Part 7 — Critical & Complex NEET-style Questions (Q151–175)

Q151.

If a tomato plant growing in saline soil shows wilting, the primary cause is:
A. Excess transpiration
B. Low root hair density
C. Soil water potential lower than root water potential ✅
D. Excessive photosynthesis

Explanation:

A. May aggravate but not the root cause.
B. Reduces absorption but not main factor here.
C. (Correct) Saline soils → ψsoil becomes very negative, water moves out of roots → plasmolysis/wilting.
D. Photosynthesis not directly related.

Q152.

Stomata close at midday in xerophytes because:
A. Leaf temperature is low
B. ABA levels rise under water stress ✅
C. Light intensity is low
D. Xylem sap sugar content decreases

Explanation:

A. Midday temp is high, not low.
B. (Correct) Water stress → ABA accumulation → stomatal closure.
C. Midday light is high.
D. Phloem sugar content unrelated.

Q153.

Which process is common to both transpiration pull and phloem transport?
A. Cohesion of water molecules
B. Bulk flow driven by pressure gradient ✅
C. Capillarity
D. Active pumping by xylem

Explanation:

A. Cohesion applies to xylem, not phloem.
B. (Correct) Both involve bulk flow due to pressure differences (negative in xylem, positive in phloem).
C. Capillarity minor in xylem only.
D. Xylem has no pumping.

Q154.

Phloem transport is bidirectional because:
A. Sieve tubes are dead
B. Source-sink relationship changes ✅
C. Root pressure drives flow both ways
D. Casparian strip allows reversal

Explanation:

A. Sieve tubes are living, not dead.
B. (Correct) Depending on developmental stage, different tissues act as source or sink → flow in either direction.
C/D. Incorrect.

Q155.

Which mineral deficiency reduces both chlorophyll formation and nitrate reduction?
A. Magnesium
B. Iron ✅
C. Potassium
D. Calcium

Explanation:

A. Mg = central in chlorophyll, but not for nitrate reductase.
B. (Correct) Iron is required for ferredoxin, cytochromes, and nitrate reductase.
C. K regulates stomata.
D. Ca structural, not nitrate metabolism.

Q156.

A tree is girdled (phloem removed) but watered regularly. Which region dies first?
A. Region above girdle
B. Region below girdle ✅
C. Entire plant simultaneously
D. Root cap

Explanation:

A. Above girdle food accumulates, remains alive.
B. (Correct) Below girdle, roots are cut off from sugars → starve, die first.
C/D. Not accurate compared to flow direction.

Q157.

If stomata are experimentally glued shut, which process declines most rapidly?
A. Respiration
B. Photosynthesis ✅
C. Protein synthesis
D. Mineral uptake

Explanation:

A. Respiration uses internal O₂.
B. (Correct) Closed stomata → no CO₂ entry → photosynthesis drops first.
C. Depends indirectly.
D. Roots continue mineral uptake.

Q158.

If xylem vessels are blocked by air embolism, ascent of sap is disrupted because:
A. Root pressure fails
B. Cohesion-tension mechanism requires continuous water column ✅
C. Companion cells fail
D. Endodermis becomes impermeable

Explanation:

A. Root pressure minor.
B. (Correct) Air bubble breaks continuity of column → tension not transmitted.
C. Companion cells → phloem, not xylem.
D. Endodermis role is at root entry, not embolism.

Q159.

In a pressure chamber (Scholander bomb), when balancing pressure is applied, it measures:
A. Transpiration rate
B. Water potential of xylem sap ✅
C. Root pressure
D. Soil water potential

Explanation:

A. Potometer, not bomb.
B. (Correct) Pressure equal to negative ψxylem balances sap exudation.
C. Not measured here.
D. Soil ψ measured differently.

Q160.

Which adaptation reduces transpiration rate in desert plants?
A. Thin cuticle
B. Stomata on upper leaf
C. Sunken stomata and reduced leaf area ✅
D. High stomatal density

Explanation:

A/B/D. Increase transpiration.
C. (Correct) Desert xerophytes conserve water via these adaptations.

Q161.

Why is sucrose the main transport sugar in phloem instead of glucose?
A. It is insoluble
B. It is non-reducing and more stable ✅
C. It requires less ATP
D. It is produced in mitochondria

Explanation:

A. It is soluble.
B. (Correct) Sucrose is chemically stable, non-reducing, soluble → ideal for long-distance transport.
C/D. Incorrect.

Q162.

Permanent wilting occurs when:
A. Root pressure is high
B. ψsoil < ψplant and cannot be reversed ✅
C. Transpiration ceases
D. Photosynthesis is maximum

Explanation:

A. Opposite case.
B. (Correct) If soil ψ becomes too negative, roots cannot absorb → permanent wilting.
C/D. Not defining condition.

Q163.

