Cereals are deficient in . . . . . . . . amino acid.

Cereals are deficient in Lysine amino acid.
Cereals are deficient in several essential amino acids including lysine, threonine and tryptophan.
The first limiting essential amino acid in cereals is generally lysine. However, there are variations between cereals. In oats, rice, and finger millet the deficiency in lysine may be only marginal, while in sorghum, maize, and other millets it is more pronounced.