| It was conducted to study the effect of the root pruning on the alfalfa (Medicago sativa L.) at the different stages in the two years after the root was cut levelly at depth of 40cm, 80cm, 120cm, 160cm, 200cm, 240cm, 280cm, 320cm, 360cm, 400cm, 450cm and 500cm respectively at the greening stage in the field to lead to the water-absorbed stress, through the index about the growth, the canopy temperature, and the physiological process of the alfalfa. The main results were as followings:1. The effects of the root pruning on the vegetal and physiological index of alfalfa varied in the depths of the root pruning, the stages and the periods of growth after the root pruning. And the treatments according to the reactions to the root pruning could be classified into 3 groups. The first group included the shallow treatments (≤80cm), the effects of which changed greatly from the most difference with the control at the beginning of the root pruning to the least difference in the second year. The in-between treatments (120～320cm) belonged to the second group, which were worse on the growth and physiologically all the time than the control and other treatments. Then, the deep treatments (≥360cm) came into the third group, which were close to the control all time long after the root pruning. At the same time, the index changed greatly from the branching to the initial bloom stage. Furthermore, the effect of the root pruning was more obvious in the second year and during the high-temperature and dry period than in the first year and the other time, respectively.2. After the root pruning, the shoot height, the leaves area, the shoot diameter, the number of the leaves and the yield of the alfalfa all decreased; the number of node tended to decrease while the rate of the shoot/leaves reversed; however, the rate of the fresh/dry didn't changed significantly. At the same time, for the shallow (≤80cm) and deep(≥360cm) treatments, the shoots were higher; the leaves area was bigger; the shoots were thicker; the nodes were more and the rate of the shoot/leaves was lower than the in-between treatments (120～320cm). Furthermore, this trend was more obvious in the second year. And there wasn't the significant difference between the leaves of the shallow (≤80cm) treatments and the control's in the second cut of the second year and the other later cuts while there was the significant difference(P<0.05) for the others'(≥120cm). as a whole, the yield of the shallow treatments (≤80cm) was highest; the deep treatments were following and the in-between treatments (120～320cm) were lowest.3. After the root pruning, the photosynthetic rate (Pn), the stomatal conductance (Gs), the water use efficiency (WUE) all decreased; the transpiration rate (Tr) tended to increase; however, the internal CO2 (Ci) did not changed significantly. And the effect varied in the depth of the root pruning and the examined time. Pn, Gs, WUE and Tr of the shallow (≤80cm) and deep (≥360cm) treatments were relatively higher than the in-between's (120～320cm). Specificly, in the first year after the root pruning, Gs, Tr and WUE basically did not changed significantly while Pn of the in-between treatments (80～360cm) went down significantly. However, in the second year, Pn and Tr of all treatments and WUE of the treatments between 80cm and 360cm decreased significantly (P<0.05) while Tr of the in-between treatments (160～240cm ) increased significantly (P<0.01).4. In the most time of the observation, the root pruning did not cause the significant change of the PSⅡactual photochemical efficiency (ΦPSⅡ) and the electronic transfer rate ( ETR ). But, the PSⅡmaximal photochemical efficiency ( Fv /Fm ) , the ability of PSⅡreaction center to trap energy from antenna pigment (1/Fo - 1/Fm ) and the initial fluorescence (Fo) went up during the high-temperature and dry period after the root pruning. And the effect of the shallow(≤80cm) and deep (≥360cm)treatments was less than the in-between's (120～320cm). At the same time, for the lines of fluorescence parameters, the maximal fluorescence (Fm') went down gradually while the photochemical quench (qP),ΦPSⅡand nor-photochemical quenching co-efficiency (qN) reversed. Furthermore, the effect of the in-between treatments (120～320cm) and of the second year on the lines was greater than that of the shallow(≤80cm) and deep (≥360cm)ones and of the first year, respectively. Also, from 0s～280s the line of the shallow and deep treatments changed gradually while that of the in-between (120～320cm) with the dramatic drop or rise went up or down greatly. Besides, from the first year to the second year, Fm', qN,ΦPSⅡand qP of the shallow treatments changed dramatically. Their Fm'turned from the increase of the first year to the decrease of the second year;ΦPSⅡfrom the decrease to the no-significant change while the qP from the decrease