Transport in Plants: Definition, Mechanism, and Types
Transport in Plants: Introduction
Plants – Sessile, well developed tissue level grade of organization are the primary food producers for all the living organisms with least exceptions.
The energy of the sun and our waste respiratory product CO2 along with water becomes essential to produce sugars and other nutrient; vitamins and minerals which are obtained from the soil in which the plants are stationed to produce energy to plant dependent organism.
For a sessile organism to survive plants has developed many mechanisms to attain the basic needs for survival.
Tissue level organization is primitive even in higher plants with simpler but efficient structure to transport nutrients and water to the plants.
Uptake of nutrients and water is the key role in plant’s physiological process involving energy and carbon assimilation to produce Oxygen as a product.
Transportation in plants becomes essential and significant role in their physiological functions for the sedentary mode of life time employing simpler and number of techniques in combination to take nutrients from soil and water.
Specialized network of tissues connects the plants from its origin aids in nutrients translocation and distribution.
The specialized tissues are segregated and are present in all parts of body forming a connection between the nutrient receptor, storage, and synthesis regions of plants distinctly.
Major transportation in plants are water, minerals, and nutrients; distinctive pathway is defined for every transportable element; water and minerals are transported through XYLEM; Phloem another tissue transports and distributes organic matter to the sites of synthesis and their end products from synthesis site are distributed to energy utilizing site either as ATP or other sugar compounds.
Transportation in broader sense involves xylem and phloem – the vascular tissues provide a network to transfer the nutrients from source to the receiver site involves physics and chemistry to move evenly to sustain the plant.
Narrowing down; the cells also play a significant role in transportation and in energy utilization and storage.
Transport in cells plays a key role in the dynamics of cell were each cell functioning as a single unit integrates with neighbor’s cell wall and cell in integrating the flow of nutrient throughout the plant.
Transport in plant is extensively studied in earlier times by many scientists who experimented and identified the network of transporters laid out many theories for the transportation in plants and in trees.
History of Transport in Plants
The study of plant transport started back in 18th century by many scientists to understand the internal structure, circulation, and mechanical support for the plants.
It was an intriguing question on how a plant transports soil nutrients and water to the apex of the shoot and how does they synthesis their food?
Observing circulation in animals’ circulation in plants were also researched to understand the mechanism behind.
In earlier, 18th and 19th centaury many scientists observed various transport in plants by dissection and other methods and formulated theories of transportation of water and other elements.
The theory “Cohesion – Adhesion” developed by H. H. Dixon gave an acceptable mechanism in transport of materials in a general term of “Ascent of Sap”.
Transport in Plants Theories
Many theories have been put forward to understand the transport of water and other materials combinedly called as sap from the root to tip of the plant or a tree – Ascent of Sap. The theories were classified into three topics based on the ideology.
They are grouped into: 3 theories namely vital force; root pressure; physical force theories.
I. Vital Force Theory
The theory defines the movement of water or sap from root to other parts by the pulse created by the cortical cells which absorbs water from the outer side and pump them to the openings of xylem vessels.
The theory was put forth by J. C. Bose in 1923 also called a pulsation theory.
The theory stated that the cortical cells to be main cells to pump the water from the soil; which is proven wrong by eliminating roots from plant and the plant; stem in specific; still absorbed water.
II. Root Pressure Theory
The theory defines the movement of water or sap from root to other parts by the pulse created by the cortical cells which absorbs water from the outer side and pump them to the openings of xylem vessels.
The theory was put forth by J. C. Bose in 1923 also called a pulsation theory.
The theory stated that the cortical cells to be main cells to pump the water from the soil; which is proven wrong by eliminating roots from plant and the plant; stem in specific; still absorbed water.
III. Passive or Physical Force Theories
Theories corresponding to passive forces such as capillary force, collision force, atmospheric pressure, imbibition, diffusion pressure are factors upon which these theories are built upon.
Numerous scientists over a period have produced theories for identifying the transport.
They are:
a) Capillary Theory, 1863 by Boehm
Capillary movement is transport of substance over a small surface area over a small distance is acted upon by a small capillary force sufficient to pull water to a small extent.
Concept of capillary theory is based on the small force created by a narrow surface area to move a small amount of water.
The force is passive based on the physical properties of pressure and force and does not cost any energy.
Plant vascular system consists of tracheid and trachea which are longitudinal tubes of vascular bundles which when empty builds a force against gravitational force to pull the water from the roots and transport it to stem.
But this force is not sufficient to move to water column to the entire length of the plant.
b) Imbibition Theory, 1868 by Unger
Imbibition absorbs water on contact with water; swells by the pressure built up in a packed wood increase in surface area.
