Question

Which of the following statement substantiates the mass flow hypothesis?
I. Sugar is translocated from sink to source.
II. Glucose is converted to sucrose at the sink.
III. Osmosis enables the movement of sucrose into the sieve tubes, which generates a hypertonic condition in phloem.

• A. I and II
• B. Only I
• C. Only III
• D. I, II and II

Answer 1

The correct option is C Only III

The mass flow hypothesis proposed by Ernst Munch is the most accepted hypothesis that explains the translocation of phloem sap i.e. water and solutes from source such as leaves to different parts of the plant which act as sink eg. roots. According to this hypothesis, the glucose prepared at the source is converted to sucrose in the mesophyll cells of the leaves. Sucrose is then transported into the sieve tubes through companion cells, this process is called as loading. Entry of sucrose increases the solute concentration in the phloem thereby decreasing the water potential in it. This facilitates the movement of water from xylem into the phloem increasing the water potential and turgor pressure. The phloem sap (sucrose and other solutes in water) moves cell to cell along the concentration gradient. This is possible due to the development of the osmotically generated pressure gradient between source and sink. Due to this loading process at the source, a hypertonic condition is produced in the phloem.