The effect of colchicine on embryogenesis induction and chromosome doubling during microspore culture was evaluated in two F1 hybrids of winter oilseed rape (Brassica napus L.). Colchicine treatment (50 and 500 mg/L) of isolated microspores during the firt 15 h in culture stimulated embryogenesis and produced large amounts of healthy-looking embryos. These normal embryos germinated well at 24℃ after being transferred to solid regeneration medium and an initial period of low temperature (2℃) for 10 days, and could directly and rapidly regenerate vigorous plants. A high doubling efficiency of 84-88% was obtained fronl 500 mg/L colchicine treatment for 15h with low frequency of polyploid and chimeric plants. A colchicine treatment duration of 6h was less effective on embryogenesis and doubling efficiency. The present experiment also showed that changing of induction medium 15h after microspore isolation produced higher spontaneous doubling efficiency, as compared with medium change 6h after isolation.

The effect of colchicine on induction of embryogenesis and chromosome doubling during microspore culture was evaluated in two F1 hybrids of spring oilseed rape (Brassica napus L.). Immediate colchicine treatment of isolated microspores with the concentrations 50 and 500 mg/L for 15 h stimulated embryogenesis and produced large amounts of healthy-looking embryos. These normal embryos germinated well at 24 ◦C after being transferred to solid regeneration medium and an initial period of low temperature (2 ◦C) for 10 days, and could directly and rapidly regenerate vigorous plants. A high doubling efficiency of 83-91% was obtained from 500 mg/L colchicines treatment for 15 h with low frequency of polyploid and chimeric plants. The present experiment showed that a treatment duration of 30 h revealed less positive effects on embryogenesis and doubling efficiency, especially at higher colchicine concentration (1000 mg/L). Poor embryogenesis and embryo germination were observed from ordinary microspore culture without change of induction medium and colchicine treatment, and several subcultures were required for induction of secondary embryogenesis and plant regeneration.

Using three varieties of Brassica rapa, cv. Hauarad (accession 708), cv. Maoshan-3 (714) and cv. Youbai (715), as the maternal plants and one variety of B. oleracea cv. Jingfeng-1 (6012) as the paternal plant, crosses were made to produce interspecific hybrids through ovary culture techniques. A better response of seed formation was observed when ovaries were cultured in vitro at 9-12 days after pollination on the basal MS and B5 media supplemented with 6-benzylaminopurine (BA) and naphthylacetic acid (NAA). The best response was observed for cross 714×6012 with the rate of seeds per ovary reaching 43.0%. Seeds for cross 715×6012 showed the best germination response (66.7%) on the regeneration medium (MS + 1.0 mg l−1 BA + 0.05 mg l−1 NAA). In all three cross combinations, good response in terms of root number and length of plants was observed on the root induction medium (MS+1.0 mg l−1 BA + 0.1 mg l−1 NAA). A better response was observed for the regenerated plants cultured for 14 days than for 7 days. The ovary-derived plants with well-developed root system were hardened for 8 days and their survival rate reached over 80%. Cytological studies showed that the chromosome number of all plants tested was 19 (the sum of both parents), indicating that these regenerated plants were all true hybrids of B. rapa (n=10)×B. oleracea (n=9). The regenerated plants were doubled with colchicine treatment, and the best response in the crosses 708×6012, 714×6012 and 715×6012 was observed when treated with 170 mg l−1 colchicine for up to 30 h and their doubling frequency reached 52, 56 and 62%, respectively.

A substantial amount of seed is left in the fields before and during harvest of oilseed rape. Although this crop exhibitslittle or no primary dormancy, the absence of certain environmental cuesthat promote germination of imbibed seeds induces secondary dormancy. The work reported investigated the extent to which environmental stress conditions, including osmotic stress, low oxygen stress and anaerobiosis, induce secondary dormancy in oilseed rape, and examined the ariation in development of secondary dormancy between and within genotypes. Osmotic stress was most effective in inducing dormancy. Anaerobic treatment produced very few dormant seeds, asdid an atmosphere low in oxygen and high in nitrogen. The development of secondary dormancy under osmotic stress varied considerably between and within genotypes. Dormancy ranged from almost zero to about 60% for winter genotypesand about 85% for spring types. Within genotypes, variations occurred between seed lots and years of harvest. Temperature variations affected the percentage of dormant seeds. More dormant seeds were likely to be produced with incubation under water stress at 20℃ than at 12℃. In winter genotypes, fewer dormant seeds were produced when incubation temperature and germination test temperaturesdiffered. Thus, incubating at 20℃ and 12℃, followed by germination tests at 20℃and 12℃, respectively, produced most dormant seeds. Also, in the winter genotypes, the potential development of secondary dormancy waspos itively correlated with the pattern and speed of germination of untreated seeds.

