When plants photosynthesise, they utilise carbon dioxide and water and convert to carbohydrate and oxygen with the aid of sunlight energy. They also require mineral nutrients. Without certain minerals, plants would show deficiencies such as leaf drying, discolouration, wilting and bud death. It is a lot like the human requirement for minerals. Without certain minerals we would show signs of deficiency and be prone to different disorders.
Plants require 13 essential mineral nutrients and 3 non-essential nutrients which certain plants require. The nutrients are classed as either macronutrients or micronutrients. Macronutrients are required in larger amounts (1000-30 mean relative concentration in dry tissue (MRC)) than micronutrients (3-1 x 10-3 MRC). Macronutrients include nitrogen (anion), potassium, calcium and magnesium (cations), phosphorous and sulfur (anions); micronutrients include chlorine, boron, molybdenum (-), manganese, zinc, iron, copper, (+) and the 3 non-essential nutrients, sodium, silicon and nickel. These minerals are taken up by plants as ions or molecules in the form of cations and anions.
The minerals are classed as essential if they satisfy the following properties:
o Without the element, a plant would not be able to complete its life cycle
o The element’s function cannot be subsidised by another
o The element is directly involved in plant metabolism
Mineral nutrients are used for a variety of purposes including:
o Tissue components, example, protein synthesis
o Cellular control
o Enzyme components
o Electron transfer
o Metabolic processes
o Regulatory control systems.
Although some ions can be absorbed by leaves through foliar feeding, the majority are taken up from solutions surrounding the roots of the plant. The main route is through the epidermis to the stele and the conduction cells of the xylem. Apoplastic transport is via the interlinked cell walls through the plant root – the main pathway for nutrient absorption.
The apoplast is the plant cell walls and intercellular spaces. Absorption is also achieved by symplastic transport. The symplast is living cells connected by the plasmodesmata. The nutrients do not pass through any membrane with apoplastic transport until they reach the endodermis, where they are prevented from entering the stele by waxy casparian strips. Before the ions reach the casparian strips, apoplastic transport is non-selective, with cations being more readily absorbed due to the negatively charged cell walls.
In order for ions to enter the stele (pericycle + xylem + phloem) with apoplastic transport, they have to cross membranes. This is where the system becomes selective in which ions are allowed to pass into the xylem. The selective parts of the membranes are the transport proteins.
Ions are also taken up by the selective symplastic transport system, again via transport proteins in the membranes.
As ions are increasingly entering the xylem cells by active transport, water follows by osmosis, (by passive transport). The liquid now in the xylem is known as the xylem sap. The transport proteins are proton pumps.
If the concentration of ions in the soil is higher than the plant, they would enter by facilitated diffusion, a passive process. However, ions are generally at a lower concentration in soil and the proton pump, powered by ATP, expels hydrogen ions from the inside to the outside of the cell, causing an electric charge across the membrane. This membrane potential causes cations to enter the cell, for example K+ ions, through potassium channel proteins. Symport protein channels accept anions to be transported into the cell.
Once the nutrients are in the xylem, they are transported to other cells, such as leaves, by the Transpiration-Cohesion-Tension mechanism:
When water transpires from the leaves’ stomatas, a tension occurs within the leaf, pulling nutrients from the veins of the leaves into the apoplastic cells. The tension pulls nutrients from the xylem into the leaves’ veins. This in turn pulls on the nutrients in the xylem of the stem, which draws the nutrients to move upwards and outwards. The motion of water and nutrients is aided by the force of cohesion of water molecules sticking together by hydrogen bonding of the water molecules.
The absorption of nutrients is regulated by several plant/environmental factors:
o pH. The more acidic the soil, the less nutrients are available to the plant, apart from iron. The higher the pH, certain ions are not available, eg, Cu, Zn, Mn, Fe, Ca, Mg, N. Although P, K, S and B are available.
o Most plants prefer a pH of around 6.5.
o Plants have a negative feedback mechanism which regulates the uptake of nutrients. If the plant has sufficient of a particular nutrient, the mechanism prevents further uptake of that nutrient.
o Nutrient foraging enables a plant to obtain optimum nutrients from the soil. For example root caps of plants exude various exoenzymes and mucilage, which makes elements more available.
o Root geometry, such as the size, growth rate of the root system, root thickness, root branching and root hairs all influence the availability and rate of nutrient uptake.
Conclusions:
Elemental ions are absorbed by plants in solution form. The plant regulates which nutrients it requires by a selective process. To analogise, the plant is similar to the London underground. People (ions) move to the platform (membranes) waiting for the train (xylem system) The train transports the people to all locations (tissues) on that particular line.
