Micronutrients: Functions; Deficiency & Toxicity Symptoms (Lecture 16 & 17)
Micronutrients
Micronutrient elements
◦Iron (Fe)
◦Manganese (Mn)
◦Boron (B)
◦Zinc (Zn)
◦Molybdenum (Mo)
◦Copper (Cu)
◦Chlorine (Cl)
Usually supplied by irrigation water and soil
Deficiency and toxicity occur at pH extremes
- Micronutrients
A. Iron (Fe)
– Component of cytochromes (needed for photosynthesis; releases
energy from sugars and starches)
– Essential for N fixation (nitrate reductase) and respiration
Deficiency Symptom:
– Interveinal chlorosis on new growth 
– Fe is immobile
– Iron chlorosis develops when soil pH is high
– Leaves yellowish or white
(young leaves first)
– affected leaves curl up
Remedy for iron chlorosis:
1) Use iron chelates
FeEDTA – Stable at pH < 7.0
FeEDDHA – Stable even when pH > 7.0
2) Lower soil pH
Iron is in more useful form (Fe2+)
B. Manganese (Mn)
– Required for chlorophyll synthesis,
– Serves as an activator for enzymes in growth processes
– Assist iron in chlorophyll formation,
– Generally required with zinc in foliar spraying of citrus.
–Deficiency:
Mottled chlorosis between main veins of new leaves 
(Mn is immobile), similar to Fe chlorosis
–Toxicity:
Chlorosis on new growth with small,
numerous dark spots
Deficiency occurs at high pH
Toxicity occurs at low pH
– Fertilizers:
Manganese sulfate (MnSO4)
Mn EDTA (chelate) for high pH soils
C. Boron (B)
– Involved in carbohydrate metabolism
– Essential for flowering, pollen germination, N
metabolism
– Aids in the assimilation of calcium; amount 
required is extremely small
– Deficiency:
-New growth distorted and malformed,
-roots tubers distorted
-Death of terminal growth, causing lateral buds to develop
and produce a “witches’ broom” effect.
-Thickened, curled, wilted and chlorotic leaves.
-spots in fruit or tubers.
-Reduced flowering or improper pollination.
– Toxicity:
Twig die back
Fruit splitting
Leaf edge burns
– Fertilizers:
Borax (Na2B4O710H2O)
Calcium borate (NaB4O7 4H2O)
D. Zinc (Zn)
-Involved in protein synthesis
-An essential constituent of several enzymes
-Controls synthesis of indole acetic acid – an important growth regulator.
-The micronutrient most often needed by – trees, grapes, beans, onions, tomatoes, cotton & rice.
– Deficiency
-Decreased stem length and rosetting of terminal leaves. 
-Reduced fruit bud formation
-Mottled leaves and stripping of corn leaves.
-Growth suppression- reduced internode lengths,
-Interveinal chlorosis on young leaves (Zn is immobile in tissues)
– Toxicity:
(occurs at low pH) Growth reduction, leaf chlorosis
E. Molybdenum (Mo)
– Required for nitrate reductase activity, vitamin synthesis
Nitrate reductase
NO3– ————————————— NH2
Mo
Root-nodule bacteria also requires Mo
Legumes cannot fix atmospheric N symbiotically without Mb.
–Deficiency:
-very similar to N deficiency due to the key role Mb plays in N utilization
– Pale green,
-Marginal cupping and scorching of leaves. (Mo is immobile) 
-Strap leaf in broad leaf plants
-Stunting and lack of vigor
-Whiptail in cauliflower and yellow spotting in citrus
Occurs at low pH
–Toxicity:
Chlorosis with orange color pigmentation
–Fertilizer:
-Sodium molybdate
F. Copper (Cu)
– Essential component of several enzymes of chlorophyll synthesis,
– Carbohydrate metabolism
– Promotes formation of Vitamin A
–Deficiency:
– Rosette or ‘witch’s broom’ 
– Stunted growth
– dieback of terminal shoots in trees
– poor pigmentation
– wilting and eventual death of leaf tips
– formation of gum pockets
around central pith in oranges
–Toxicity:
-Chlorosis
–Fertilizers:
-Copper sulfate (CuSO4)
G. Chlorine (Cl)
–Absorbed as Cl- along with sodium and potassium ions.
–Maintains solute concentration and anion cation balance in the cell
–Required in photosynthetic reactions of plants
– Deficiency:
– Normally not existing; is not seen in the field due to its
universal presence in nature (Only experimentally induced)
– Symptoms:
– Wilting, followed by chlorosis 
– Excessive branching of lateral roots
– Bronzing of leaves
– Chlorosis and necrosis in tomatoes and barley
– Toxicity:
–Leaf margin chlorosis, necrosis on all leaves
– Fertilizer:
–Never applied
(Cl- is ubiquitous!)
Roots and mineral nutrient acquisition
The uptake of nutrients occurs at both the roots 
and the leaves.
◦Roots provide large surface area for nutrient uptake, through mycorrhizae
and root hairs, absorb water
and minerals from the soil.
◦Carbon dioxide diffuses into
leaves from the surrounding
air through stomata.
Roots are able to absorb minerals somewhat selectively, enabling the plant to accumulate essential elements that may be present in low concentrations in the soil.
◦However, the minerals in a plant reflect the composition of the soil in which the plant is growing.
◦Therefore, some of the elements in a plant are merely present, while others are essential.
Depletion zones – regions of lower
nutrient concentration -develop
around roots
Once all the nutrients have been used up, this area becomes depleted of nutrients that is known as the “Zone of Depletion” and the plant becomes incapable of obtaining food unless it receives an application of fertilizer or it forms an association with mycorrhizae fungi.
With this association, the hyphae will extend far beyond the zone of depletion and obtain moisture and nutrients unlike non mycorrhizal plants.
Non mycorrhizal plants include many weed species that will naturally be outcompeted.
The mycorrhizal native species will then establish and grow faster.
- Roots and mineral nutrient acquisition
Fine roots and root hairs “mine” the soil for nutrients.
Mycorrhizal hyphae do this even better.
Mycorrhizae and Plant Nutrition
Mycorrhizae: are modified roots consisting of mutualistic associations of fungi and roots
The fungus benefits from a steady supply of sugar donated by the host plant
In return, the fungus increases the surface area of water uptake and mineral absorption and supplies water and minerals to the host plant
Agricultural importance: Farmers and foresters often inoculate seeds with spores of mycorrhizal fungi to promote the formation of mycorrhizae
Ectomycorrhizae
In ectomycorrhizae the mycelium of the fungus forms a dense sheath over the surface of the root
A Ectomycorrhizae. The mantle of the fungal mycelium ensheathes the root. Fungal hyphae extend from the mantle into the soil, absorbing water and minerals, especially phosphate. Hyphae also extend into the extracellular spaces of the root cortex, providing extensive surface area for nutrient exchange between the fungus and its host plant.
Endomycorrhizae
In endomycorrhizae the microscopic fungal hyphae extend into the root
B Endomycorrhizae. No mantle forms around the root, but microscopic fungal hyphae extend into the root. Within the root cortex, the fungus makes extensive contact with the plant through branching of hyphae that form arbuscules, providing an enormous surface area for nutrient swapping. The hyphae penetrate the cell walls, but not the plasma membranes, of cells within the cortex.
