ABSORPTION AND ASSIMILATION OF NITRATE AND AMMONIUM-IONS BY JACK PINE-SEEDLINGS

Jack pine (Pinus banksiana Lamb.) seedlings were grown in a shaded or unshaded light regime with either NO3-- or NH4+-N as the sole N source. After three months, seedlings grown with NH4+-N were larger than seedlings grown with NO3--N. Irradiance had a greater effect on growth of ammonium-fed seedlings than on growth of nitrate-fed seedlings. At all times from 6 to 24 h following incorporation of N-15, soluble, insoluble, and total N-15 contents of shoots and roots were higher in ammonium-fed seedlings than in nitrate-fed seedlings. The pattern of N-15 accumulation in shoots was similar to that in roots. After 6 and 24 h of N-15 incorporation, unshaded, ammonium-fed seedlings had 8.8 and 2.8 times greater total N-15 contents, respectively, than unshaded, nitrate-feds seedlings. In response to shading, ammonium-fed seedlings increased their total uptake of N-15 per unit root weight, whereas nitrate-fed seedlings did not. No nitrate or (NO3-)-N-15 was detected in any plant tissue. Nitrate-fed plants had higher NH4+, Asp, and Gln concentrations in needles and higher gamma-aminobutyric acid and Arg concentrations in stems. Accumulation of 1 N-15 in roots was riot affected by the pH of the N-15 solution or by the N source fed to the seedlings before the period of N-15 incorporation. Thus NO3- transport into roots, rather than its reduction or transport within the plant, seems to be the factor limiting the growth of jack pine supplied with NO3--N as the sole N source.