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

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Abstract
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 15N, soluble, insoluble, and total 15N contents of shoots and roots were higher in ammonium-fed seedlings than in nitrate-fed seedlings. The pattern of 15N 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 2.8 times greater total 15N contents, respectively, than unshaded, nitrate-fed seedlings. In response to shading, ammonium-fed seedlings increased their total uptake of 15N per unit root weight, whereas nitrate-fed seedlings did not. No nitrate or 15NO3− was detected in any plant tissue. Nitrate-fed plants had higher NH4+, Asp, and Gln concentrations in needles and higher γ-aminobutyric acid and Arg concentrations in stems. Accumulation of 15N in roots was not affected by the pH of the 15N solution or by the N source fed to the seedlings before the period of 15N 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.

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