Total and Soluble Nitrogen in Douglas Fir in Relation to Plant Nitrogen Status

Concentrations of total N, ninhydrin-positive compounds, and monosubstituted guanidino compounds were determined in current and 1-year-old foliage and twigs, and in roots of 3-year-old Douglas fir (Pseudotsugamenziesii (Mirb.) Franco.). These plants were grown in a trial where levels of N (0, 255, and 675 kg N/ha), and sources of N (nitrate, ammonium, and urea) had been applied factorially. N fertilizer treatment increased soluble N levels by as much as 19 times control values, whereas total N was increased at most 40%. Total N in 1-year-old foliage, current twigs, and roots, and guanidino compounds in 1-year-old twigs were consistently different for different N treatment levels. Sixty percent of the variance in plant size measurements could be explained by six N measurements, including total N, in multiple regression.In a second experiment, concentrations of arginine and proline, in addition to concentrations of ninhydrin-positive compounds, guanidino compounds, and total N, were determined in foliage of 3-year-old Douglas fir grown in growth chambers at 10 and 21 °C. Four provenances, to which three moisture-stress treatments were factorially applied, were represented at each temperature. Concentration of soluble N compounds was higher at 21 °C than at 10 °C, but not concentration of total N. Moisture stress increased concentration of soluble N compounds, in particular proline, and there were some differences between provenances in concentrations of soluble N compounds and total N.