Studies On The Evaluation Methods And The Mechanism Of Resistance Of Chinese Cabbage (Brassica Campestris L.) To Diamondback Moth(Plutella Xylostella)

In China, diamondback moth (Plutella xylostella) is one of the most destructive pests and its harmness has risen rapidly especially for cruciferous vegetables. Because of its resistance to insecticide including B. thuringiensis., it has made management of this pest more and more difficult. Thus, it will be very important to screen resistant germpilasm and develop resistant crop varieties. Eight Chinese cabbage inbred lines with different resistance to diamondback moth were used as materials in this experiment. The evaluation methods of pest-resistance and the the morphological, biochemical and genetics mechanism of the resistance of Chinese cabbage (Brassica campestris L.) to diamondback moth were studied. The main results are as follows:Eight Chinese cabbage inbred lines with different characters were evaluated in vitro and net from the aspects of antibiosis and avoidance, respectively. The results showed that there were no obvious differences between two methods. The correlation coefficient of the two methods was 0.96 and at extremely significant level. The resistance to diamondback moth was significantly different among eight Chinese cabbage inbred lines in two methods: 599-3-7 high resistant; 519-3-4 medium resistant; 114-4-4 and 116-4-4 high susceptive. In two methods, the resistance of some Chinese cabbage inbred lines to diamondback moth was not identical to some extent. The evaluation method in vitro was faster and less environmental influence than the evaluation under net conditions. Moreover, the method in vitro can be used to observe the growth and development of diamondback moth's larvae in details.The resiatance of four Chinese cabbage inbred lines at different growth stages was identified in vitro. The result indicated that the resistance change were different based on different genotypes. Generally, the resistance of plants at different growth stages to the diamondback moth had no change and keep stable.By the aid of scanning electron microscopy, it was shown that there was closely correlation between the stoma density on Chinese cabbage leaf and the resistance to the pests. High resistance inbred lines had less stomata than susceptive lines, and the difference of stoma number among different lines was significant. There was more wax on the epiderm and around stomata in high resistance lines. The total Glucosinolates content in eight Chinses cabbage lines was significantly different. High resistant lines had less content than susceptive lines. By analyzing polyamine content, it could be found that the resistant inbred lines had higher content of putrescine (put) than susceptible linse, and the contents of other two polyamines spermidine (spd) and spermine (spm), showed no regular changes relative to the resistance.The complete diallel cross II design only including direct crosses baesd on eight Chinese cabbage inbred lines were used in this experiment. Combining ability, genetic component and genetic model were estimated by using Griffing II, Hayman methods and joint analysis of six generations. The results told us that when parents were insect-resistant, their descendants would be resistant, even the resistance of hybrids may be beyond the parents. When parents were susceptive, their descendants would be still susceptive. When parents had different resistance, the hybrid resistance between the parents trended to the susceptive parents. The resistance to diamondback moth was controlled by recessive genes and the susceptibility was partial dominant. The resistance to diamondback moth were significantly different among the combinations.Estimate of combining ability for insect-resistance showed there were significant differences in GCA and SCA. Estimates of GCA and SCA effects of different inbred lines or combinations showed that the parents with high GCA could produce good F1 with high SCA. Selection should be focused on both the parents with high GCA and the higher SCA combinations in heterosis breeding on insect-resistance improvement in Chinese cabbage.Inheritance of insect-resistance in Chinese cabbage fitted to the "additive-dominant" model. There were additive effect and dominant effect. However, the additive effect played a major role. The the narrow sense and the broad sense heritability on the pest-resistance were 65.71% and 77.87% respectively. Because the high narrow sense heritability, it implied the effectiveness of selection in early generations.With joint analysis methods of six generations, the best fitted model of the resistance inheritances was one major additive-dominant gene plus additive-dominant polygenes (D-1 model), the heritability of the major gene in BC₁P₁, BC₁P₂, F2 populations were respectively 58.26%. 3.27% and 41.11%, and the heritability of polygenes were separately 0%, 34.11% and 5.9%, The additive effect of major gene was higher than the dominant effect. Therefore, breeding for the resistant varieties, it was recommended to make full use of major gene of BC₁P₁ and F2. However, it should not be overlook the minor gene effects in order to get the persistent resistance.