Anatomical And Cytological Studies On Stem Swelling In Brassica Juncea Var. Tumida Tsen Et Lee

Tuber mustard (Brassica juncea var. tumida Tsen et Lee) is an important variant of Brassica juncea in the Cruciferae family and its swollen stems are used as excellent raw materials for processing and pickling. However, there are many problems in agricultural practices about how to improve its morphological styles and quality. Researches were mainly paid attention to the selection of superordinary varieties of lines and the improvement of cultural measures. Very little knowledge has been attained about the fundamental mechanisms of stem swelling. In order to indicate this aspect, tuber mustard was used as the material. The anatomical structure changes were investigated during tuber growth and development. Meanwhile, changes of cell ploidy level and metabolic substance were studied. Besides, some tentative researches were carried out to investigate the molecular regulation of stem swelling and partial conserved sequences of CyclinD were cloned. The main results were as follows:1. The anatomical structure changes were studied to reveal the swelling mechanism of tuber mustard. The results show that the swollen stem was mainly ascribed to cell number increase and cell enlargement in cortex and pith regions. The existence and activities of the perimedullary region were not observed. Tangential divisions firstly occurred in cambium to increase cell layers; then, longitudinal cell divisions were observed to form the long stem axis; and then, cells divide transversely or randomly oriented in all directions. Cells expanded during the whole process making the tuber swell completely. Due to the unmedial divisions of cambium layer to inner/outer tissues and the different degree of cell expansion in cortex and pith regions, this two parts contribute significantly different to the final size of the swollen stem.2. The changes of nuclear relative DNA contents were investigated during the growth and development stages of tuber mustard. The results revealed cells with multiple ploidy levels (also called endopolyploidy) because of endoreduplication and the highest cell ploidy level is 16C (The whole DNA content of haploid cell is 1C). Flow cytometric analysis showed that MCV (Mean C-value) increases form 3.14C to 5.18C, revealing that endopolyploidy level was characteristic of the development stages. Tissues from different positions of fully swollen stem demonstrated that MCV first decreased then increased from outer to inner part. The lowest MCV occurred near cambium and the highest is in the innermost layer of pith. Combined with the anatomical analysis, we found a close relationship between endopolyploidy level and cell size. Cells in cortex and pith expanded apparently, while cells in cambium and epidermis kept relatively small and there was a gradient existing in pith region. When the stem exhibited as swollen and not swollen features under different growth conditions, the degree of endopolyploidy distincted significantly. When the pithes of stems at different development stages were cultured under proper conditions, callus were induced but there was no endopolyploidy phenomenon investigated. Buds are redifferentiated from piths of stems at young growth stages. Our results also found that the distribution of cells with different ploidy levels was organ-specific and species- specific based on the detection of tuber mustard organs and its closely related species in Brassica genus.3. The changes of metabolic substance contents were studied. The results demonstrated that endopolyploidy often need a high metabolic activity and had a relationship with the accumulation of storage substance.4. We investigated cell cycle-related genes in relation to cell division and endoreduplication. According to the cluster results of CyclinD gene family in Arabidopsis thaliana, conserved homologous sequenceof CycD was cloned in tuber mustard.