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How does phloem transport sugars from leaves to other plant parts?
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Plant
Grandidier's baobas (Adansonia grandidieri) in Madagascar.
Phloem is the specialized vascular tissue responsible for transporting the products of photosynthesis—primarily sugars—from leaves to other parts of the plant, such as growing tissues, flowers, fruits, and storage organs like roots and tubers.123
The primary mechanism for this transport is widely considered to be the pressure-flow hypothesis. According to this model:
- Source Regions: In areas of photosynthesis (sources), sugars are loaded into the sieve tubes, which increases the osmotic pressure within those tubes.1
- Sink Regions: In areas of growth or storage (sinks), sugars are removed from the sieve tubes, which lowers the osmotic pressure.1
- Pressure Gradient: This difference in pressure between the source and the sink establishes a gradient that allows the solution to flow through the sieve tubes.1
Evidence for this solution flow includes the observation of exudate from injured sieve tubes and the behavior of phloem-feeding aphids, whose feeding patterns indicate that transport occurs as a continuous flow of solution.1