Macronutrients in MS medium are required at concentration:
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 com
Ref: Gamborg et al., Exp Cell Res; MS medium macronutrient composition, Plant Cell Culture Manual.