Genotypic Difference In Response To Continuous Cropping And Its Mechanism For Tobacco(Nicotiana Tabacum L.)

Tobacco (Nicotiana tabacum L.) is one of the important cash crops in china, and it shows a continuous cropping degradation, mainly due to increased plant diseases and insect pests, deteriorating plant growth, and as a result, leading to reduced yield and quality. Plant auto-toxicity and soil microbes were concerned as the key reasons for continuous cropping of the other crops. Therefore, several experiments were carried out to study the effects of plant root exudates on the growth of tobacco seedings and soil microbe communities in different cropping systems. The main objects of the current study were to determine the mechanisms relevant to the bad issues caused by continuous cropping, and to find effective methods for solving the problems and also to provide scientific information for improving the yields and qualities of tobacco.Firstly, the experiment was conducted to collect and extract plant root exudates, and to investigate the effect of these extracts on germination rate of seeds and the growth of tobacco seedlings. Secondly, the experiments were designed to study the effects of different cropping systems on soil microbe communities. At last, the experiment was done to understand the effects of different crop root exudates on tobacco. The main results are as follows:(1) The effects of tobacco root exudates on germination rate differed with tobacco genotypes. Thus, there was no significant difference between the treatments and control for Chunlei3, Nanjiang3and Qianxi1, while other genotypes showed increase under lower root exudate concentration and significant decrease at higher concentration, being similar to the effect of root exudates on seedling growth. In addition, there was an obvious difference in the response to the treatments between the two genotypes. Qianxi1was inhibited and while1Hao was promoted with the increased concentration of the root exudates. Thus, it may be concluded that these two cultivars differed in the auto-toxicity trait.(2) In the various crop rotation systems, the more plant species there were, the more diversity of the bacterial community in soil there was, including Shannon diversity index, equitability index and the Sorensen’s similarity index among soils. In addition, the beneficial bacteria were increased. Under the continuous cropping, the bacterial community diversity and richness were decreased with the increased cropping over the years, and the bacterial community structure became simpler, with a few of pathogen being predominant. Therefore, the reasonable crop rotation is an effective approach to alleviate the deteriorated influence of tobacco continuous cropping.(3) The seeds and seedlings of tobacco genotypes Qianxi1and1Hao were exposed to rape and ryegrass root exudates. The results showed that there was no significant effect on the root exudates on tobacco germination rate, while root length, plant height and fresh weight were increased. Meanwhile, the root exudates did not change membrane lipid peroxide, as expressed by malondialdehyde (MDA) content and stimulated superoxide dismutase (SOD) activity. Moreover, rape root exudates had stronger effect on Qianxi1than ryegrass. The MDA content and SOD activity in the seedlings of1Hao were increased by rape root exudates. Thus it may be suggested that rape is a good preceding crop for tobacco planting.