Effects of nitrate and light intensity on photosynthesis and nitrogen fixation in alfalfa plants

Abstract

Alfalfa plants (Medicago sativa, L. cv. Aragón) inoculated with Rhizobium meliloti weregrown with a N-free-nutrient solution in controlled environmental chambers. After 5 weeks plants were placed under 2 irradiance regimes (120 and 60 μE · m-2 · s-1) and different N concentrations were supplied to treatment plants as Ca(NO3)2. Rates of acetylene-dependent ethylene production and CO2 uptake were determined 2 days later. The addition of 1.75 and 3.5 mM N03- to plants in each level of irradiance decreased both nitrogenase activity and CO2 assimilation regardless of the light intensity. The addition of 7 and 14 mM NO3- resulted in an enhanced leaf photosynthesis. Simultaneously, the C2H2 reduction rate by nodulated plants at 60 μE was lower than that of N2-dependent plants, while plants at 120 μE had similar acetylene reduction values. Higher levels of combined nitrogen, 28 and 56 mM NO3-, reduced nitrogenase activity of plants under both of the two light intensities and CO2 uptake by plants at 60 μE. However, current photosynthesis was not affected in plants at 120 μE. These results are interpreted as showing that alfalfa plants can utilize nitrate to improve CO2 assimilation without inhibiting N2 fixation when plants are under conditions in which light is not saturating.