Q.54 Glyphosate is a broad spectrum herbicide. Upon application to leaves, it is translocated to
meristematic areas and underground rhizomes by
(A) phloem (B) xylem (C) border pits (D) tracheids

Glyphosate translocates to meristematic areas and underground rhizomes primarily through the phloem after foliar application. This systemic movement enables its effectiveness as a broad-spectrum herbicide by targeting growing tissues. The correct answer is (A) phloem.

Option Analysis

Glyphosate enters leaves via the cuticle and apoplast, then loads into phloem for transport to sinks like meristems and rhizomes.​
Phloem conducts organic solutes downward or bidirectionally via pressure flow, matching glyphosate’s path to roots and shoots.​
Xylem moves water upward unidirectionally, unsuitable for foliar-applied herbicides reaching underground parts.​
Border pits are xylem structures regulating water flow between tracheids, not involved in herbicide translocation.​
Tracheids form xylem conduits for water, lacking role in systemic organic transport like glyphosate.​

Introduction
Glyphosate translocation in plants occurs primarily through phloem after leaf application, reaching meristematic areas and underground rhizomes effectively. This broad spectrum herbicide’s systemic action inhibits EPSPS in the shikimate pathway at growth sites. Understanding phloem vs xylem transport aids CSIR NET aspirants in mastering herbicide mechanisms.​

Translocation Mechanism

Glyphosate absorbs into leaves, moves symplastically into phloem sieve tubes, and travels source-to-sink under pressure flow. Up to 70% relocates from treated leaves to roots and apices within 48-72 hours before self-limiting. This enables control of perennials with rhizomes.​

Role of Vascular Tissues

• Phloem: Living tissue transports sugars and systemic herbicides like glyphosate bidirectionally to sinks.​

• Xylem: Dead tissue conducts water/minerals upward from roots; minimal foliar herbicide role.​

• Border pits: Valve-like xylem features prevent embolism, irrelevant to solute translocation.​

• Tracheids: Xylem cells for water conduction, not organic herbicide movement.​