Growth and fine structure of cultured plant cells.

Tissue cultures of Acer pseudoplatanus L. and Atropa belladonna cultivar Lutea Doll have been used as experimental material for ultrastructural and growth studies. Cytoplasmic changes accompanying the growth of batch propagated Acer suspensions in synthetic medium are described, special attention being given to the structure of cell walls, mitochondria, plastids, and microbodies. The structural changes accompanying transfer of cells to liquid medium containing elevated sucrose concentrations, and/or enhanced levels of growth hormones are described. A study has been made of the general ultrastructural features of chlorophyllous Atropa root callus cells at the time of sub-culture, during division, and when senescent. By plating of suspensions of root callus, variant clones were isolated which differed in their morphology and pigmentation, and these differences were retained when the variants were propagated on semi-solidified medium and in suspension. Differences in chlorophyll and carotenoid biogenesis were apparent between the variants following exposure of the dark-grown callus cultures to light. The accumulation of pigments was correlated with chloroplast ontogeny. Investigations of the photosynthetic and autotrophic capabilities of green callus, together with the effect of auxins on growth and pigmentation of suspension cultures are reported; the latter experiments were aimed at enhancing chlorophyll production in cultured cells as a step towards establishing autotrophic cultures. The variant clones differed in their rate of cell division and the maximum cell density attained in suspension. The minimum inoculum density for sustained growth of these cultures was higher on agar than in liquid medium. The clones showed pH optima for growth at low densities. The differing responses of the cultures to "conditioned" medium suggested nutritional differences between the clones. Suspension cultures of green and ''hyaline" clones became aggregated on transfer to medium lacking auxin, or with a low auxin level. This aggregation was accompanied by expression of the totipotency of these cells by organogenesis, even though the clones had previously been propagated for a prolonged period in liquid medium. Cellular aggregates contained meristematic regions overlying highly vacuolated cells, and primordia arising from these meristematic regions differentiated into roots of normal structural organisation.