Abstract:

In order to explore the effects of nodulation and nitrogen fixation on the photosynthetic characteristics， respiratory metabolism， and plant biomass accumulation of alfalfa （Medicago sativa） plants in alfalfa-rhizobium symbiotic system， two Sinorhizobium meliloti strains， LL2 and QL5， were used to infect the M. sativa cultivar‘Gannong No. 9’， and the differences in nodulation and nitrogen-fixation ability， the phenotype， photosynthetic characteristics， and respiratory metabolism of alfalfa plants after inoculation with LL2 and QL5 were compared. The results showed that： 1） Compared with plants inoculated with S. meliloti QL5， those inoculated with S. meliloti LL2 formed significantly more effective nodules per plant （7）， and the nodules showed significantly higher nitrogenase activity （0.29 μmol·g^-1·h^-1） and leghemoglobin content （0.76 mg·g^-1）. The aboveground dry weight， plant height， and leaf area of alfalfa plants were greater in those inoculated with S. meliloti LL2 than in those inoculated with QL5， indicating that S. meliloti LL2 was a well-matched strain for M. sativa ‘Gannong No. 9’ and S. meliloti QL5 was a poorly matched strain. 2） The values of gas exchange and the contents of chlorophyll and chlorophyll fluorescence parameters and photosynthetic products were significantly higher in plants inoculated with the high-efficiency strain LL2 than in those inoculated with the low-efficiency strain QL5. 3） The adenosine triphosphate （ATP） content and the activities of ATP synthase， hexokinase， isocitrate dehydrogenase， 6-phosphogluconate dehydrogenase， and glucose-6-phosphate dehydrogenase were significantly higher， and the respiration rate was significantly lower， in plants inoculated with strain LL2 than in those inoculated with strain QL5. 4） There was a significant positive correlation between aboveground dry weight and transpiration rate （correlation coefficient， 0.72）， starch content （0.83）， and ATP synthase activity （0.72） （P<0.05）； and a negative correlation between the aboveground dry weight and respiration rate （0.66）. The nodule nitrogen-fixation ability was extremelly significantly positively correlated with net photosynthetic rate （0.83） and soluble sugar content （0.79） （P<0.01）； significantly positively correlated with chlorophyll （a+b） content （0.76） and glucose 6-phosphate dehydrogenase activity （0.85） （P<0.05）； and negatively correlated with the respiratory rate. 5） The dry weight per unit light time was significantly higher in plants inoculated with the highly efficient rhizobium strain LL2 （0.06 g·h^-1） than in plants inoculated with QL5 （0.03 g·h^-1）. In summary， the well-matched， efficient rhizobial strain promoted nodulation and nitrogen fixation， providing a nitrogen source for enhanced photosynthesis capacity and respiration. This increases the production of photosynthetic products， reduces the respiration rate， and promotes plant metabolism， leading to increased biomass accumulation.

Key words: alfalfa, rhizobium, symbiotic nitrogen fixation, photosynthetic characteristics, energy, respiratory metabolism