EFFECTS OF MEDIA-N CONTENT AND RHIZOBIAL STRAIN ON N-2 FIXATION AND PARTITIONING IN LEUCAENA SEEDLINGS

Research was conducted to examine the effect of Rhizobial race and N f ertilization on N-2 fixation and N partitioning in seedlings of the ge nus 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 containin g 0, 2, or 8 mM nitrate N labeled with 0.1% N-15. The seedlings were i noculated with Rhizobia loti race 94A3 or R. loti race 9408. After 12 weeks the plants were harvested and the effects of N fertilization and Rhizobial race on nodulation N-2 fixation, and N partitioning were ex amined. Using the N-15-dilution method total N fixed by dinitrogen fix ation was determined as well as distribution of fixed versus fertilize r N within the plant. Both Rhizobial races infected both Leucaena spec ies resulting in an effective symbiosis. However, the 2 Leucaena speci es responded quite differently to N fertilization. The addition of 2 m M N to the nutrient solution effectively eliminated nodulation in L. r etusa yet increased both nodulation and N-2 fixation in L. leucocephal a. The 8 mM N rate eliminated nodulation in both species. Due to the e limination of nodulation and N-2 fixation by N fertilization in L. ret usa, 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 ret ained in the root and nodules than fertilizer N. Although the 2 mM N t reatment increased the amount of N-2 fixed in L. leucocephala compared to the 0 mM treatment, the proportion of fixed N-2 was reduced to les s than one third of the total N in the plant.