Root parasites of the genus Orobanche are serious weeds in agriculture. An aseptic infection system of host roots using calli of three Orobanche species was developed for the study of host

The effects of culture media, explants and genotypes on shoot regeneration in oilseed Brassica species were examined in this study. The maximum shoot regeneration 3frequency was obtained in Murashige and Skoog medium supplemented with 3 mg l)1 6-benzylaminopurine and 0.15 mg l)1 1-naphthaleneacetic acid. The addition of 2.5mg l)1 AgNO3 was very beneficial to shoot regeneration in B. napus and Ag2S2O3 (10mg l)1) was even superior to AgNO3 (2.5 mg l)1). Explant age, explant type and carbon source also significantly affected shoot regeneration. Four-day-old seedlings of cotyledonary explants showed the maximum shoot regeneration frequency and number of shoots per explant. Of the four explants-peduncles, hypocotyls, cotyledons and leaf petioles-cotyledons produced the highest shoot regeneration frequency (56.67%). Four carbon sources-glucose, maltose, starch and sucrose-were compared for their respective effects on shoot regeneration from cotyledonary explants. Sucrose appeared to be the best carbon source for shoot regeneration with the highest shoot regeneration frequency (76.00%). Considerable variation in shoot regeneration from cotyledonary explants was observed both between and within Brassica species. The shoot regeneration frequency ranged from 10.00% for cv. R5 (B. rapa) to 83.61% for cv. N1 (B. napus). Two B. napus, one B. carinata and one B. juncea cultivars exhibited shoot regeneration frequency higher than 70 %. In terms of the number of shoots produced per explant, B. rapa showed the highest variation, ranging from 5.64 for cv. R3 to 1.33 for cv. R5. Normal plantlets were regenerated from all induced shoots and developed normally. The 4regenerated plants were fertile and identical with the source plants.

Crosses were made to produce interspecific hybrids between Brassica napus • B. juncea and their reciprocals with the aid of embryo culture techniques. A better response of hybrid embryo culture was obtained from 4two cross combinations of B. juncea • B. napus (Ames 24521 • Huyou 15 and Vittasso • Zheshuang 72) than from their reciprocals. Embryo culture was more effective in terms of plant regeneration when embryos were cultured in vitro at 15 days after pollination (DAP), while more calli were initiated when embryos were excised and cultured at 10 DAP. A better response was observed on the MS medium with 0.3 mg l)1 naphthylacetic acid (NAA) + 1.5 mg l)1 6-benzylaminopurine (BAP) and with 0.3 mg l)1 NAA + 2.0 mg l)1 BAP. Callus formation and plant regeneration on these two media reached 55.43 and26.65 %, and 66.98 and 24.61 %, respectively.

Stress is an essential component during embryogenesis induction in microspore culture. Cold pretreatment has been used in cereal microspore culture but very seldom attempted in Brassica microspore culture. The effect of cold pretreatment of flower buds subjected to a liquid medium on microspore embryogenesis was investigated in spring and winter Brassica napus, as well as in B. rapa and B. oleracea. Cold pretreatment significantly enhanced microspore embryogenesis (by 1-7 fold) compared to commonly used microspore culture protocol in B. napus, while it was less effective in B. rapa or even negative in B. oleracea. The appropriate duration of cold pretreatment was found to be 2-4 days, which stimulated the best microspore embryogenesis. Cold pretreatment was also able to promote embryo development including the improvement of embryo quality and acceleration of embryogenesis. When incorporating with medium refreshing, coldpretreatment could initiate the most microspore embryogenesis than any other treatment used. With further improvement cold pretreatment method may have a positive potential in Brassica breeding programmes.

Field experiments were conducted to study genotypic variation for potassium uptake and utilization efficiency in sweet potato for 2 years. In 1997, a field experiment was conducted using 84 genotypes under no potassium fertilization. In 1998, two field experiments were carried out. In experiment I, eight good genotypes evaluated from the previous experiment were tested at two K application levels: 0 and 270 kg ha-1 of K2SO4. In experiment II, three special varieties with respect to pigment content were tested at five K levels from 0 to 600 kg ha-1. A split-plot design with three replications was used. Variation existed among genotypes in K concentration, accumulation and potassium efficiency ratio (KER) in the field. Among various plant parts, petiole contained the highest K concentration and storage roots had the highest K accumulation at maturity. These changed considerably with genotype and K level. KER had significant positive correlation with root weight per plant, root:top ratio and harvest index (HI); and significantly negative correlation with K concentration and accumulation in the roots or whole plant at maturity. Based on KER, genotypes were identified as efficient and inefficient in potassium utilization. Yield increased generally in all genotypes with K application. The increase, however, varied among different genotypes. It was also found from this study that the fertilizer level that gave the highest yield was K2 which is 300 kg ha-1 of KzSO4. The yield increase as a result of K application was mainly due to the increase in root:top ratio which led to greater amount of photosynthate translocation to the storage roots causing their increase in size. Most of the quality parameters tested increased with increasing levels of K nutrition. Root dry matter (%), Brix (%), carotene content, anthocyanin content increased with K application. The extent of increase, however, differed with genotypes. Protein content generally tended to reduce with increase in K level. The degree of reduction also varied with genotypes.