Plant mineral requirementsEdexcel A-Level Biology A: Revision notes

Water makes up most of the mass of a plant and has several roles:

• Support: water enters cells by osmosis and the vacuole swells, pushing the cell contents against the cellulose wall. The cell becomes turgid and the turgor pressure holds non-woody stems and leaves rigid. If water is lost the cells become flaccid and the plant wilts.
• Reactant in photosynthesis: water is split by photolysis in the light-dependent reactions, providing hydrogen ions and electrons and releasing oxygen.
• Transport medium and solvent: mineral ions move up the xylem dissolved in water, sucrose moves in the phloem in water, and reactions in the cytoplasm take place in water.
• Cooling and cell expansion: evaporation in transpiration cools the leaf, and water uptake produces the turgor pressure that makes cells expand during growth.

Plants take up nitrate (NO₃⁻) from the soil. It is the main source of nitrogen, which the plant uses to make amino acids (and so proteins, including enzymes), nucleotides (for DNA and RNA) and chlorophyll.

Without enough nitrate the plant makes less protein and cannot divide cells quickly, so growth is stunted, and it cannot make enough chlorophyll, so leaves are pale yellow (chlorosis). Nitrate is mobile, so the plant moves it from older leaves to young growing tissue, and the older leaves show the symptoms first.

Calcium ions (Ca²⁺) combine with pectin to form calcium pectate, which is the main component of the middle lamella, the layer that cements neighbouring plant cell walls together. They are therefore needed for cell wall formation and for cell division. Calcium also helps to keep cell membranes stable and works as a signalling ion.

A deficiency causes weak cell walls, stunted growth and distorted or dying young leaves and root tips. Calcium is not mobile in the plant, so it cannot be moved from older tissue and it is the young tissue that shows the symptoms first.

Magnesium ions (Mg²⁺) are found at the centre of every chlorophyll molecule, so they are needed for light absorption in photosynthesis. Magnesium also activates a number of enzymes.

A deficiency leads to chlorosis, with yellowing between the veins of the older leaves first, because magnesium is mobile and is moved to young leaves. Photosynthesis and so growth are reduced.

Grow identical seedlings (for example barley, wheat or tomato) in culture solutions: a complete solution as the control, and solutions each lacking one ion (minus nitrate, minus magnesium, minus calcium). Each tube should contain the same volume of solution and the same type, number and size of seedlings.

Cover the tubes in black paper to stop algae growing and taking up the ions, aerate the solutions with an air pump so that roots can respire aerobically and take up ions by active transport, and keep light and temperature the same. After two to three weeks record leaf colour, height, number of leaves and root length, and if possible dry mass. Repeat with several seedlings in each solution so that anomalies can be spotted and a mean found.

Evaluate the method: seedlings may differ in their stored reserves, the ions in the seed may delay symptoms, and colour is a subjective measure. Dry mass is quantitative but kills the seedlings.