The pressure and increase in surface area pull the water but the force built up is exceedingly small to water to ascend the apex.
c) Cohesion - Tension Theory, 1894 by Dixon and Jolly
This is also called as Cohesion – Tension Transpiration Pull Theory.Â
The cohesive force and Transpiration pull combines to attract the water and other elements to move through the column of vascular tissues are now moved to the apex of the plant.
The cohesive force between the walls of xylem and hydrophilic water molecules attracts the molecules to the xylem wall.
Alongside xylem cohesive pull; water transpires when stomata open; which attracts water from nearby cells causing a Diffusion Pressure Deficit gradient between the guard cells and surrounding cells which causes diffusion from water filled cells to water deficit cells.
This theory was widely accepted by the scientific community thereby resolving a question of how a plant conducts water along its length.
Vascular Tissues of Transportation
Vascular tissues comprising xylem and phloem provides mechanical strength and support the plant in its sedentary lifestyle by transporting and distributing the assimilates from the site of synthesis to other parts.
The primary and secondary growth provides development of the vascular tissues to support even larger plants like trees where they reach a greater height.
The basic mechanism in transport is same in all organisms but distributing the synthesis and the network differs from species to species.
Phloem transports organic substance and nutrients from synthesis site and then to other storage sites and assimilation sites with network of vascular bundles which are well differentiated into various components such as fibers and other storage parenchymal cells.
Xylem and Phloem which are present in vascular bundles has many components.
Phloem comprised sieve tubes, companion cells, parenchymal cells, and fibers; transport organic substances for energy synthesis.
Xylem costs of tracheid’s, vessels, parenchyma, and fibers responsible for water and mineral uptake.
Transportion Classification
Transport in plants is classified to short distance transportation and long-distance transportation.
Short Distance Transport
The plants transport nutrients or water over a small distance.
Example: Water from soil is taken up by root hairs present in the soil by the process of diffusion where the root hairs are rich in solutes and when water is high in the soil; the difference in the concentration between root and exerts a pull for water from soil to the root hairs. The water then travels through the cells enter xylem.
Long Distance Transport
The plants and the organizing tissues transports water and nutrients to longer distance.
For Example: Water entering root xylem by diffusion is carried to the shoot apex of the plant by combined efforts of the diffusion, imbibition, transpiration, cohesion, and adhesion. Transpiration is the determining factor directly proportional to uptake of water.
Mechanism of Transportion
Mechanism of transport in plants as classified are of 2 main vascular bundles which transports water, minerals and other organic compounds by xylem and phloem, respectively.
Phloem transports organic compounds such as Sucrose and other carbohydrates from the region of synthesis; distributed to other plant parts of requirement to store or to be utilized in the process of energy production for growth and development.
For Example: Carbohydrates; sucrose synthesized in Leaves of plants; rich in chloroplasts and stomata assimilates CO2 and converts into carbohydrates (Sucrose) carried again through phloem to other parts of the plants such as stem, flower and to developing shoots through network of vascular bundles connecting plant parts.
The phloem vessels and cells present from the root hair to the apex facilitates the assimilate transport from the synthesis point to other regions of plant.
Phloem transports organic materials by a mechanism of mass flow of substance through circulation.
Similarly; Xylem transports water and minerals from roots to the apex of the plants through various mechanisms in combination with Cohesive and Transpiration to other parts of plant.
Xylem transports water and minerals unidirectionally; varies from phloem circulating the metabolic products bidirectionally (i.e.) Root to Apex, Leaves; Leaves to Stem, flower, root.
Water is taken from the soil through root hairs through the process of osmosis; from roots the water is further taken up by the combined action of Transpiration, Capillary Action, and root pressure.
Changes in any one factor has a respective contribution and determining role in uptake of nutrients.
Minerals are also taken up by xylem by a different action of Diffusion; uptake of nutrients passively by the difference between the concentration gradient; and Active Transport utilize ATP, an energy molecule to move a molecule using transmembrane protein from extracellular matrix to the cell’s internal environment to mediate many metabolic processes.
Each mechanism and processes have its significant role in uptake of nutrients and act as a limiting factor in controlling the transport.
Transport in Plants Citations
- Molecular mechanisms of boron transport in plants: involvement of Arabidopsis NIP5;1 and NIP6;1. Adv Exp Med Biol . 2010;679:83-96.
- Molecular mechanisms of urea transport in plants. J Membr Biol . 2006;212(2):83-91.
- Strategies for optimization of mineral nutrient transport in plants: multilevel regulation of nutrient-dependent dynamics of root architecture and transporter activity. Plant Cell Physiol . 2014 Dec;55(12):2027-36.
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