If a phloem-feeding insect (aphid) is removed while still attached to phloem, sap continues to exude. This proves:
A. Root pressure drives phloem flow
B. Phloem is under positive hydrostatic pressure ✅
C. Phloem is under tension
D. Xylem pushes sugars

Explanation:

A. Not root pressure.
B. (Correct) Exudation shows positive pressure.
C. Tension is in xylem, not phloem.
D. Wrong tissue.

Q164.

Why does guttation occur mainly at night in some plants?
A. Root pressure is high when transpiration is low ✅
B. ABA levels are high
C. Stomata remain closed
D. Photosynthesis stops

Explanation:

A. (Correct) Low transpiration at night → root pressure pushes liquid water via hydathodes.
B-D. Not primary cause.

Q165.

Which nutrient deficiency causes death of root tips due to impaired cell wall development?
A. Potassium
B. Calcium ✅
C. Nitrogen
D. Iron

Explanation:

A. Marginal necrosis.
B. (Correct) Ca is structural and immobile, deficiency → meristem/root tip death.
C/D. Cause other symptoms.

Q166.

Which force directly drives movement of sugars from phloem to sink tissues?
A. Root pressure
B. Pressure gradient between source and sink ✅
C. Capillarity
D. Active water pumping

Explanation:

A. Xylem force.
B. (Correct) Source high pressure, sink low pressure → mass flow.
C/D. Not relevant.

Q167.

A sunflower plant shows drooping at midday but recovers at night. This is:
A. Permanent wilting
B. Temporary wilting ✅
C. Plasmolysis
D. Hydration

Explanation:

A. Permanent wilting = irreversible.
B. (Correct) Water loss > absorption during day, but recovers later.
C. Plasmolysis is cell-level phenomenon.
D. Not correct term.

Q168.

When sucrose is actively loaded into sieve tubes, what immediate effect occurs?
A. Water potential in sieve tube increases
B. Water enters sieve tube osmotically ✅
C. Pressure potential decreases
D. Xylem water potential rises

Explanation:

A. ψ decreases, not increases.
B. (Correct) Lower ψ in sieve → water entry.
C. Pressure increases, not decreases.
D. Localized, not systemic.

Q169.

If all xylem of a young herbaceous plant is removed (ring experiment), the plant:
A. Survives normally
B. Dies due to no upward water supply ✅
C. Accumulates sugars above cut
D. Shows root death first

Explanation:

A. Impossible.
B. (Correct) Without xylem, no water → wilting, death.
C. Sugars accumulate when phloem removed, not xylem.
D. Whole plant suffers.

Q170.

The transport proteins in plant membranes responsible for ion gradients are:
A. Proton pumps (H⁺-ATPases) ✅
B. Sodium pumps
C. Aquaporins only
D. Potassium channels

Explanation:

A. (Correct) Proton pumps establish gradients used for active transport.
B. Plants don’t use Na⁺ pumps like animals.
C. Water transport only.
D. K⁺ channels facilitate, not create gradients.

Q171.

Water potential gradient in a transpiring plant follows:
A. Air < leaf < stem < root < soil ✅
B. Soil < root < stem < leaf < air
C. Root < soil < stem < air < leaf
D. Stem < leaf < root < air

Explanation:

A. (Correct) Most negative in air, least in soil → gradient drives upward flow.
B-D. Wrong order.

Q172.

Which of the following can act both as source and sink depending on plant stage?
A. Root only
B. Mature leaf only
C. Storage organ like tuber ✅
D. Flowers

Explanation:

A. Roots usually sink.
B. Mature leaves are sources only.
C. (Correct) Tubers store (sink) when young, remobilize (source) during sprouting.
D. Flowers are sinks only.

Q173.

In CAM plants, transpiration is minimized because:
A. Stomata close at night
B. Stomata open at night ✅
C. Thick cuticle only
D. Absence of guard cells

Explanation:

A. Incorrect (they open at night).
B. (Correct) Night stomatal opening conserves water.
C. Additional adaptation but not main.
D. Guard cells present.

Q174.

Which condition will most severely reduce translocation in phloem?
A. Low oxygen supply to roots ✅
B. Reduced transpiration
C. Low cuticle thickness
D. Increased guttation

Explanation:

A. (Correct) Phloem loading/unloading require ATP → low O₂ inhibits respiration, reduces ATP, halting transport.
B-D. Affect xylem or transpiration, not phloem directly.

Q175.

Which one links transpiration, mineral uptake, and photosynthesis together?
A. Root pressure
B. Opening of stomata ✅
C. Casparian strip
D. Capillarity

Explanation:

A. Root pressure minor.
B. (Correct) Stomatal opening → CO₂ entry (photosynthesis), water loss (transpiration), mineral flow (transpiration stream).
C/D. Not integrative.

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Part 7 — Critical & Complex NEET-style Questions (Q151–175)