internodal to nodal and embryonic tissue increased, and the zinc concentration of the entire plant dropped. The concentration in the plant again increased when more zinc was added.

The zinc concentration in actively growing parts of plants is higher than in older tissue. Zinc tends to accumulate in and around the primary veins of the leaf blade in actively growing corn leaves. The highest zinc concentration in cornstalks is at the node; concentration falls off quite rapidly both above and below the node. In oats, 20 to 30 percent of the zinc in the plant is in the leaves, mainly in the chloroplasts. Zinc is bound to the protein material in the chloroplasts along with their colored pigments.

A deficiency of zinc in a plant may affect its content of other elements. Leaves of corn plants that show zinc deficiency have more potassium and phosphorus than leaves of normal plants.

ZINC is a necessary component of several enzyme systems, which regulate various metabolic activities within plants. It is a part of the enzyme carbonic anhydrase, which regulates the equilibrium between carbon dioxide, water, and carbonic acid. It also functions as a part of two enzymes, dehydopeptidase and glycylglycine dipeptidase, which have a part in specific aspects of protein metabolism.

Zinc also is needed for the formation of auxins, which are growth-promoting substances in plants. Because one of the symptoms of zinc deficiency is a failure of the stem tissue between the nodes to elongate, resulting in rosetting, it was thought that zinc deficiency might be related to a low content of auxin. This phase of the function of zinc in plants has been studied by F. Skoog and his associates at the University of Wisconsin. They learned that plants deficient in zinc were low in auxin. The auxin content of the plants increased when zinc was applied. Further investigation showed that zinc seems to limit the production of one of the compounds that serves as a building block in the formation of the complex auxin compound.

Zinc is associated with water relations in plants. High osmotic pressures resulting from zinc deficiency are due to reduced water uptake, which was restricted by the failure of cell walls to grow because of lack of auxin.

The observation that zinc deficiency does not develop so readily in mild sunlight as it does in bright sunlight may be associated with auxin activity. Plants grown under blue light exhibit zinc deficiency symptoms more readily than those grown under red light. Light of high intensity and of short wavelengths inactivates auxin.

Plants differ in their ability to extract zinc from the native soil supply. Crops that followed crotalaria as a cover crop in Florida exhibited zinc deficiency, but crops that followed a cover crop of native weeds did not. Analysis of the plants showed that the weeds accumulated much more zinc from the limited soil supply than the crotalaria did.

Dr. Viets and his coworkers studied the relative ability of 26 different crops to utilize native soil zinc. They classified the plants in three groups.

In the very sensitive group (plants that cannot get enough zinc from the soil) are beans, soybeans, corn, hops, grapes, lima beans, flax, and castor beans.

The mildly sensitive group includes potatoes, tomatoes, onions, alfalfa, sorghum, Sudangrass, sugar beets, and red clover.

Insensitive plants are peppermint, the cereals, peas, asparagus, mustard, carrots, safflower, and grasses.

ZINC DEFICIENCY of trees is known as rosette, mottle leaf, littleleaf, and yellows. The terms tend to describe the usual symptoms of zinc deficiency although such symptoms are not always brought about by too little zinc.

The deficiency in citrus is usually known as mottle leaf. Affected leaves have irregular chlorotic areas between the leaf veins. The leaves may be normal in size in the early stages of the disease, but new leaves as they develop become progressively smaller. Resetting does not usually occur in citrus.

Pecans that get too little zinc have a bronzing of the leaves and rosettes of small leaves on shortened branches. Yellowish mottled areas appear on young leaves. They turn reddish brown on older leaves and may die and cause many small holes in the leaf.

The disease is called little leaf or rosette in deciduous fruit trees, such as apple, cherry, peach, apricot, and plum. Resetting occurs, and dense clusters of small, yellowish leaves grow at the end of twigs that are bare of normal lateral leaves. The internodes are shortened. Little leaf produces small, chlorotic, narrow leaves. Chlorotic mottling progresses inward from the margin in the interveinal tissue. Considerable dieback of the branches occurs in severe stages of little leaf. The number of blossoms and fruits set usually is greatly reduced, and fruits that are produced are of poor quality.

A typical symptom of lack of zinc in most field crops is chlorotic of the inter-veinal tissue, particularly of the lower leaves. The younger leaves in severe cases may also be chlorotic. The lower leaves may turn brown to purple and die. The internodes may become short, and the plants are stunted. In severely affected corn the bud may become almost white.

Zinc deficiency disease usually can be cured by applying zinc to the plants in an available form.

Application of zinc-containing materials to the soil either as a soluble salt or in a chelated form is a common way. The effectiveness of soil applications is determined by how strongly the soil fixes the applied zinc. For that reason the application of soluble zinc salts may not be effective on soils with a high zinc-fixing capacity. Sometimes band placement of the zinc-containing fertilizer has increased its effectiveness.

The use of zinc chelates is an effective way of providing zinc to plants under high soil-fixing conditions. The chelating agent is made up of complex molecules capable of reacting with the zinc and combining it into the complex molecular structure. The zinc therefore is not fixed by the soil and remains in a soluble and available form to the plants. Often applications of chelated zinc have corrected deficiencies where applications of soluble salts, such as zinc sulfate, have not been effective.

Foliar sprays have been effective in correcting zinc deficiency, particularly on tree crops. For citrus a single spray of a solution containing about 5 pounds of zinc sulfate per 100 gallons of water corrected zinc deficiency for 1 to 3 years. Foliage injury is reduced by adding 2 to 3 pounds of hydrated lime, soda lime, or lime sulfur to the spray. Wetting and adhesive agents sometimes have given a beneficial effect at low concentrations. Sprays applied just before flush growth give a longer effect than those applied before dormancy.

Concentrated sprays of 25 pounds of zinc sulfate to 100 gallons of water are effective on apples and pears when applied as a dormant spray just before the buds open. Zinc dusts have been less effective than sprays.

Grapes are usually treated by daubing the freshly cut pruning wound with a solution containing 2 pounds of zinc sulfate to a gallon of solution.

Sometimes, especially on sweet cherries and walnuts, when sprays are not effective, injections of zinc-containing materials are helpful. Zinc-coated nails and pieces of galvanized iron may be driven into the trunk and the main branches. Another means of injecting zinc into the tree has been to bore several holes into the trunk about 3 inches apart and to pack 2 to 3 grams of zinc sulfate into each hole. The holes are then filled with wax.

Field crops usually are fertilized with zinc fertilizers in the row or in bands slightly below and to the side of the seed at the time of planting. Top-dressing and sidedressing with zinc salts after the crop is growing have not been satisfactory. Fertilizer manufacturers in some areas add zinc sulfate to the mixed fertilizer so that farmers can apply the necessary fertilizer elements in one operation without having to mix zinc sulfate with the fertilizer.

Zinc sulfate sprays have also been satisfactory on field crops when the spraying is done early in the season before serious deficiencies arise.