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Amaranthus caudatus L. seedlings produce less amaranthin in the presence of gibberellic acid A3 (GA3). This phenomenon has been tested as a bioassay for gibberellic acid and has been compared to the bioassay based on lettuce hypocotyl extension. The inhibition of pigment synthesis is much more sensitive to GA3 at concentrations from 1 to 0.01 μg/ml. Of the gibberellins tested GA1, GA3, GA4, and GA7 are the most active in the described bioassay.

Exogenous gibberellic acid, A3 (GA3) inhibits phytochrome mediated betacyanin synthesis in seedlings of Amaranthus caudatus. The growth retardants, β-chloroethyl-trimethylammonium chloride (CCC), 'isopropyl-4'-(triethylammonium chloride)-5'-methylphenyl piperidine carboxylate (AMO 1618) and tributyl-2,4,-dichlorobenzylphosphonium chloride (phosphon D) enhance pigment synthesis. Retardant stimulation of pigment synthesis is overcome by GA3 application. Besides lowering endogenous GA levels the retardants inhibit protein synthesis by as much as 25%. Retardant inhibition of protein synthesis is not overcome by GA3. The results suggest that amaranthin synthesis in Amaranthus caudatus can be directly controlled by endogenous GA. GA3 has no effect on kinin induced dark pigment synthesis. Kinins, however, do not overcome GA3 inhibition of pigment synthesis in the light.

Both gibberellic acid and abscisic acid inhibit the light induced synthesis of amaranthin in Amaranthus tricolor seedlings. The auxin, indolyl-3-acetic acid has no effect. The protein/RNA inhibitors, cycloheximide and 8-azaguanine, also reduced the levels of amaranthin produced.