The effect of various plant growth substances as single agents was evaluated in a complex tissue culture system: whole embryo culture of early differentiating barley embryos. Callus formation in unsupplemented medium derives from the mesocotyl and is uniquely characteristic of cultures initiated at this stage of embryonic development. This phenomenon could be prevented or reversed by incorporation of gibberellic acid in the medium resulting in plantlet formation. Indoleacetic acid enhanced callus growth, whereas kinetin did not promote either callus or meristematic development. Callus tissue markedly accumulated starch, effectively lowering the cellular osmolarity, while inducing a corresponding rise in the osmolarity of the culture medium. This osmotic pattern was reversed by gibberellic acid induction of shoot formation. These osmotic-hormonal interactions are interpreted relative to in vivo, in situ normal embryogeny or developmental lesions such as tumors.

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