Practice question

In plant tissue culture classification, macronutrients are defined as elements required at concentration greater than 0.5 millimolar, reflecting their roles as building blocks and osmotic regulators. They include nitrogen incorporated into amino acids, nucleotides, chlorophyll porphyrin and alkaloids; potassium acting as cofactor for over 40 enzymes, osmoregulator for stomatal opening; calcium essential for middle lamella calcium pectate crosslinking cell walls and as secondary messenger binding calmodulin during stress signaling; magnesium central atom of chlorophyll and bridge for ATP Mg complexes needed by kinases; phosphorus in nucleic acids phospholipids and high-energy ATP; sulfur in cysteine, methionine, coenzymes and Fe-S clusters. In MS medium macronutrients occur at tens of millimolar, for example 20.6 mM NH4+, 18.8 mM NO3-, 20 mM K+, 3 mM Ca2+, 1.5 mM Mg2+. Deficiency below 0.5 mM rapidly causes chlorosis necrosis or hyperhydricity. Micronutrients conversely needed below 0.5 mM as catalytic cofactors, highlighting quantitative distinction guiding stock preparation where macro stock prepared at 10X and micro at 100X to prevent precipitation and toxicity.