Q.51 Which of the following summarizes the role of Casparian strip in transport of water in the root?
(A) Symplast to Apoplast
(B) Apoplast to Symplast
(C) Phloem to Xylem
(D) Xylem to Phloem
The Casparian strip forces water movement from the apoplast pathway to the symplast pathway in plant roots, acting as a selective barrier in the endodermis.
Correct Answer
Option (B) Apoplast to Symplast is correct. The Casparian strip, a suberin-impregnated band in endodermal cell walls, blocks apoplastic water flow through cell walls and intercellular spaces, compelling water and solutes to cross endodermal cell membranes into the symplast for regulated entry into the vascular tissue.
Apoplast Pathway
Water moves freely via the apoplast—through cell walls and spaces—until reaching the endodermis, where the Casparian strip halts this passive route. This prevents unregulated solute entry from soil.
Symplast Pathway
The symplast involves cytoplasm-to-cytoplasm movement via plasmodesmata; post-Casparian strip, water enters endodermal cells selectively via membrane transport. Endodermal cells then release contents into the stele symplast.
Why Other Options Fail
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(A) Symplast to Apoplast: Incorrect, as the strip blocks exit from symplast back to apoplast, not vice versa.
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(C) Phloem to Xylem: Wrong; this involves no root water uptake and ignores Casparian role in radial root flow.
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(D) Xylem to Phloem: Incorrect; xylem carries water upward, phloem transports sugars bidirectionally—neither relates to root apoplastic block.
| Option | Pathway Direction | Matches Casparian Function? |
|---|---|---|
| A | Symplast → Apoplast | No |
| B | Apoplast → Symplast | Yes |
| C | Phloem → Xylem | No |
| D | Xylem → Phloem | No |
