Absorption and assimilation of nitrate and ammonium ions by jack pine seedlings

Jack pine (Pinns hamksiana Lamb.) seedlings were grown in a shaded or unshaded light regime with either NOTor NH$-N as the sole N source. After three months, seedlings grown with NH$-N were larger than seedlings grown with NOT-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 15N, soluble, insoluble, and total”N contents of shoots and roots were higher in ammonium-fed seedlings than in nitrate-fed seedlings. The pattern of “N accumulation in shoots was similar to that in roots. After 6 and 24 h of 15N incorporation, unshaded. ammonium-fed seedlings had 8.8 and 2.X times greater total “N contents, respectively, than unshaded, nitrate-fed seedlings. In response to shading, ammonium-fed seedlings increased their total uptake of ‘“N per unit root weight, whereas nitrate-fed seedlings did not. No nitrate or 15N02 was detected in any plant tissue. Nitrate-fed plants had higher NH$, Asp, and Gln concentrations in needles and higher yaminobutyric acid and Arg concentrations in stems. Accumulation of 15N in roots was not affected by the pH of the “N solution or by the N source fed to the seedlings before the period of “N incorporation. Thus NOT 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 NOT-N as the sole N source.