Initial Stages of Callus Development on Internodal Stem Segments of Pea and Faba Bean In Vitro

ABSTRACT

We have investigated the morphology and the cytogenetics in cultivated stem cells of pea (Pisum sativum L.J and faba bean (Vicia faba L.) at the initial stages of callus development in liquid nutrient medium containing different concentrations 2, 4-D (2,4-dichlorophenoxyacetic acid) of 0, 2, 4, 8, 16 and 32 mgl⁻¹ The earliest stage of callus development in both species was indicated by the formation of specific morphological patterns—the so-called dividing rows (DR). These arose after periclinal divisions of medium-sized parenchyma cells from the superficial layers in the neighbourhood of the vascular bundles. DR consist of small meristematic cells with high mitotic activity. At the end of the second week of treatment, their structure was lost and meristematic regions of cells with varying shape and size remain in the actively proliferating tissue. Callus induction and development were obtained in the presence of 2 mgl⁻¹ of 2,4-D only and this process was initiated by the formation of DR. Xylem and cambium were not involved in callus development in both species. That was true for the epidermis tissue in pea too. The faba bean epidermis was integrated in the proliferation process. In cultures lacking 2,4-D no callus was observed. Concentration of 32 mgl⁻¹ was found to inhibit the mitosis. The most frequent aberrations in the two species were polyploidy and aneuploidy. Other anomalies were the enlargement of cells and/or nuclei, bi- and multinuclear cells, increased number of nucleoli, unstained nuclei and loosen nuclear structure. The mitotic aberrations included laggard formation, chromatin bridges besides irregular chromatin distribution and chromosomes with decreased integrity in the metaphase. The observed morphological and cytogenetical anomalies have been arisen under the influence of the cultural conditions. Their distribution by the two species suggests different genotype response in vitro. Deep invaginations in the nuclei of dedifferentiated cells and changes in the wall thickness were observed electron microscopically.