Q.33 Which of the following is a primary sugar source in translocation of phloem sap.
- Young leaf
- Mature leaf
- Young fruit
- Shoot tips
Mature leaf is the primary sugar source in translocation of phloem sap (Option B).
In phloem translocation, sugars like sucrose produced via photosynthesis move from source organs (high sugar concentration) to sink organs (growth/storage sites) through pressure-flow mechanism. Mature leaves export the bulk of this sap after peaking photosynthesis.
Option Breakdown
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Young leaf: Net importer/sink; imports sugars for its own expansion/growth, not yet exporting to phloem network.
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Mature leaf: Correct. Fully expanded leaves at peak photosynthesis act as primary sources, loading sucrose into phloem sieve tubes for bulk flow.
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Young fruit: Sink organ; consumes imported sugars for development, creating low pressure to pull sap.
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Shoot tips: Active sinks for meristematic growth/apical dominance, relying on imported photoassimilates.
The primary sugar source in translocation of phloem sap is the mature leaf, where photosynthesis peaks to load sucrose into sieve elements. This drives bulk flow from sources to sinks like roots/fruits in the pressure-flow model.
Phloem Loading Basics
Mature source leaves actively transport sucrose (via SUT transporters) into companion cells/sieve tubes, lowering water potential and drawing xylem water for turgor-driven sap movement (up to 1 m/hr).
Source-Sink Dynamics Table
Option Role in Translocation Sugar Status Young leaf Sink Imports for growth Mature leaf Primary source Exports sucrose Young fruit Sink Consumes sap Shoot tips Sink Meristem demand -
