Study On The Relationship Between Root Growth And Plant Growth In Alfalfa Under Seasonal Cultivation

Alfalfa(Medicago sativa L.) improved the situation of insufficient supply of high quality forage under the model of seasonal cultivation in southern farming regions of China, where alfalfa was introduced. There are obvious difference of soil conditions and climate between south and north in China, which affected the characteristic change of forage root systems, consequently affecting the growth of shoot. Eight high fall dormancy alfalfa cultivars were carried out in Nanjing in this study. Morphological characteristics of root system, storage nutrient capacity of taproot, water use efficiency, net photosynthetic rate and the growth characteristic of shoot were observed by pot and field experiment, respectively, the relationship among of which were analyzed. The main results are as follows.1. Taproot diameter, taproot dry weight and first lateral number were significant difference among eight alfalfa cultivars (P<0.05). The contents and reserves of soluble sugar, starch and nitrogen were significantly different among eight alfalfa cultivars (P<0.05). Saidie7, WL525HQ and Eureka had advantage on taproot diameter, taproot dry matter and first lateral number than other cultivars. Saidie7, WL525HQ, Sequel and WL903had higher contents and reserves of soluble sugar, starch and nitrogen.2. In field experiment, alfalfa was planted in autumn. In initial flowering stage next spring, study on root system at0～15cm closed collar was conducted. The taproot diameter of alfalfa cultivars was3.16～3.52mm. The taproot dry weight was0.73～1.43g. The first lateral root number was34.2～46.4. Most first lateral roots, taking of50.57%～55.16%, grew mainly at0～5cm closed collar. The contents of soluble sugar, starch and nitrogen were241.10～319.92mg/g,126.10～180.05mg/g and17.28～21.86mg/g, respectively. The reserves of soluble sugar, starch and nitrogen were221.77～445.48mg/plant,123.75～226.17mg/plant and14.31～30.92mg/plant, respectively.3. Bigger taproot diameter and dry weight promoted water use efficiency and growth of plant. In pot experiment, taproot diameter had a significant positive correlation with plant height, shoot diameter and leaf dry weight (P<0.05). In field experiment, the taproot diameter had a significant positive correlation with shoot diameter and shoot dry weight (P<0.10), and had a significant positive correlation with primary shoot number and leaf dry weight (P<0.05). In pot experiment, taproot dry weight had a significant positive correlation with shoot diameter, shoot dry weight and leaf dry weight (P<0.05). In field experiment, taproot dry weight had a significant positive correlation with primary shoot number, shoot dry weight, leaf dry weight, and leaf area (P<0.05).4. More first lateral root and closer to collar promoted water use efficiency and plant growth. In pot experiment, first lateral root number had a significant positive correlation with plant height and shoot diameter (P<0.05). Lateral root position had a significant negative correlation with shoot dry weight and leaf dry weight (P<0.05). In field experiment, the proportion of first lateral root number at0～5cm closed collar was positively correlated with water use efficiency, plant height, shoot diameter, primary shoot number, shoot dry weight, leaf dry weight, and leaf area. But the proportion of first lateral root number at5～10cm or10～15cm closed collar had no significant correlation with plant growth.5. Better plant growth and higher net photosynthetic rate promoted the nutrient accumulation in taproot. Nitrogen in taptoot was a basis of plant growth and photosynthesis. In pot experiment, the reserves of soluble sugar and starch basically had a significant positive correlation with shoot diameter, shoot dry weight, leaf dry weight (P<0.05). The reserves of nitrogen had a significant positive correlation with shoot dry weight and leaf dry weight (P<0.05). In field experiment, the reserves of soluble sugar and starch in taproot basically had a positive correlation with net photosynthetic rate, primary shoot number, shoot dry weight, leaf dry weight and leaf area (P<0.05). The reserves of nitrogen had a significant positive correlation with net photosynthetic rate, shoot diameter primary shoot number, leaf dry weight and leaf area (P<0.05). Root morphology was closely related with storing nutrients in taproot. The reserves of soluble sugar, starch and nitrogen in taproot had a promote relationship with the growth of taproot and lateral roots.6. Nonstructural carbohydrates in taproot contributed to the shoot regrowth during the early time after cutting. Five days after cutting, the contents of soluble sugar and starch in taproot decreased by53.17～163.56mg/g and24.49～95.88mg/g, respectively. Higher contents of soluble suger and starch promoted the regrowth of plant. The contents of soluble sugar and starch in taproot before cutting were positively correlated with plant regrowth rate in0～6day after cutting and regrowth grasses morphological traits. Soluble sugar contents had a significant positive correlation with primary shoot number (P<0.10). Starch contents had a significant positive correlation with primary shoot number, shoot dry weight and leaf dry weight (P<0.05).