Plant Nutrition: Essential Elements

• All living things need food to carry out their life processes.
• Animals get energy-rich biomolecules from plants or other sources.
• Saprophytic fungi and bacteria get food by breaking down dead plants and animals.
• Plants create their own food from simple elements, using chlorophyll and the sun to make glucose and amino acids.
• Plant nutrition was studied starting in the 1800s, with botanists and chemists finding inorganic elements in plants from the environment.
• By the late 1800s, about 10 inorganic elements were found to be needed for plant growth; without them, plants grew and reproduced abnormally, and so were named essential inorganic nutrients.
• More than 16 elements are considered essential for plant nutrition.

Macronutrients vs. Micronutrients

• Macronutrients are needed by plants in high quantities and are found in high concentrations in plants.
• Micronutrients are needed by plants in very small quantities and are also known as trace elements.

Nutrient Concentrations in Maize

• Macronutrient percentages:
  • Nitrogen 2.81%
  • Potassium 1.86%
  • Calcium 0.40%
  • Phosphorus 0.28%
  • Magnesium 0.27%
  • Sulphur 0.18%
  • Oxygen, carbon, and hydrogen are present in large proportions.
• Micronutrient percentages:
  • Iron 0.000110%
  • Chloride 0.003100%
  • Copper 0.000006%
  • Manganese 0.000080%
  • Zinc 0.000027%
  • Molybdenum 0.000001%
  • Boron 0.000014%
• Some plants require specific elements; legumes, for example, grow better with cobalt in their rooting medium.
• Cobalt is used by bacteria in a symbiotic relationship with legumes where they convert gaseous atmospheric nitrogen to nitrogenous compounds.
• Halophytes can tolerate sodium, but it's not proven to be essential for their metabolism, though some consider it so.
• Soybeans need nickel, which, if absent, leads to a toxic buildup of urea, causing necrosis (browning due to cell death) at leaf tips and reduced growth.

Element Function Types

• Element functions can be specific or non-specific, where some elements are replaceable by others.

Non-specific Functions

• Maintenance of osmotic pressure by inorganic ions like sodium, potassium, or chlorine is necessary for water uptake.

Specific Functions

• Elements carry out various functions in a plant’s life processes, and can act as structural, non-structural, metabolic, and catalytic agents.

Structural Roles

• These elements make up the plant cell's structures.
• Calcium and pectic acid form the middle lamella.
• Phosphorus makes phospholipids of the plasma membrane.
• Glucose chains of cell walls are formed from carbon, oxygen, and hydrogen.

Non-Structural Roles

• These elements are components of biomolecules that are not part of the cell structure.
• Phosphorus is a component of nucleic acids (genes), energy compounds (ATP, ADP), and coenzymes.
• Magnesium is part of the chlorophyll molecule.
• Nitrogen makes up amino acids (proteins), nucleic acids, chlorophylls, and coenzymes like NAD.
• Carbon, oxygen, and hydrogen make up carbohydrates (glucose, sucrose, starch).
• Lipids include oleic acid, linoleic acids, oils, and waxes.
• Proteins include glycine, alanine, and proline.
• Sulphur is a component of some amino acids and proteins like coenzyme A.

Metabolic Activity

• Chlorine (Cl) and manganese (Mn) are involved in the light reactions portion of photosynthesis which leads to the release of oxygen.

Catalysis

• Most micronutrients are components of enzymes required for their function.
• Iron (Fe) is found in cytochrome oxidase which transfers electrons to oxygen during respiration and nitrogenase, which converts nitrogen to ammonia.
• Other essential elements activate or regulate enzymes by changing the shape of the enzyme, either exposing or obstructing the catalytic site.
• Magnesium (Mg 2+) activates ATPase.

Osmotic Activity

• Some essential elements maintain osmotic potential and ionic balance in plant cells.
• Potassium (K) affects stomatal opening and closure, which depends on osmotic potential in guard cells.
• Water uptake causes them to swell and open the stomata, which happens when the guard cells experience an influx of potassium ions.
• During efflux of potassium ions, the osmotic potential reduces, water moves out, and the guard cells lose turgidity, shrinking and closing the stomata.

Cell Permeability

• Calcium (Ca) directly affects the cell membranes' physical structure.
• Membrane integrity depends on calcium which impacts the selective uptake of substances into the cell.

Plants require both macro and micronutrients to survive. Macronutrients are needed in large quantities, while micronutrients are only needed in trace amounts. More than 16 elements are considered essential for plant nutrition, with deficiencies leading to abnormal growth and reproduction.