Difference in δ15N signatures among plant parts of perennial species subjected to drought stress with special reference to the contribution of symbiotic N2-fixation to plant N

The N-15 natural abundance method has been widely used for evaluation of symbiotic N-2-fixation. The method inevitably requires a reference plant that reflects soil derived delta(15)N similar to that in a N-2-fixing target plant, for estimating the contribution of fixed N-2. However, it is often difficult to select a suitable reference plant. Recently, an alternative method was proposed using the difference in delta(15)N values between shoots and nodulated roots, which did not require a reference plant per se. Whether this Method is applicable to a wide range of N-2-fixing plants having different, growth habits and symbiosis types remains to be verified. To test the applicability of this method for perennial plants, we examined the difference in delta(15)N values between shoot and nodulated root (Delta delta(15)N(s-nr)), and that between Shoot and root (Delta delta(15)N(s-r)) in 6-month-old plants grown in pots with different soil moisture regimes. The relationships between Delta delta(15)N(s-nr) and the percentage of N derived from atmospheric N-2 (%Ndfa) calculated from the conventional N-15 natural abundance method, and between Delta delta(15)N(s-r) and %Ndfa were analyzed in N-2-fixing legume Lespedeza cuneata and N-2-fixing non-legume Elaeaguus pungens and Myrica rubra. A close correlation was found between Delta delta(15)N(s-nr) and %Ndfa as well as between Delta delta(15)N(s-r) and %Ndfa in Lespedeza cuneata, while no correlation was found in N-2-fixing non-legume species. The results indicated that Delta delta(15)N signatures could be useful for estimating %Ndfa for N-2-fixing perennial legume (Lespedeza cuneata) in the first growth season but might not be applicable for N-2-fixing actinorhizal plants.