EFFECTS OF AMMONIUM AND NITRATE ON NUTRIENT-UPTAKE AND ACTIVITY OF NITROGEN ASSIMILATING ENZYMES IN WESTERN HEMLOCK

Western hemlock seedlings were grown in nutrient solutions with ammoni um, nitrate or ammonium plus nitrate as nitrogen sources. The objectiv es were to examine ( 1 ) possible selectivity for ammonium or nitrate as an N source, (2) the maintenance of charge balance during ammonium and nitrate uptake, and (3) the activity of the nitrogen assimilating enzymes, nitrate reductase, glutamine synthetase, and glutamate dehydr ogenase, in relation to the uptake of different nitrogen sources. The uptake studies revealed that western hemlock takes up ammonium faster than nitrate and that ammonium partially inhibits nitrate uptake. Effl ux of H+, 1.26 muequiv H+ muequiv -1 NH4+, occurred to maintain charge balance in plants utilizing ammonium nitrogen whereas potassium serve d as a counter-ion in the uptake of nitrate. Nitrate reductase activit y varied with nitrate availability in root tissue, but showed no respo nse in needles. There was no glutamine synthetase response to differin g nitrogen sources or uptake rates in root tissue, and little response in needles. Glutamate dehydrogenase activity in root tissue varied si gnificantly with nitrogen source, being greatest in treatments contain ing ammonium. Results indicate that western hemlock may be adapted to sites where NH4+ is the predominant N source.