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How do stems transport carbohydrates from leaves to roots?
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Plant
Grandidier's baobas (Adansonia grandidieri) in Madagascar.
Stems transport carbohydrates, primarily sugars produced during photosynthesis, from leaves to roots through a specialized vascular tissue called phloem.12
While the exact mechanism remains a subject of study, the most widely supported explanation is the pressure-flow hypothesis. According to this model:1
- Source to Sink: Sugars are actively transported into the phloem at "source" regions (such as mature leaves), which increases the osmotic pressure within the sieve tubes.1
- Pressure Gradient: At "sink" regions—areas of growth or storage like roots—sugars are removed from the phloem, which lowers the osmotic pressure.1
- Mass Flow: This difference in pressure creates a gradient that allows the solution of sugars to flow en masse from the source to the sink.1
Evidence for this solution flow includes the observation of exudate from sieve tubes and the behavior of phloem-feeding insects like aphids, whose feeding patterns indicate that transport occurs as a continuous flow of liquid.1