The ambiguous role of 2,4-dichlorophenoxyacetic acid in wheat tissue culture

The very basal, highly immature regions of dissected young leaves of Triticum aestivum L. cv. Kite formed adventitious roots on a nutrient medium supplemented with comparatively low concentrations (0.16 to 0.63 μ M ) of 2,4-dichlorophenoxyacetic acid (2,4-D). Higher concentrations (up to 640 μ M ) had to be applied to stimulate growth from more mature regions higher up the leaf. Yet, already at 2.5 μ M roots were less distinct and more callus-like, and eventually (at 10 to 640 μ M ) only a subculturable callus of apparently suppressed, slowly proliferating root primordia developed. Furthermore, at the most basal, highly immature regions growth was significantly retarded when the auxin concentration was raised. The leaf culture system appears to reflect the dual action of 2,4-D known from herbicide research, namely growth stimulation from differentiating (or differentiated) cells, but growth suppression at or in the vicinity of apical meristems. Correspondingly, when the callus of apparently suppressed, slowly proliferating root primordia was transferred to media without 2,4-D or with low concentrations (0.16–2.5 μ M ) rapid proliferation commenced, leading to profuse root outgrowth. The system demonstrates the ambiguous role which this auxin appears to have, at least in wheat tissue culture.