Micronutrients play a critical role in citrus production and tree health

While a lot of attention has been focused on foliar nutrition as a means of promoting tree health in the face of HLB, many growers are learning that application of key micronutrients makes sense even if by some miracle the disease disappeared.

In what may be the irony of all ironies, some farmers have even claimed the onset of HLB has made them even better growers. “To paraphrase a quote I’ve heard from some growers is, ‘Yes, HLB is a bad deal, but in the process we’ve really learned how to grow citrus,”‘ says product representative for KeyPlex. “Some of these guys have reported yield increases as high as 30% because they have initiated an aggressive foliar nutrition campaign.”

While citrus trees require a firm macronutrient foundation, a strong compliment of micronutrients will enhance yields and tree health. It has been shown that most micronutrients (except iron) are more effective when applied in foliar form.

So what exactly do key micronutrients do in citrus trees? Here is a rundown of the biology of micronutrients in citrus.

Functions of iron (Fe)

• Catalyzes the production of chlorophyll.
• Involved in some respiratory and photosynthetic enzyme systems.
• Involved in the reduction of nitrates and sulfates.

Functions of zinc (Zn)

• Involved in plant carbon metabolism.
• A necessary component of several enzyme systems that regulate various metabolic activities within plants.
• Part of an enzyme that regulates the equilibrium among carbon dioxide, water, and carbonic acid.
• Part of two enzymes that play a role in protein metabolism.
• Essential for the formation of chlorophyll and function of normal photosynthesis.
• Needed to form auxins, which are growth-promoting substances in plants.
• Associated with water relations in plants and improves water uptake.

Functions of manganese (Mn)

• Involved in the production of amino acids and proteins.
• An activator of several enzymes.
• Plays an essential role in respiration and nitrogen (N) metabolism.
• Necessary for the reduction of nitrates and helps make them usable by plants.
• Plays a role in photosynthesis and in the formation of chlorophyll.

Functions of boron (B)

• Important in sugar translocation and carbohydrate metabolism.
• Particularly needed at the location of active cell division.
• Plays an important role in flowering, pollen-tube growth, fruiting processes, N metabolism, and hormone activity.
• Maintains calcium (Ca) in a soluble form, thus ensuring its proper utilization.
• Deficiencies may be aggravated by severe drought conditions, heavy lime applications, or irrigation with alkaline water.

Functions of copper (Cu)

• Part of several enzyme systems.
• Has a role in photosynthesis and chlorophyll formation.
• May have an important function in root metabolism. (Cu appears to be concentrated more in the rootlets of plants than in leaves or other tissues. Cu in citrus fibrous roots may be five to 10 times greater than in leaves.)
• Regulates several biochemical processes within the plant.
• Important in the utilization of proteins in the growth processes of plants. (The photosynthesis rate of Cu-deficient plants is abnormally low.)
• May also be involved in oxidation-reduction reactions in plants.
• Heavy fertilization with N tends to increase the severity of Cu deficiency.

Functions of molybdenum (Mo)

• Assists in the formation of plant proteins.
• Helps starch, amino acid, and vitamin formation.
• Considered a catalyst that aids the conversion of gaseous N to usable forms by nitrogen-fixing microorganisms.
• A constituent of the plant enzyme that converts nitrate to ammonia.

Functions of chlorine (Cl)

Although the essentiality of Cl has been established for higher plants, its need for fruit crops has not yet been demonstrated. The plant requirement for Cl is quite high as compared with other micronutrients, but its exact role in plant metabolism is still obscure. Chlorine is:

• Associated with turgor in the guard cells through the osmotic pressure exerted by imported potassium (K) ions.
• Involved with oxygen production in photosynthesis.
• Involved in chlorophyll and photosynthesis because its deficiency causes chlorosis, necrosis, unusual bronze discoloration of foliage, and reduction in growth.

Functions of nickel (Ni)

Within the last decade, Ni has been established as an essential element in higher plants. Although well-defined enzymatic functions are known to be associated with Ni in legumes, apparently the need for Ni exists in other plants as well. Nickel deficiency in soil-grown plants has not been observed.

Source: Information on micronutrient functions was taken from “Plant Nutrients For Citrus Trees” by Mongi Zekri, UF/IFAS, and Thomas Obreza, UF/IFAS.