SEED DEVELOPMENT, TESTA STRUCTURE AND PRECOCIOUS GERMINATION OF CHINESE-CABBAGE (BRASSICA-RAPA SUBSP PEKINENSIS)

On the basis of embryo and seed colour, morphology, accumulation of fr esh and dry weight, seed development of Chinese cabbage (Brassica rapa subsp. pekinensis) can be divided into 10 discrete but contiguous sta ges. Precocious germination (PG) occurs in the seeds of a Chinese cabb age mutant on the parent plant mainly during the maturation period (fr om stage 5 to 8), with either the radicle or the cotyledon protruding out of the testa. All plants of the mutant line produce some PG seeds, but among the seeds, only 18% (maximum) germinate precociously. The d eveloping mutant seeds have higher water contents and lower dry weight s than those of the wild-type and are less tolerant of desiccation. Th e testa structure of Chinese cabbage seed is similar to that of other Brassica species. In developing seeds, it consists of the epidermis, s ubepidermis, palisade layer and pigmented layers, while in the mature dry seeds, all the cell layers except the palisade layer are crushed i nto non-cell structures and are stacked on the outer and inner faces o f the palisade layer; the aleurone layer is fused to the testa. An alt eration in testa structure of the mutant seeds (both non-germinated an d precociously-germinated seeds) is probably related to PG. The second ary cell wall materials are less, or not deposited on the radial and b asal walls of the palisade cells on the adaxial side of the testa, whi ch could result in a weaker mechanical restraint, thus leading to PG.