When plants photosynthesize, they utilize water and carbon dioxide and convert them to oxygen and carbohydrates with the assistance of sunlight energy. They also need mineral nutrients. Without specific minerals, plants would show sad deficiencies such as discoloration, leaf drying, bud death, and wilting. It is a lot like our requirement for minerals. Without some minerals, humans would show extreme signs of deficiency and be prone to various disorders.
Plants need 3 non-essential nutrients and 13 essential mineral nutrients. The nutrients are classed as either micronutrients or macronutrients. Macronutrients are needed in more significant amounts (1000-30 mean relative density in dry tissue (MRC)) than micronutrients (3-1 x 10-3 MRC). Macronutrients include anion (nitrogen), calcium, potassium, and magnesium (cations), sulfur and phosphorous (anions); micronutrients include boron, chlorine, molybdenum (-), zinc, manganese, copper, (+), iron, and the three non-essential nutrients, sodium, sodium, and nickel. Plants take up these essential and non-essential minerals as molecules or ions in the form of anions and cations.The minerals are classified as essential if they meet the below properties:
- Without the particular element, a plant would fail to complete its life cycle.
- The particular element is involved in plant metabolism directly.
- The element’s function cannot be subsidized by anything else.
Mineral nutrients are used for numerous purposes, including:
- Cellular control
- Tissue components, for example, protein synthesis
- Electron transfer
- Enzyme components
- Regulatory control systems.
- Metabolic processes
Although leaves can consume some ions through foliar feeding, the bulk is taken up from solutions encompassing the roots of the plant. The central route is through the necessary epidermis to the stele and the xylem’s direct conduction cells. Apoplastic transportation is via the curly interlinked cell walls through the plant root – the principal pathway for nutrient absorption.
The essential apoplast is the plant cell walls and intercellular areas. Absorption is also performed by symplastic transport. The symplast is living cells united by the plasmodesmata. The nutrients do not move through any membrane with apoplastic transport until they enter the endodermis, where they are stopped from entering the stele by waxy Casparian strips. Before the ions enter the Casparian strips, apoplastic transport is non-selective, with cations being more quickly absorbed due to the negatively charged cell walls.
In order for ions to access the stele (xylem+pericycle+phloem) with apoplastic transportation, they have to cross membranes. This is where the system becomes selective, in which ions are permitted to pass into the xylem. The careful parts of the membranes are the transport proteins.
Ions are also brought up by the selective symplastic transport system, again via carrier proteins in the membranes.
As ions frequently enter the xylem cells by active transport, water follows by osmosis (by passive transport). The liquid now in the xylem is prized as the xylem sap. The transport proteins are proton pumps.
If the density of ions in the soil is higher than the plant, they will enter by facilitated diffusion, a passive process. However, ions are usually at a lower concentration in soil. The proton pump, powered by ATP, discharges hydrogen ions from the inside to the outside of the cell, causing an electric charge over the membrane. This membrane potential enables cations to enter the cell, for example, K+ ions, through potassium channel proteins. Symport protein channels allow anions to be transported into the cell.
Once the nutrients are in the xylem, they are moved to other cells, such as leaves, by the Transpiration-Cohesion-Tension mechanism:
When water ensues from the leaves’ stomata, a tension transpires within the leaf, extracting nutrients from the leaf’s veins into the apoplastic cells. The tension draws nutrients from the xylem into the leaf’s veins. This, in turn, pulls on the nutrients in the stem’s xylem, which draws the nutrients to move outwards and upwards. The motion of nutrients and water is supported by the force of union of water molecules sticking together by hydrogen bonding of the water particles.
Several environmental/plant factors control the absorption of nutrients:
- pH: The more acidic the soil, the fewer nutrients are possible to the plant, apart from iron. The higher the pH, specific ions are not available, e.g., Cu, Mn, Zn, Ca, Fe, N, Mg. Although K, P, B, and S are available. Most plants love and enjoy a pH of around 6.5.
- Mechanism: Plants have a negative feedback mechanism that regulates the nutrients uptake. If the plant has enough of a particular nutrient, the mechanism stops further uptake of that nutrient.
- Foraging: Nutrient foraging allows a plant to achieve optimum nutrients from the soil. For example, root caps of plants emit various mucilage and exoenzymes, which makes elements more convenient.
- Structure: Root geometry, such as the growth rate, size of the root system, root branching, root thickness, and root hairs, all influence the rate and availability of nutrient uptake.
Elemental ions are consumed by plants in solution form. The plant controls which nutrients it needs by a selective process.