MORPHOGENETIC BEHAVIOR OF TOBACCO TISSUE CULTURES AND IMPLICATION OF PLANT SENESCENCE

Decreases in the growth and organ‐forming capacities characterized continuously cultured tobacco (Nicotiana tabacum ‘Wisconsin 38‘) callus. The root‐initiation ability was completely lost in 1 1/2‐year‐old cultures. The rate of shoot formation decreased to a low plateau in cultures that reached 1 1/2‐3 years of age since the explanting. An inverse relationship between callus growth and in vitro clonal age was also observed. Studies with callus clones started from individually isolated pith cells showed that the growth and organ‐forming potentials of somatic cells varied, signifying that cell alterations had occurred in vivo. Both totipotent and non‐totipotent cell lines were obtained. Subculturing the single‐cell lines through several passages disclosed that the morphogenetically depressed state was irreversible and instability was characteristic of the totipotent lines. In the latter, a change toward the morphogenetically repressed level was observed. These findings are discussed in relation to the phenomenon of senescence. It is suggested that an accumulation of somatic mutations, i.e., genetic alterations resulting in reductions in the morphogenetic potential of cells, is a basis underlying senescence.