16. Which of the following is/are trace element(s)?
(A) Mn
(B) P
(C) S
(D) Zn
Trace Elements in Plants Explained: Manganese (Mn) and Zinc (Zn)
Introduction
Mineral nutrition is one of the fundamental topics in plant physiology because plants require a continuous supply of essential mineral elements for normal growth, metabolism, reproduction, and survival. These mineral nutrients are absorbed primarily from the soil through the root system and participate in numerous biochemical and physiological processes, including photosynthesis, respiration, enzyme activation, nucleic acid synthesis, protein synthesis, osmotic regulation, and hormone production.
Based on the quantity required by plants, essential mineral elements are divided into macronutrients and micronutrients (trace elements). Macronutrients are needed in relatively large amounts because they form structural components of cells and participate directly in major metabolic pathways. Micronutrients, also called trace elements, are required only in minute quantities, yet they are equally essential because they function mainly as enzyme cofactors, activators, and regulators of metabolic reactions. Even though their requirement is very small, deficiency of trace elements severely affects plant growth and productivity.
Correct Answer
Correct Answers: (A) Mn and (D) Zn
Detailed Explanation
Trace elements, also known as micronutrients, are mineral nutrients required by plants in extremely small concentrations, generally less than 100 mg per kilogram of dry plant tissue. Despite their low requirement, they are indispensable because they participate in enzyme activation, electron transport, chlorophyll synthesis, antioxidant defense, and numerous metabolic pathways.
Manganese (Mn) is an essential micronutrient involved in photosynthesis, respiration, nitrogen metabolism, and enzyme activation. It is a crucial component of the oxygen-evolving complex (OEC) of Photosystem II, where it plays a central role in splitting water molecules during photosynthesis and releasing oxygen.
Zinc (Zn) is another essential trace element that functions as a structural and catalytic component of numerous enzymes. It is involved in protein synthesis, carbohydrate metabolism, auxin biosynthesis, membrane stability, and gene expression through zinc-finger proteins.
In contrast, phosphorus (P) and sulfur (S) are macronutrients. They are required in much larger quantities because phosphorus forms ATP, nucleic acids, and phospholipids, while sulfur is a constituent of amino acids such as cysteine and methionine, vitamins, and several coenzymes.
Therefore, only Mn and Zn are classified as trace elements.
Explanation of Each Option
Option (A): Manganese (Mn)
This option is correct. Manganese is an essential micronutrient required in trace amounts. It activates numerous enzymes and is indispensable for the oxygen-evolving complex of Photosystem II during photosynthesis.
Option (B): Phosphorus (P)
This option is incorrect. Phosphorus is a primary macronutrient required in relatively large quantities. It forms ATP, DNA, RNA, phospholipids, and many phosphorylated intermediates essential for energy metabolism.
Option (C): Sulfur (S)
This option is incorrect. Sulfur is a secondary macronutrient required for the synthesis of sulfur-containing amino acids, vitamins, and coenzymes. It is not considered a trace element.
Option (D): Zinc (Zn)
This option is correct. Zinc is a micronutrient involved in enzyme activation, protein synthesis, auxin metabolism, membrane integrity, and transcriptional regulation through zinc-finger proteins.
Why Options (A) and (D) are Correct
Manganese and zinc are required only in minute concentrations but perform essential catalytic and regulatory functions in plant metabolism. Since they are needed in trace quantities, they are classified as micronutrients or trace elements.
Why the Other Options are Incorrect
Why Option (B) is Incorrect
Phosphorus is a macronutrient because plants require it in large quantities for energy transfer and nucleic acid synthesis.
Why Option (C) is Incorrect
Sulfur is also a macronutrient because it forms essential amino acids and numerous sulfur-containing biomolecules.
Comparison of All Options
| Option | Element | Classification | Correct or Incorrect |
|---|---|---|---|
| A | Manganese (Mn) | Micronutrient (Trace Element) | Correct |
| B | Phosphorus (P) | Macronutrient | Incorrect |
| C | Sulfur (S) | Macronutrient | Incorrect |
| D | Zinc (Zn) | Micronutrient (Trace Element) | Correct |
Classification of Essential Plant Nutrients
| Category | Elements |
|---|---|
| Primary Macronutrients | Nitrogen (N), Phosphorus (P), Potassium (K) |
| Secondary Macronutrients | Calcium (Ca), Magnesium (Mg), Sulfur (S) |
| Micronutrients (Trace Elements) | Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B), Molybdenum (Mo), Chlorine (Cl), Nickel (Ni) |
Functions of Important Trace Elements
| Trace Element | Major Function |
|---|---|
| Manganese (Mn) | Water splitting in Photosystem II and enzyme activation |
| Zinc (Zn) | Auxin synthesis and enzyme activation |
| Iron (Fe) | Electron transport during photosynthesis and respiration |
| Copper (Cu) | Oxidation-reduction enzymes |
| Molybdenum (Mo) | Nitrate reduction and nitrogen fixation |
Difference Between Macronutrients and Micronutrients
| Feature | Macronutrients | Micronutrients |
|---|---|---|
| Quantity Required | Large amounts | Very small amounts |
| Main Function | Structural and metabolic roles | Enzyme activation and regulation |
| Examples | N, P, K, Ca, Mg, S | Fe, Mn, Zn, Cu, B, Mo, Cl, Ni |
Biological Significance
Although trace elements are required in minute quantities, they are indispensable for plant survival. Manganese ensures efficient photosynthetic oxygen evolution, while zinc activates numerous enzymes involved in protein synthesis, hormone production, and gene regulation. Deficiencies of these micronutrients can result in chlorosis, stunted growth, reduced crop yield, impaired photosynthesis, and decreased resistance to environmental stress. Balanced mineral nutrition is therefore essential for sustainable agriculture and optimum plant productivity.
Final Answer
Correct Answers: (A) Mn and (D) Zn
Manganese (Mn) and Zinc (Zn) are trace elements (micronutrients) because plants require them only in minute quantities for enzyme activation, photosynthesis, hormone synthesis, and various metabolic processes. In contrast, Phosphorus (P) and Sulfur (S) are macronutrients that are needed in much larger amounts.
