NDA (II) 2016 GAT Q101: A plant having yellow leaves with dead spots has the

Q: What is the answer to NDA (II) 2016 GAT Q101?

A. Correct — A, potassium. The stem gives two symptoms at once, and only one nutrient is the standard textbook answer for the pair. Yellowing of the leaf is chlorosis, the loss of chlorophyll; dead spots are necrosis, patches of tissue that have actually died. Potassium is highly mobile inside the plant, so when supply runs short the plant pulls it out of the older lower leaves to feed the growing tip. Those older leaves first go mottled and yellow, and then the tips and margins scorch into brown dead patches while the rest of the leaf is still yellow — which is exactly the picture the question describes.

NDA (II) 2016 General Ability Test · Question 101 of 150

Question text verified against the official NDA question paper.

A plant having yellow leaves with dead spots has the deficiency of

(a)Potassium
(b)Magnesium
(c)Nitrate
(d)Phosphate

Answer

Why

Correct — A, potassium. The stem gives two symptoms at once, and only one nutrient is the standard textbook answer for the pair. Yellowing of the leaf is chlorosis, the loss of chlorophyll; dead spots are necrosis, patches of tissue that have actually died. Potassium is highly mobile inside the plant, so when supply runs short the plant pulls it out of the older lower leaves to feed the growing tip. Those older leaves first go mottled and yellow, and then the tips and margins scorch into brown dead patches while the rest of the leaf is still yellow — which is exactly the picture the question describes.

Why the others are wrong

(b)Magnesium — The closest rival, and worth taking seriously. Magnesium sits at the centre of every chlorophyll molecule, so its shortage certainly yellows leaves — but the signature pattern is interveinal chlorosis, in which the tissue between the veins turns yellow while the veins themselves stay a clear green. Dead spots are not what the classic magnesium picture leads with; the scorched-margin-plus-yellowing combination is the potassium one.
(c)Nitrate — Nitrogen shortage produces the most familiar yellowing of all, but it is a uniform, even pallor that spreads over the whole of the older leaves, together with stunted growth and thin stems. There is no necrosis in the classic description — the leaf simply fades and, if the shortage continues, drops off intact.
(d)Phosphate — Phosphorus shortage does the opposite of yellowing in its early stages: leaves go dark, dull green and often develop purple or reddish tints from anthocyanin build-up, along with stunting and delayed flowering. A plant short of phosphorus does not present as yellow with dead spots.

Concept

Plants need seventeen essential elements; the three they need in the largest amounts, and the three that fertiliser bags advertise, are nitrogen, phosphorus and potassium. A deficiency shows itself as a visible symptom, and where on the plant the symptom appears depends on how freely the element moves inside it. Mobile elements such as nitrogen, phosphorus, potassium and magnesium are withdrawn from old leaves and sent to young ones, so their deficiency symptoms appear first on the older lower leaves. Immobile elements such as calcium and boron cannot be moved once fixed, so their symptoms appear first on the young leaves at the top.

Read the stem as two clues rather than one. Yellowing alone would not separate the options, because several elements can cause chlorosis; it is the dead spots that do the separating, and the standard pairing of mottled yellowing with scorched, necrotic tips and margins on older leaves is potassium. Be honest about one wrinkle a careful student will notice: the NCERT Class 11 chapter on mineral nutrition lists both potassium and magnesium under chlorosis, and also lists magnesium among the elements whose lack causes necrosis. So the two options overlap on paper. What breaks the tie is the shape of the symptom — magnesium's calling card is the green-veined, yellow-between-the-veins leaf, while potassium's is the yellow leaf with a burnt edge. The key takes the standard potassium description.

Key facts

Chlorosis means loss of chlorophyll and shows as yellowing; necrosis means the death of tissue and shows as brown or grey dead patches.
Potassium, nitrogen, phosphorus and magnesium are mobile within the plant, so their deficiency symptoms appear on the older, lower leaves first.
Calcium and boron are immobile once deposited, so their deficiency symptoms appear on the young leaves and growing points first.
Magnesium is the metal atom at the centre of the chlorophyll molecule, which is why its shortage bleaches the tissue between the veins.

Reading a deficiency off the leaf

Mottled yellowing of older leaves with brown, scorched dead patches at the tips and margins→Potassium — chlorosis and necrosis together, which is the combination this question describes (the key's answer)

Yellow between the veins while the veins themselves stay green, on older leaves→Magnesium — the metal sits at the centre of chlorophyll, so the tissue bleaches but the vascular strands hold their colour

Even, all-over pale yellow-green of the older leaves, with stunted growth→Nitrogen — a uniform fade, with no dead spots

Dark dull green leaves with purplish or reddish tints, stunting, late flowering→Phosphorus — the plant darkens rather than yellows

Yellowing narrows the field; it is the dead spots that pick out potassium.

Study next

Macronutrients and micronutrients — the criteria for calling an element essentialMobile against immobile nutrients, and why that decides where a symptom appearsThe roles of nitrogen, phosphorus and potassium in the plant, and the meaning of the NPK numbers on a fertiliser bagNitrogen fixation — biological, industrial and atmospheric routesHydroponics and the experiments that established which elements are essential

Common traps

Marking magnesium on the strength of yellowing alone, without asking what pattern the yellowing takes.
Forgetting that the stem describes two symptoms; the second one, necrosis, is doing all the work.
Assuming every deficiency symptom starts on new growth — for the mobile nutrients it starts on the old leaves.

NDA and the state services like to hand you one or two symptoms and ask for the missing element, so learn the deficiencies as picture-pairs — which colour change, and where on the plant.

Related PYQs

UPSC_1995_GS1_Q1011995Open & attempt →

Which one of the following is a mixed fertilizer?

(a) CAM
(b) Urea
(c) Ammonium Sulphate
(d) NPK

Answer(d) NPK

The same three primary nutrients seen from the supply side — nitrogen, phosphorus and potassium are exactly the elements whose shortage the NDA item asks you to diagnose from the leaf.

Practice

practice — not a real PYQ
Interveinal chlorosis of older leaves, in which the veins remain green while the tissue between them turns yellow, is the classic symptom of a shortage of
- (a)nitrogen
- (b)magnesium
- (c)phosphorus
- (d)calcium
Answer(b) magnesium — it is the central atom of chlorophyll, so the leaf tissue bleaches while the veins hold their colour.
practice — not a real PYQ
Deficiency symptoms of which one of the following elements appear first on the young leaves and growing tips rather than on the older leaves?
- (a)Nitrogen
- (b)Potassium
- (c)Calcium
- (d)Magnesium
Answer(c) Calcium — it is immobile once deposited in the plant, so it cannot be withdrawn from old leaves to supply new growth.