Functions of 5 Micronutrients in Horticultural Plants
Micronutrients are essential elements required by plants in very small quantities (usually less than 100 mg/kg of dry matter), but they play a crucial role in plant growth, development, and productivity. In horticultural crops like fruits, vegetables, and ornamental plants, a deficiency or excess of any micronutrient can severely affect yield and quality. Below are the functions of five important micronutrients in horticultural plants:
1. Iron (Fe)
Functions:
- Iron is essential for chlorophyll synthesis, although it is not a part of the chlorophyll molecule itself. Without iron, plants show symptoms of chlorosis (yellowing) especially in young leaves.
- It plays a key role in the electron transport chain during photosynthesis and respiration.
- Iron is a component of enzymes and proteins like cytochromes and ferredoxin, which are involved in energy transfer.
- It assists in nitrogen fixation in leguminous plants by supporting the nitrogenase enzyme activity.
Deficiency Symptoms:
- Interveinal chlorosis (yellowing between leaf veins), starting from the younger leaves.
- Poor growth and development, especially in fruiting plants like guava and citrus.
2. Zinc (Zn)
Functions:
- Zinc is a co-factor in over 300 enzymatic reactions, especially those related to protein and carbohydrate metabolism.
- It is crucial for the synthesis of auxin, a growth hormone, hence supports shoot and root growth.
- Zinc activates enzymes that regulate gene expression and protein synthesis.
- It is involved in the formation of chlorophyll and some carbohydrates.
Deficiency Symptoms:
- Shortened internodes, causing a rosette appearance.
- Leaf chlorosis, bronzing, and necrotic spots, especially in mango, citrus, and grapes.
- Poor fruit setting and malformed fruits.
3. Manganese (Mn)
Functions:
- Manganese is essential for photosynthesis, especially in the photolysis (splitting) of water molecules during the light reaction.
- It activates various enzymes involved in nitrogen metabolism and the synthesis of some amino acids.
- Helps in pollen germination and growth of pollen tubes, thereby contributing to reproductive development.
- It also participates in lignin synthesis, which strengthens plant cell walls.
Deficiency Symptoms:
- Interveinal chlorosis in younger leaves, similar to iron deficiency but with grey or tan specks.
- Weak stems and delayed flowering, common in vegetables like beans and tomatoes.
4. Boron (B)
Functions:
- Boron is important for cell wall formation and stability.
- It plays a vital role in carbohydrate transport within the plant, particularly from leaves to growing parts like fruits and roots.
- Essential for flowering, pollen germination, and fruit and seed development.
- Boron regulates membrane integrity and hormonal activity, influencing plant growth.
Deficiency Symptoms:
- Cracking of fruits (especially tomatoes and apples), and hollow stems.
- Deformed and brittle leaves, flower abortion, and poor fruit set.
- “Heart rot” in beets and “brown heart” in turnips.
5. Copper (Cu)
Functions:
- Copper is a component of several enzymes such as polyphenol oxidase, which are involved in lignin synthesis and defense against pathogens.
- It is important for photosynthesis (component of plastocyanin protein) and respiration (cytochrome oxidase enzyme).
- Supports reproductive growth, particularly in pollen viability and seed formation.
- Enhances disease resistance in plants.
Deficiency Symptoms:
- Wilting of leaf tips, chlorosis, and necrotic areas on young leaves.
- Dieback of shoots in fruit trees like citrus and apple.
- Poor flower and fruit development.
Conclusion
Micronutrients, though required in minute quantities, are indispensable for the optimal growth and productivity of horticultural plants. Their balanced supply ensures proper metabolic functions, better quality produce, enhanced resistance to diseases, and improved yields. In the Indian context, many horticultural soils suffer from micronutrient deficiencies, especially zinc, iron, and boron, which need to be corrected through soil or foliar application to sustain healthy crop production. Regular soil testing and use of micronutrient-enriched fertilizers can help maintain the desired balance and support horticultural development.