to the increase. At the same time, for the deep treatments, qN decreased significantly; qP increased all the time and Fm'和ΦPSⅡdid not change significantly. But, when it came to the in-between ones (120～320cm), there were the significant difference in Fm', qN,ΦPSⅡand qP with the control(P<0.05). All in all, in the second year there was a tend that Fm'andΦPSⅡof the shallow treatments (≤80cm) decreased, that of the in-between (120～320cm) increased while that of the deep ones (≥360cm) did not change significantly. Whereas, qP of the shallow (≤80cm)and deep (≥400cm)treatments increased while that of the others (120～360cm)reversed while qN of all treatments decreased.5. The root pruning at different depth of the soil caused the different-level water stress and the treatments between 200cm～320cm strengthened the water stress of the soil.The root pruning made the activities of the nitrate reductase and the glutamine synthetase of the alfalfa decreased. Basically, the activities of the two enzymes of the shallow(≤80cm) and deep (≥400cm)treatments were higher than that of the in-between(120～360cm). The activities of the nitrate reductase in the all stages decreased significantly (P<0.05). However, when it came to the activities of the glutamine synthetase, some treatments decreased and others increased in the first year while all the treatments generally deceased significantly(P<0.05) in the second year.6. The root pruning made the activities of the SOD, POD and CAT decreased and the malondialdehyde content (MDA) increased. And the effects on the activities and the MDA content varied in the depth of the root pruning and the period after the root pruning. As a whole, the activities and the MDA content of the shallow (≤80cm) and deep (≥360cm) treatments were higher and lower than that of the in-between ones (120～320cm), respectively.7. The content of the soluble protein and sugar of the alfalfa leaves decreased after the root pruning. And the contents of the shallow (≤80cm) and deep (≥360cm) treatments were higher than that of the in-between (120～320cm). However, there was no distinct regulation of the effect on the proline content. In the most of the time, the proline content of some treatments decreased while that of others increased. Only a little of the time, did the proline content of all treatments decrease or increase, and were the contents of the shallow (≤80cm) and deep (≥400cm) treatments higher than that of the in-between (120～360cm). But, for the content of the soluble protein, some treatments increased while that of others decreased in the branching and budding stages of the first and second cut in the first year. And that of all treatments decreased in the initial bloom stage of the first cut of the first year, the third cut of the first year, and the first cut of the second year. Furthermore, that of the in-between treatments (120～360cm) decreased while that of the shallow (≤80cm) and deep (≥400cm) ones increased in the third cut of the second year. Whereas, in the forth cuts of the two years the soluble protein contents of all treatments did not changed significantly. However, when it came to the soluble sugar content, all treatments decreased all the time, except some treatments decreased while others increased in the branching and budding stages of the first cut in the first year.8. The root pruning made the temperature of the canopy of the alfalfa increased. Basically, the temperature of the shallow (≤80cm) and deep (≥320cm) treatments was significantly (P<0.05) lower than the in-between ones (120～280cm). After the root pruning, there were little difference in the canopy temperature between the treatments in the first and forth cuts of the first year and after-raining period of the second cut in the two years. But, that of the in-between increased significantly (P<0.05) in the third cut (high-temperature and dry) and the forth cut of the second year while that of all treatments increased significantly (P<0.05) in the third cut of the second year (high-temperature and dry).9. Under the water stress caused by the root pruning, the electronic transfer rate of the photosynthetic center were influenced; the photosynthetic rate and the water use efficiency went down; the activities of the enzymes closely related to the nitrogen metabolism decreased; the activities of the protective enzymes and the content of the osmotic adjustment substances decreased; the content of MDA increased; the physiological process were inhibited; the resistance ability to the stress decreased. Lastly, that led to the inhibition of the growth of alfalfa, the decrease of the yield, the increase of the canopy temperature, and the decrease of the ability of soil water maintenance. Thus, the ability of the alfalfa to maintain the self-beneficial surroundings decreased.
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