BROMIDE AS A TRACER FOR NITRATE-N UPTAKE IN ALFALFA HERBAGE

The N-15 isotope is a reliable way to measure nitrate-N (NO3--N) uptak e in N-2-fixing legumes, but is too expensive for plant breeding progr ams. Our goal is to develop an affordable method of selecting alfalfa (Medicago saliva L.) for NO3--N uptake under field conditions. In this research, we tested whether the anion bromide (Br-), which is used to trace NO3--N movement in soils and is inexpensive to analyze, reflect s NO3--N uptake in alfalfa. 'Webfoot' (1994 and 1995), 'Agate' (1995), and 'Ineffective Agate' (1995) alfalfa were established in pots in th e greenhouse. After removing herbage, daily additions of solutions con taining Br- and N-15-labeled NO3- were made to the pots. Uptake of Br- and N derived from NO3- was determined in herbage regrowth sampled af ter 15, 25, and 35 d and the molar ratio of NO3--derived N:Br- was cal culated. In both years, when a constant molar ratio in solution was pr ovided at variable tracer concentrations, NO3--N uptake and Br- uptake both increased with increasing applied tracer concentration, but rati os in herbage were not constant for treatment, germplasm, or days of r egrowth, These results imply that selection must be carried out under well-controlled tracer supply and crop management. At a constant NO3-- N concentration, but increasing solution Br- concentration, NO3--N upt ake remained constant and Br- uptake increased, Molar ratios of NO3--d erived N:Br- in herbage directly reflected applied solution ratios in a soil-sand mixture (1994), but were less responsive in sand (1995) ab ove molar ratios of 100:1 in solution. Individual plant analysis showe d close agreement in Brand NO3--N uptake among plants in all germplasm s across a wide range of tracer supply, and indicated that selection f or NO3--N uptake using Br- uptake would result in minimal error. We co nclude that Br- is a promising alternative tracer for use in selecting alfalfa for NO3- uptake.