Plant mineral requirementsEdexcel International A Level Biology: Revision notes

Water is taken up by the roots and has several important roles.

• Turgor: water fills the vacuoles and pushes the membrane against the cell wall, so the cells are turgid. Turgid cells support leaves and non-woody stems. When cells lose water they become flaccid and the plant wilts.
• A reactant in photosynthesis and in hydrolysis reactions.
• A solvent and transport medium: mineral ions and assimilates dissolve in it and travel in the xylem and phloem.
• Evaporation from the leaves (transpiration) cools the plant and moves mineral ions up in the transpiration stream.
• Turgor of the guard cells opens the stomata, and turgor drives cell expansion in growth.

Plants take up nitrate ions (NO₃⁻) from the soil by active transport. The nitrogen is used to make:

• amino acids, which are joined to form proteins, including enzymes
• nucleotides, used to make DNA and RNA
• chlorophyll, which contains nitrogen

Nitrogen is therefore needed for growth, cell division and photosynthesis.

Deficiency: stunted growth and chlorosis (yellowing), particularly in the older leaves, because nitrate is mobile and is moved to the young growing leaves.

Calcium ions (Ca²⁺) are used to make calcium pectate, which forms the middle lamella between the walls of neighbouring plant cells. It sticks the cells together and gives strength.

Deficiency: weak cell walls and stunted growth, with distorted young leaves and poor growth of the root tips, because calcium is not moved easily from old tissue to new.

Magnesium ions (Mg²⁺) form part of the chlorophyll molecule. Without them, less chlorophyll is made, so less light is absorbed and photosynthesis is slower.

Deficiency: chlorosis (yellowing) between the veins of the older leaves, poor growth and low yield because less glucose is made.

Link the symptom to the ion's role.

• Nitrate lacking: stunted growth and yellow older leaves (less protein and chlorophyll).
• Magnesium lacking: yellowing between veins of older leaves (less chlorophyll).
• Calcium lacking: weak walls, distorted young leaves and stunted growth.

Nitrate and magnesium are mobile, so they are moved to the young growing tissue and the older leaves show symptoms first. Calcium is not mobile, so the young tissue shows symptoms first.

Grow seedlings with their roots in culture solutions made with distilled water and chemicals containing the required ions.

1. Prepare a complete solution and solutions lacking one ion (for example nitrate, magnesium or calcium).
2. Place seedlings of the same species, age and size in each, with several seedlings per solution.
3. Aerate the solutions, and cover them to keep out light and prevent algae.
4. Keep light and temperature the same for each.
5. After a set period, observe leaf colour and measure height, root length and dry mass.

The complete solution is the control. Percentage decrease in dry mass = (control − test) ÷ control × 100.