Question

Suggest an experiment to prove that organic food is translocated through phloem.

Answer 1

Ernst Munch (1930), first proposed this hypothesis of solute transport in phloem tissue also called as the pressure flow hypothesis or the Munch hypothesis.

According to this theory, organic solutes are translocated through the sieve tubes from the source (leaves) to the consumption end or sink(is the part which stores the food). But, the source and sink may be reversed depending on the seasons or plant needs. Therefore the direction of movement in phloem can be bidirectional.

Movement of solutes such as sucrose through the phloem of a plant. Three Stages are involved namely,

A) Phloem loading - here sugar moves from the photosynthesizing cells into a chain of mesophyll cells, transfer cells, to the phloem of the vascular bundle.

B) Translocation - pressure inside the sieve tubes is greatest at the source and lowest at the sink It pushes sucrose, etc, from source to sink.

C) Unloading - it is a passive transport mechanism. Unloading of the sieve tube takes place at the sink. The solute is removed for use, thus maintaining the pressure gradient in the sieve tubes.