Q.How many elements have been found to be essential for plants?

17 elements are currently recognised as essential for normal plant growth and development.

An element is called 'essential' only if it satisfies Arnon and Stout's criteria: the plant cannot complete its life cycle without it, its function cannot be replaced by another element, and it is directly involved in the plant's metabolism. Based on these criteria, 17 elements are classified as essential -- 6 macronutrients needed in large amounts already known from earlier study (C, H, O, N, P, K, and also S, Ca, Mg making 9 macronutrients in total) and micr …

The mismatch is Zn linked to ferredoxin -- ferredoxin actually contains iron, not zinc.

Checking each option against known mineral element functions:

• K (Potassium) - stomatal movement: CORRECT. K+ ion flux into/out of guard cells drives their turgor changes, opening and closing stomata.
• Mo (Molybdenum) - Nitrate reductase: CORRECT. Molybdenum is an essential metal cofactor of the enzyme nitrate reductase, which reduces nitrate to nitrite during nitrogen assimilation.
• B (Boron) - translocation of sugars: CORRECT. Boron is known to be involved in the translocation of sugars, and in cell elongation, pollen germination, and cell wall synthesis. …

Essential elements are divided into macronutrients (needed in larger quantities, e.g. N, P, K, Ca, Mg, S) and micronutrients/trace elements (needed in very small quantities, e.g. Fe, Mn, Zn, Cu, B, Mo, Cl, Ni). Nitrogen is a macronutrient.

Plants require nitrogen in comparatively large amounts because it is a core component of amino acids/proteins, nucleic acids, and chlorophyll -- this places it firmly among the macronutrients, alongside phosphorus, potassium, calcium, magnesium and sulphur. Copper, zinc, and iron, by contrast, are needed in trace amounts, mainly as cofactors for enzymes (e.g. Fe in cytochromes, Cu in plastocyanin, Zn in several dehydrogenases), so they are classed as micronutrients.

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