Functions of macronutrients

In this chapter (Chapter 6), the roles of the macronutrients nitrogen (N), sulfur (S), phosphorus (P), magnesium (Mg), calcium (Ca), and potassium (K) in plant metabolism and growth are described together with the consequences of deficiency and toxicity. After carbon, N is the element required in the largest quantity by plants and is a major driver for crop yield; it plays a central role in plant metabolism as a constituent of proteins, nucleic acids, chlorophyll, coenzymes, phytohormones, and secondary metabolites. After uptake from the environment mostly as ammonium or nitrate, N is assimilated into amino acids in either roots or shoots. Within the plant, N may be translocated as nitrate or amino acids. Sulfur is taken up as sulfate and assimilated into S-containing amino acids such as cysteine which are used to synthesize S-containing enzymes and coenzymes as well as secondary compounds such as phytochelatins (detoxification of metals) or allicins and glucosinolates (feeding deterrents). Nitrogen and S are both important constituents of seed storage proteins. Phosphorus is a structural element in nucleic acids and plays key roles in energy transfer as a component of adenosine phosphates and in the transfer of carbohydrates between organelles in leaf cells. Phytate is a typical storage pool of P, particularly in seeds. Magnesium is a component of chlorophyll and is required for photosynthesis, transport of photoassimilates, and protein synthesis. Calcium is important for cell wall and membrane stabilization, osmoregulation, and as a second messenger allowing plants to regulate responses to environmental stimuli. The main role of K is osmoregulation, which is important for cell extension and stomata movement. Potassium affects loading of sucrose and the rate of mass flow-driven solute movement within the plant.