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 fresh and dry weight, seed development of Chinese cabbage (Brassica rapa subsp. pekinensis) can be divided into 10 discrete but contiguous stages. Precocious germination (PG) occurs in the seeds of a Chinese cabbage mutant on the parent plant mainly during the maturation period (from 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 developing mutant seeds have higher water contents and lower dry weights than those of the wild-type and are less tolerant of desiccation. The testa structure of Chinese cabbage seed is similar to that of other Brassica species. In developing seeds, it consists of the epidermis, subepidermis, palisade layer and pigmented layers, while in the mature dry seeds, all the cell layers except the palisade layer are crushed into non-cell structures and are stacked on the outer and inner faces of the palisade layer; the aleurone layer is fused to the testa. An alteration in testa structure of the mutant seeds (both non-germinated and precociously-germinated seeds) is probably related to PG. The secondary cell wall materials are less, or not deposited on the radial and basal walls of the palisade cells on the adaxial side of the testa, which could result in a weaker mechanical restraint, thus leading to PG.