TRANSPORTATION IN PLANTS
Transportation : The transport of water, food, minerals, hormones and other substances from one part to another part inside the body of an organism is known as transportation.
Plants take in CO2, photosynthesize and store energy in leaves. Besides CO2, plants need other raw materials for growth. These raw materials like nitrogen, phosphorous etc., they get from the soil by the process of absorption by roots. These raw materials need to be transported to each and every part of the plant, mainly to leaves.
Diffusion : The movement of molecules is in the direction of concentration gradient i.e., from a region of higher concentration to that of lower concentration.
In unicellular organisms like Chlamydomonas, transport of material mainly takes place by the process of diffusion.
Osmosis : Osmosis is the flow of water molecules from the region of higher water potential to the region of lower water potential through a semi-permeable membrane.
If the distance between roots and leaves is small, then these raw materials can reach to the leaves easily by the process of diffusion.
In case distance between roots and leaves is more, then proper system of transportation is required.
In plants, transportation is done by a specialized vascular system which is present in the root, stem and leaves.
Vascular system is made up of two types of vascular tissues:
In plants the transport of materials can be divided into two parts:
(A) Transport of water and minerals through xylem tissue.
(B) Transportation of food and other substances through phloem tissue.
(A) Transport of water and minerals [Ascent of sap].
The plants require water and minerals for making food and performing other functions.
The upward movement of water and minerals from the roots to various parts of the plant is known as ascent of sap.
Ascent of sap is carried out by xylem tissue which consists of :-
1. Xylem vessels
2. Xylem tracheids
3. Xylem fibre
4. Xylem parenchyma.
In flowering plants, xylem vessels and tracheids conduct water and minerals upward.
In non-flowering plants, tracheids are the only conducting cells.
Mechanism of ascent of sap :-
Absorption of water and minerals :- The water and minerals are absorbed by land plants from the soil where it is present in the form of soil solution.
The main water and minerals absorbing organs are root hairs. The water is absorbed actively due to the water potential difference between soil solution and root hairs.
Water molecules move from higher water potential to lower water potential and then migrate from cell to cell passing from epidermis to cortex, from cortex to endodermis and from endodermis to xylem vessels and tracheids. This is called lateral transportation.
The mineral salts move from higher partial pressure to lower partial pressure along the concentration gradient by passive absorption or against the concentration gradient by active absorption. Finally the water and minerals reach the vessels and tracheids of xylem from where they move upward by the process called ascent of sap.
Many theories have been put forward to explain the upward movement of water by distinguished scientists. Two among them are important :
(a) Root pressure theory :- According to root pressure theory, the roots absorbs water and exert a pressure, the root pressure, which pushes the water upward. The root pressure develops in the tracheary element of xylem (i.e., tracheids and vessels) as a result of metabolic activities of roots. The root pressure theory is applicable in small herbs but not in tall tress. The effect of root pressure in transport of water is more important at night.
(b) Transpiration pull and cohesion-tension theory :-
Transpiration pull and cohesion-tension theory explains the upward movement of water in tall trees. According to this theory, the main force responsible for upward movement of water is transpiration pull generated in the leaves which pulls the water column filled in the xylem tracheids and vessels.
During day period, Cohesion force and Transpiration pull help in the upward movement of sap from roots to leaves.
Functions of transpiration :
The minimum transportation is found in succulent xerophytes and no transportation in submerged hydrophytes.
Potometer is an instrument used to measure transpiration.
Guttation : The loss of water from the uninjured part or leaves of the plant in the form of water droplets is called as guttation. Exuded liquid of guttation is not pure water. The process of guttation takes place due to the root pressure.
Bleeding :- Fast flowing of liquid from the injured or cutting parts of the plants is called bleeding or exudation.
Wilting :- Drooping of soft parts of the plants due to loss of turgidity in their cells is called wilting. Wilting is caused due to high rate of transpiration during mid-day or deficiency of water in soil and also in prolonged drought conditions.
(B) Transportation of food and other substances
Sugar, amino acid and other substances are translocated from site of synthesis to site of storage through the phloem.
Transport of food from leaves to different parts of plant is termed as translocation.
Translocation may be in upward or downward direction depending on the need of the plant.
Translocation of food takes place in the sieve tubes with the help of adjacent companion cells by phloem tissue.
Phloem tissue consist of four components:
1. Sieve tubes
2. Companion cells
3. Phloem parenchyma
4. Phloem fibre
Mechanism of translocation
Girdling experiments are also used to demonstrate that the transportation of sugar takes place through phloem tissue.
Various theories have been proposed to explain translocation. Most widely accepted one is the mass flow hypothesis proposed by Munch in 1931.
In this way food material is translocated between the source of food material to the sink which is the site of utilisation. This is downward translocation of food.
In spring, sugar stored in root or stem tissue would be transported to the buds which need energy to grow.