Is it possible for water to move DOWN and sugars to move UP in plant vascular tissue?

Question

BlackTom AI · Accepted Answer

Kicking off with a broad view of plant vascular transport, we know two separate systems are at play: xylem primarily moves water and minerals upward from roots, while phloem moves photosynthates (like sugars) from sources to sinks, and this flow can go in multiple directions depending on where sugars are produced and needed.
Option A claims that this downward water movement with upward sugar movement is most common. In reality, the classic pattern is water moving upward in the xylem due to transpiration pull, while sugars are transported through the phloem from source to sink, which is not described as 'most common' as a single direction for both substances. This makes A unlikely to be correct.
Option B says gravity causes both sugars and water to go downward. This is incorrect because the primary forces driving xylem transport are transpiration-generated tension and root pressure, and phloem transport is not governed by gravity in a simple downward-only manner; sugars can move both up and down depending on source-sink demands.
Option C states: yes, phloem contents may need to reach ovaries, and water may cycle downwards in phloem. This aligns with the pressure-flow model: phloem sap moves from sources to sinks and can travel in directions that deliver sugars to growing tissues (like ovaries or developing seeds) that are located in various parts of the plant, including downward. Water movement with phloem sap can also occur in various directions, including downward, as needed to maintain turgor pressure and sap flow.
Option D claims water sinks are always in the leaves and sugars sinks are always in the roots. This is incorrect because sinks for sugars can be developing fruits, seeds, or storage tissues—not exclusively roots, and sinks can be above or below sources depending on plant needs.
Option E suggests this downward water movement with upward sugar movement only happens in CAM photosynthesis. CAM plants refer to a particular carbon fixation pathway and do not dictate this directional transport pattern in the vascular system; therefore E is not a correct general statement.
Overall, the reasoning shows that while xylem typically moves water upward, the phloem can transport sugars in directions dictated by source-sink relationships, and associated water movement in phloem can occur downward as part of sap flow, making option C the coherent choice among the presented statements.

Is it possible for water to move DOWN and sugars to move UP in plant vascular tissue?单项选择题

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