Nitrate content, amino acid composition and growth of yellow birch seedlings in response to light and nitrogen source

Yellow birch (Betula alleghaniensis Britt.) seedlings were grown for three months in a greenhouse at two radiant flux densities-full light (FL) and 50% shade (LL)-and with three nitrogen sources- ammonium only (NH(4) (+)), nitrate only (NO(3) (-)) and a 1:1 mixture of ammonium and nitrate (NH(4) (+)/NO(3) (-))-in a completely randomized factorial design. The total biomass of seedlings grown under low light (LL) did not vary significantly with nitrogen source; although NO(3) (-)-treated seedlings were smaller and had a significantly lower (P </= 0.05) net assimilation rate (NAR) than NH(4) (+)-, and NH(4) (+)/NO(3) (-)-N-treated seedlings. Under full light (FL), NW-treated seedlings had a lower leaf area ratio (LAR) and a significantly lower total biomass and relative growth rate (RGR) than seedlings grown with either NO(3) (-) or NH(4) (+)/NO(3) (-). Under LL, nitrate concentrations in xylem sap and leaves of plants grown with NO(3) (-) or NH(4) (+)/NO(3) (-) were many times greater than under FL. Amino acid profiles were affected by nitrogen source but were not affected by radiant flux density. When NH(4) (+)-N was absent from the nutrient solution, the percentage of NH(2)-N carried in the xylem sap decreased for aspartic acid but increased for alanine and glycine. The results indicate that it is the initial reduction of NO(3) (-) in roots, rather than the later stages of amino acid synthesis, that is limited by light in yellow birch seedlings.