Abstract:

In order to study the response of alfalfa （Medicago sativa） leaves and roots to water and N addition， two contrasting water availability regimes were set up under greenhouse conditions： water stress （WS） with soil maintained at 35%±5% field water capacity （FWC） and well-watered （WW， 70%±5% FWC）. Treatments of 0， 5 and 10 mmol·L^-1 N （Nn， Nm and Nh， respectively） were established under each watering regime. The responses of physiological parameters of alfalfa leaves and roots and their C and N characteristics to different water conditions and exogenous N addition were then studied at the three N levels. It was found that WS and exogenous N increased malondialdehyde content in alfalfa leaves， but had no significant effect on the root system. WS and N addition did not affect C content in alfalfa leaves， but N addition increased C content in roots. The content of N in alfalfa leaves did not differ between water treatments， but root N content was increased under the WS treatment. Exogenous N addition not only increased leaf N content， but also increased root N content. However， leaf N content was more sensitive to exogenous N addition under the WW treatment， while root N content was more sensitive to exogenous N addition under the WS treatment， which indicates that the response of alfalfa leaf and root C and N status to N addition was regulated by soil moisture conditions. The C∶N of alfalfa roots was higher than that of leaves， and was more sensitive to soil water status and exogenous N addition. The WS treatment resulted in significantly less negative root δ¹³C， but had no significant effect on leaf δ¹³C. δ¹⁵N tended to be reduced with exogenous N addition， but this effect was statistically significant in roots under WS treatment and in leaves under WW treatment. In conclusion， compared with leaves， alfalfa root physiological parameters and C and N characteristics displayed more active responses to water and exogenous N addition， and played a more important role in growth. The results are helpful to fully understand the response strategies of alfalfa organs to water and exogenous N addition， and provide a theoretical basis for formulation of water and fertilizer management systems that more precisely meet alfalfa requirements in dry farming areas of China.

Key words: alfalfa, root-shoot interaction, δ¹³C, δ¹⁵N, water deficit