Plant culture media

A plant culture medium has six parts: inorganic nutrients, organic supplements, a carbon source, growth hormones, a gelling agent, and antibiotics. The exact recipe varies with the species, and even with the tissue.

• Inorganic nutrients: macronutrients, needed in millimolar amounts (C, Ca, H, K, Mg, N, O, P, S), and micronutrients, needed in micromolar amounts (B, Co, Cu, Fe, Mn, Mo, Zn).
• Organic supplements: amino acids (arginine, asparagine, glycine, proline) and vitamins (thiamine, nicotinic acid, pyridoxine).
• Carbon source: cultured tissues cannot make enough food by photosynthesis, so they are given sugar. Sucrose, at about 2-5%, is the best.
• Hormones: of the five groups of plant hormones, auxins and cytokinins are used most; their ratio decides which organs form.
• Gelling agent: usually agar.
• Antibiotics and antifungals keep down contaminating microbes.

The pH is set to about 5.8-6.0. It matters in several ways: a higher pH makes the medium harder, a lower pH makes it set poorly, and pH also affects how well nutrients are taken up and how well salts dissolve.

The two classic media differ a lot. MS medium has far more nitrogen than White's medium (potassium nitrate 1900 mg and ammonium nitrate 1650 mg per litre), plus iron-EDTA (which keeps iron available) and myoinositol. That is one reason it suits so many plants.