Effects Of Continuous Cropping Obstacle On Growth Of Rehmannia Glutinosa And Its Physio-logical Characteristics

Taking Rehmannia glutinosa Libosch, a medicinal plant with typical continuous cropping obstacles as the testing material, using the field experiment combined with lab analysis as the method. This research explained the harm of continuous cropping to Rehmannia glutinosa Libosch systematically by studying its physio-ecological changes such as emergence of seedlings, growing, nutritional situation and quality in continuous cropping, in comparison with the main crop. Based on the special phenomenon of Rehmannia glutinosa Libosch in continuous cropping, this research attach most importance to the changes of hormone contents, photosynthesis, photosynthetic product transporting, investigate the effects of on the plant growing as well as mechanism of the continuous cropping obstacles of Rehmannia glutinosa Libosch. The main results as follows:1. The effect of continuous cropping obstacles to Rehmannia glutinosa Libosch begins at the seedling stage, but has no influence on the emergence of seedlings. The two most important effect of the obstacles are: the lower growth rate and the imbalanced R/T which to bring about what the growing of continuous cropping plant manifest as: the smaller of the overground plant and the improper intumescentia of the underground root, the imbalanced R/T, the shorter growth season, low quality and eventually has no commodity value.2. The content of endogenous hormone is the first indicator of change in the body of plant during the continuous cropping. ABA content in Rehmannia glutinosa Libosch of continuous cropping is significantly higher that the main plant, that is 1.2 times than the main plant. Endogenous hormone is also speculated as one of the main factors which initially decreased the photosynthesis and caused the output stream slack. The change of the endogenous hormone content is the starting point of the obstacles effects, and ABA is one of the most important factors.3. The MDA content in the leaves of continuous cropping Rehmannia glutinosa Libosch is remarkably higher than the main plant in the 70 days after sowing. This indicates that the leaves of continuous cropping plant produces lots of active oxygen and free radicals which induced the membrane lipid peroxidation and damage to the membrane structure and function.4. The decrease of the photosynthetic rate is the main reason for the lower growth rate of continuous cropping plant. Closed Stomatal closure and reduction of chlorophyll content are the initial factors of lower photosynthetic rate. While the destruction of the latter half of the cell structure makes the photosynthetic rate even lower. C14 isotope tracer test shows that the imbalanced R/T of the continuous cropping Rehmannia glutinosa Libosch is caused by the poor downward flow output of the photosynthate. It is confirmed that poor photosynthate flow and low efficiency transportation of photosynthate are the main reasons for restrains the swelling of Rehmannia glutinosa Libosch's tuberous root.5. The ATPase activity of tuberous root from Rehmannia glutinosa Libosch of continuous cropping is lower than that from the main crop. Two significant differences can be found between the 70th day and 95th day samples after the planting. ATP inhibitions of enzyme activity in the continuous cropping plant are as follows: 38.4% and 13.6%. A preliminary conclusion can be drawn that it is the decrease of the ATPase activity that affects cell division of the tuberous root, and thus leads to the extended swelling of it. When the activity of continuous cropping Rehmannia glutinosa Libosch's root system is restrained, it affects the root system's absorption of water and mineral elements, which in turn causes double water and mineral stress at its growing stages, and greatly slows down its growing speed.6. The analysis indicates that during the whole growing stage, Rehmannia glutinosa Libosch of continuous cropping shows potassium stress. Thus we can infer that potassium stress affects the transport of photosynthate and is one of the main reasons that restrain the swelling of its tuberous root.To sum up, we can draw the following conclusions: On about the 50th day after the sowing of continuous cropping plant, when the seedlings just come out, its growing adjusting system is greatly influenced by allelochemicals, and its endogenous hormones changes apparently, especially the rapid increase of the level of ABA. The high ABA level restrains the transport of photosynthate, and reduces the stomatal conductance of the leaves as well, thus affects the photosynthesis. That is the starting point of disorder effect of Rehmannia glutinosa Libosch of continuous cropping. Meanwhile with the growth of the plant, the amounting of time and power of adversity stress under continuous cropping, the cell permeability in its leaves increases, its defense system is damaged, the chlorophyll level declines, all these lead to the lowering of photosynthesis speed. At the middle and late growing stage, the cell structure of continuous cropping plants begins to change, which greatly slows down the photosynthesis speed, reduces the activity of its root system, causes double water and nutrition stress. Since potassium stress may restrain the transport of photosynthate and the choking of the current may lead to the feedback-inhibition effect of photosynthesis, thus causing the reduction of photosynthesis products and even more reduction for that of the underground. This chain goes on and on, forming a vicious circle. Finally, it causes the slow growing and the unbalanced root cap ratio of continuous cropping dihuang, and on the surface the plant above ground is uaually of a small size, the tuberous root underground can not boom irregularly, the root cap ratio is unbalanced, and in the end the plant will not grow abnormally.