Question

Briefly describe how materials are hypothesized to move within the phloem of a plant, including the name of the hypothesis, the structures and processes involved, and the direction of flow.

Answer 1

Ans: Xylem and Phloem are the main transportation channels of a plant. While xylem transportation was tried to be explained using cohesion- tension theory in 1894, the phloem mechanism hypthesis came only in 1930. But the phloem hypothesis still stands without much modifications or challenges.

* xylem transport water and nutrients absorbed through roots to the leaves to facilitate photosynthesis.

* phloem transport the products of photosynthesis into various parts of the plant body to facilitate cellular respiration and to store the excess sucrose as starch.

* German scientist Ernest Munch proposed pressure flow hypothesis or mass flow hypothesis to explain transportation of nutrients through phloem.

* The process altogether known as "translocation " of nutrients.

* The translocation mainly involves loading and unloading of phloem.

* phloem is made up of interconnected sieve tubes, which conduct the nutrients.

* From where the loading of phloem happens, is termed as 'source' . Usually it will be leaf tissue.

* To where the unloading of phloem nutrients happens, is known as the 'sink'. Usually they are the storage parts of a plant body and tissues other than leaves, which can not synthesize glucose on their own.

* Source - leaf mesophyll - synthesize glucose and convert it to sucrose - sucrose is actively transported( ie:- against concentration gradient) to companion cells - sucrose diffuses through plasmodesmata - sucrose enters sieve tubes of phloem - water diffuses from xylem into phloem - create pressure flow - reaches the sink - unloading happens.

* Direction of phloem transportation is bi-directional . Some experiments have revealed that same sieve tube element can handle bi-directional transportation of nutrients like sucrose to various parts of plant body. ( But in xylem flow is unidirectional, ie :- from roots to leaves/photosynthetic tissue.

* For phloem, the photosynthetic tissue will be usually the source, but the sink can be any part in the plant body , which is in need of energy in the form of sucrose . In fruits and tubers, the sucrose will be converted into starch and will get stored. Fruits will always be sink, but the tubers can act as source also, when the stored food is needed for the plant growth. eg:- a new plant growing from a tuber.