Which plant structure causes water to be reduced by sucrose and move from the xylem to the phloem?

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Question: Which plant structure causes water to be reduced by sucrose and move from the xylem to the phloem?

Option a: Sieve tube elements. While sieve tube elements are the conduits in the phloem that transport phloem sap, they are not the structures that cause the initial water movement into the phloem. The loading of water into the phloem occurs due to osmosis as sugars are loaded, which then affects the water potential around the phloem. So this option describes the transport vessels but not the causative site for the water move-ment into them.

Option b: Cuticle. The cuticle is a protective waxy layer on the epidermis that reduces water loss from the leaf surface, not a tissue involved in phloem loading or water movement between xylem and phloem. This makes it an incorrect venue for the described process.

Option c: Sink Cell. Sink cells are tissues that consume or store sugars (e.g., developing fruits, roots) and typically draw in phloem sap. While they affect phloem flow by demand, they are not the initiators of water movement from xylem to phloem caused by sucrose loading. The water movement described is driven at the source of loading, not at the sink away from loading.

Option d: Source cell. Source cells are tissues where sugars are actively loaded into the phloem (for example, photosynthetic leaves). This loading lowers the water potential in the phloem, causing water to move by osmosis from the xylem into the phloem to balance the tension and create sap transport. This fits the mechanism described: sucrose loading reduces water potential, drawing water from the xylem into the phloem.

In summary, the mechanism involves osmosis driven by sugar loading at the source region, which reduces local water potential and draws water from the xylem into the phloem. Among the options, the structure that best fits this initiating role is the source cell, which aligns with phloem loading processes. The other options either describe components of the transport pathway (sieve tube elements) or tissues not directly responsible for initiating water movement via sucrose loading (cuticle, sink cell).

Which plant structure causes water to be reduced by sucrose and move from the xylem to the phloem?

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