Question

As ___(i)___ in the phloem sieve tube increases, ___(ii)___ begins, and the sap moves through the phloem.

• A. (i)–DPD, (ii)–proton flow
• B. (i)–transpiration pull, (ii)–pressure flow
• C. (i)–root pressure, (ii)–electron flow
• D. (i)–hydrostatic pressure, (ii)–pressure flow

Answer 1

The correct option is D (i)–hydrostatic pressure, (ii)–pressure flow

The increased turgor pressure at the source drives the flow of synthesized sugars towards the sink. The high concentration of sugar in the seive tube elements drives the rush (osmosis) of water carried by xylem into the phloem cells. This generates abundant hydrostatic pressure at the source. As the hydrostatic pressure in the phloem sieve tube increases, pressure flow begins, and the sap moves through the phloem from the source to the sink.