Developmental and germinative events can occur concurrently in precociously germinating Chinese cabbage (Brassica rapa ssp. Pekinensis) seeds

cabbage, stored reserves, precocious germination, seed development. During germination in the siliquae of developing seeds of a precociously germinating (PG) line of Chinese cabbage (Brassica rapa ssp. Pekinensis), the synthesis of Introduction cruciferin and oleosins was maintained and the activit- After fertilization, the ovule of higher plants develops ies of isocitrate lyase and malate synthase increased. into a seed. During seed development there are three Thus, both developmental and germinative/post- major events: morphogenesis, maturation and desiccation germinative events coexisted within the same PG (Bewley and Black, 1994). Storage reserves are syntheseeds. Drying of the seeds resulted in the develop- sized during maturation, and deposited within storage mental events being switched off and germinative/ organs. Desiccation, the final stage of maturation, is a post-germinative ones being initiated. Only about 18% ‘switch’ from development to germination and, usually, of the seeds of the PG line completed germination in orthodox seeds germinate upon subsequent imbibition the siliquae, so the potential for germination of the ( Kermode, 1990; West and Harada, 1993). However, in remaining non-germinating seeds was tested during some species, developing seeds isolated from their fruits, their development. Isolation of these developing seeds or developing embryos dissected from their seeds, can from the siliquae and imbibition in water induced PG, germinate precociously if they are imbibed in water or but developmental events were terminated and the placed in culture medium, without prior desiccation, for seeds entered a germinative/post-germinative pro- example, Petunia axillaris and P. hybrida seeds (Petiot, gramme. This termination of developmental events in 1989), tomato seeds (Berry and Bewley, 1991), Phaseolus the induced PG seeds was not permanent, however, vulgaris ( Walbot et al., 1972) and cotton (Ihle and Dure, and they could be re-induced by incubating the seeds 1969) embryos. Furthermore, seeds of precociously gerin ABA on Murashige and Skoog (MS) medium. If incub- minating mutants (e.g. those of maize, Arabidopsis and ated on MS medium plus a low concentration of ABA, tomato) can germinate prior to maturation desiccation both developmental and germinative/post-germinative while still on their parent plants ( Koornneef and Jorna, events could be induced to coexist in the PG seeds 1982; McCarty, 1995). This has raised the question as to up to the time of establishment of the seedlings. whether both developmental and germinative events occur Control of the developmental and germinative/post- simultaneously in precociously germinating (PG) seeds germinative events in the seeds was influenced by the or whether developmental events must cease before gerenvironment in which they were developing, which minative ones can commence. could be mimicked by manipulating the nutritional Chinese cabbage (Brassica rapa ssp. Pekinensis) seeds status and ABA content in which the seeds were in are non-dormant or slightly dormant. Isolated developing contact. seeds have a poor germination percentage and germination rate; after desiccation, seeds can achieve a high