Abstract
In this chapter, the role of the macronutrients nitrogen (N), sulphur (S), phosphorus (P), magnesium (Mg), calcium (Ca) and potassium (K) in plant metabolism and growth are described as well as symptoms of deficiency and toxicity. After carbon, N is the element required in largest quantity by plants; it plays a central role in plant metabolism as a constituent of proteins, nucleic acids, chlorophyll, co-enzymes, phytohormones and secondary metabolites. Upon uptake as ammonium or nitrate, N is assimilated into amino acids either in the roots or shoots. Within the plant, N is translocated as nitrate or amino acids. Sulphur is taken up as sulphate and assimilated into S-containing amino acids such as cysteine which are used to synthesize S-containing enzymes and co-enzymes as well as secondary compounds such as phytochelatins (detoxification of metals) or aliins and gluco-sinolates (feeding deterrents). Phosphorus is a structural element in nucleic acids and plays a key role in energy transfer as a component of adenosine phosphates. It is also essential for transfer of carbohydrates in leaf cells. Magnesium is a component of chlorophyll and is required for photosynthesis and protein synthesis. Calcium is important for cell wall and membrane stabilization, osmoregulation and as second messenger allowing plants to regulate developmental processes in response to environmental stimuli. The main role of K is osmoregulation which is important for cell extension and stomata movement. Potassium further affects loading of sucrose and the rate of mass flow-driven solute movement within the plant.