Effects of media N content and rhizobial strain on N2 fixation and partitioning in Leucaena seedlings

Abstract

Research was conducted to examine the effect of Rhizobial race and N fertilization on N2 fixation and N partitioning in seedlings of the genus Leucaena. Seedlings of Leucaena leucocephala (Lam) Dewit variety K-8 and L. retusa Gray variety Yellow Puff were grown in slender tubes filled with fritted clay and watered with nutrient solutions containing 0, 2, or 8 mM nitrate N labeled with 0.1% N15. The seedlings were inoculated with Rhizobia loti race 94A3 or R. logi race 9408. After 12 weeks the plants were harvested and the effects of N fertilization and Rhizobial race on nodulation N2 fixation, and N partitioning were examined. Using the N15-dilution method total N fixed by dinitrogen fixation was determined as well as distribution of fixed versus fertilizer N within the plant. Both Rhizobial races infected both Leucaena species resulting in an effective symbiosis. However, the 2 Leucaena species responded quite differently to N fertilization. The addition of 2 mM N to the nutrient solution effectively eliminated nodulation in L. retusa yet increased both nodulation and N2 fixation in L. leucocephala. The 8 mM N rate eliminated nodulation in both species. Due to the elimination of nodulation and N2 fixation by N fertilization in L. retusa, the effect of N fertilization on partitioning of fixed N was only examined in L. leucocephala. Increasing the N content of the nutrient solution increased the proportion of total N in the shoot and reduced the proportion in nodules. A larger proportion of the fixed N was retained in the root and nodules than fertilizer N. Although the 2 mM N treatment increased the amount of N2 fixed in L. leucocephala compared to the 0 mM treatment, the proportion of fixed N2 was reduced to less than one third of the total N in the plant.