Effect of amino acids glutamine (Gln), arginine (Arg), asparagine (Asn), γ-amino-butyric acid (GABA), 3.5 mM each, a mixture of the three amino acids Gln, Arg and Asn (3.5 mM each) and the control with or without 1% activated charcoal on cork oak somatic embryo growth per month. (A) Relative size increment in projected area per month (RAI, month⁻¹). (B) Relative fresh weight increment per month (RWI, month⁻¹). Treatments with different letters are statistically significant at the 5% level.