Practice important
Selective herbicidal action
MCQ questions and answers.
Page 1
of
1
|
10
MCQ Questions
1. A cereal field is infested with broad-leaved dicot weeds. Which treatment would most likely remove the weeds while causing comparatively little damage to the cereal crop?
A.
A dilute application of 2,4-D
B.
A concentrated application of indole-3-acetic acid
C.
A dilute application of gibberellic acid
D.
A concentrated application of cytokinin
Correct Answer:
A dilute application of 2,4-D
Explanation:
2,4-D is a synthetic auxin used as a selective herbicide. At suitable concentrations, it is more toxic to broad-leaved dicot weeds than to many monocot cereal crops.
2. The selectivity of 2,4-D in a wheat field is best explained by the fact that:
A.
Wheat lacks all auxin receptors, whereas dicots possess them
B.
Dicot weeds generally show greater physiological sensitivity to abnormal auxin levels than wheat
C.
2,4-D is absorbed only by dicot roots and never by wheat roots
D.
Wheat rapidly converts 2,4-D into a growth-promoting natural auxin
Correct Answer:
Dicot weeds generally show greater physiological sensitivity to abnormal auxin levels than wheat
Explanation:
Selective herbicidal action does not mean that wheat is completely unaffected. Rather, dicot weeds are generally more sensitive to synthetic auxins such as 2,4-D, so an appropriate dose damages them more severely than the cereal crop.
3. If the concentration of 2,4-D applied to a cereal crop is increased far beyond the recommended selective dose, the most likely result is:
A.
Only dicot weeds are affected, regardless of the dose
B.
The herbicide loses all auxin-like activity
C.
The cereal crop may also suffer injury or die
D.
The cereal crop becomes permanently resistant to auxin
Correct Answer:
The cereal crop may also suffer injury or die
Explanation:
Selectivity depends on dose as well as differential sensitivity. Excessive 2,4-D can produce toxic auxin-like effects in the crop itself and may therefore destroy its selectivity.
4. Which field situation would provide the strongest basis for using 2,4-D as a selective herbicide?
A.
A rice field containing broad-leaved weeds
B.
A wheat field containing only grass weeds
C.
A maize field containing only other monocot weeds
D.
A pasture containing only broad-leaved forage plants
Correct Answer:
A rice field containing broad-leaved weeds
Explanation:
Rice is a monocot crop, while many troublesome broad-leaved weeds are dicots. A carefully controlled 2,4-D treatment can selectively suppress the dicot weeds while relatively sparing the rice.
5. A farmer applies 2,4-D to a crop containing both wheat and broad-leaved weeds. Which observation would best indicate successful selective herbicidal action?
A.
Both wheat and weeds show equal leaf distortion
B.
The weeds show abnormal twisting and dieback, while wheat growth remains comparatively normal
C.
Wheat shows rapid flowering, while the weeds remain completely unchanged
D.
The weeds increase root growth, while wheat leaves lose chlorophyll immediately
Correct Answer:
The weeds show abnormal twisting and dieback, while wheat growth remains comparatively normal
Explanation:
Synthetic auxins cause uncontrolled and abnormal growth in sensitive dicot weeds, leading to twisting, vascular disruption, and eventual death. Relative preservation of wheat indicates selectivity.
6. Why should spraying of 2,4-D near a sensitive broad-leaved crop be avoided even when the intended target is a weed?
A.
2,4-D selectively kills only monocots near the sprayed field
B.
Vapour or spray drift can expose the non-target broad-leaved crop to a damaging auxin-like dose
C.
2,4-D becomes a nutrient when it contacts crop leaves
D.
Broad-leaved crops are completely insensitive to synthetic auxins at all concentrations
Correct Answer:
Vapour or spray drift can expose the non-target broad-leaved crop to a damaging auxin-like dose
Explanation:
Selectivity is relative, not absolute. A sensitive broad-leaved crop may be injured by accidental contact or drift because synthetic auxins can disturb its growth even when used selectively in a cereal field.
7. Which combination correctly identifies the herbicide, its principal selective target, and the crop group it can relatively spare?
A.
2,4-D; broad-leaved dicot weeds; cereal monocots
B.
2,4-D; cereal weeds; broad-leaved dicot crops
C.
IAA; all weeds equally; all crop plants
D.
Ethylene; broad-leaved weeds; all monocots without exception
Explanation:
2,4-D is a synthetic auxin herbicide commonly used against broad-leaved dicot weeds in cereal crops, which are monocots and are comparatively more tolerant at recommended doses.
8. Two plots receive the same 2,4-D treatment. Plot X contains wheat with broad-leaved weeds, whereas Plot Y contains cabbage with broad-leaved weeds. Why is greater crop damage expected in Plot Y?
A.
Cabbage is a broad-leaved dicot and is generally more sensitive to 2,4-D than wheat
B.
Cabbage cannot absorb chemicals through its leaves
C.
Wheat is a dicot and cabbage is a monocot
D.
2,4-D functions only as a nutrient in cabbage
Correct Answer:
Cabbage is a broad-leaved dicot and is generally more sensitive to 2,4-D than wheat
Explanation:
The usefulness of 2,4-D depends on a sensitivity difference between the weed and crop. A broad-leaved dicot crop such as cabbage may be damaged along with the weeds, whereas wheat is comparatively tolerant.
9. A synthetic auxin is applied at a dose that kills dicot weeds but leaves a cereal crop alive. Which conclusion is most justified?
A.
The herbicide is chemically incapable of entering cereal tissues
B.
The cereal crop has absolute immunity to auxin analogues
C.
The dose lies within a range where differential sensitivity produces selectivity
D.
The weeds were killed because auxin promotes their normal development
Correct Answer:
The dose lies within a range where differential sensitivity produces selectivity
Explanation:
Selective herbicidal action results from applying a concentration at which sensitive dicot weeds undergo harmful auxin-like effects while the cereal crop tolerates the treatment comparatively well.
10. Which change would most likely reduce the selective advantage of 2,4-D in a cereal field?
A.
Applying it at the recommended dose during suitable weed growth
B.
Targeting actively growing broad-leaved weeds
C.
Increasing the dose until the cereal receives a toxic auxin-like exposure
D.
Preventing spray contact with the cereal crop as far as possible
Correct Answer:
Increasing the dose until the cereal receives a toxic auxin-like exposure
Explanation:
Selective action depends on maintaining a dose that exploits the greater sensitivity of dicot weeds. Excessive application can injure the cereal and thereby reduce or eliminate selectivity.
Short Questions and Answers
Practice short questions and answers related to
Selective herbicidal action
.
Short questions are currently unavailable for this page.