A fully habituated (auxin- and cytokinin-independent) nonorganogenic (HNO) sugarbeet (Beta vulgaris) callus produces very little ethylene as compared with a normal (N) hormone-requiring callus of the same strain. Both callus types react by growth changes to application of inhibitors of ethylene biosynthesis and ethylene action, of 1-aminocyclopropane-1-carboxylic acid (ACC) as the immediate precursor of ethylene, to transfer from light to darkness, and also to application of exogenous ethylene or an ethylene trapper. This indicates their growth dependency upon their endogenously biosynthesized ethylene and also their sensitivity to exogenous gas. However, the sensitivity was generally higher for the HNO callus producing naturally less ethylene. The weaker reaction of the HNO callus to the exogenous ethylene was attributed to its hyperhydric status (a water layer surrounding the cells). Because low ethylene production appears as a general characteristic of habituated cell lines, the causal and/or consequential relationships of this low ethylene production with other characteristics of habituated tissues (absence of exogenous hormones in the culture media, deficiency of cell differentiation, accumulation of polyamines in neoplastic tissues) are discussed.
