Nitrogen Nutrition, Growth and Gas Exchange in Eucalyptus globulus Labill. Seedling

Abstract

Potted Eucalyptus globulus Labill. seedlings were grown in sand with added inorganic nutrients. Three treatments were applied: (1) inorganic nitrogen was added regulary (N₂), (2) in a small initial quantity only (N₁) and (3) after a period of N deficiency (N₃); other nutrients were supplied regularly. Biomass increment, foliar nutrient concentrations and gas exchange of leaves were measured.

Carbon assimilation, N uptake, growth, and leaf production and expansion were all greater at higher N. Partitioning of dry matter to roots and tops of seedlings was unaffected by treatment.

Carbon assimilation and diffusive conductance were linearly related at saturating light and were positively associated with foliar N concentrations; intercellular CO₂ partial pressures were constant at c. 246 μbar. The relationship between carbon assimilation and foliar N concentration was better when calculated per leaf weight than per leaf area. Dark respiration was positively associated with foliar N concentration. Following refertilisation of N-deficient seedlings, foliar N and carbon assimilation increased rapidly; about 20 days later N uptake declined and seedling biomass started to increase.

Instantaneous transpiration efficiency [c. 5 mmol (CO₂) mol^-1 (H₂O)] was not significantly affected by foliar N concentration or treatment. Instantaneous nitrogen use efficiency of leaves: (rate of carbon assimilation)/(leaf N content) was greater at higher N. In contrast to the literature, there was no simple relationship between nitrogen use efficiency of whole seedlings (biomass gain)/(nitrogen concentration) and seedling N. Instantaneous transpiration and nitrogen use efficiencies were generally high compared with values published for many woody plants.

A simple model predicted that, with no environmental constraints, exposed mature leaves with high N (1.5 mmol g^-1) assimilate 5.4 times more carbon than similar leaves with low N (0.5 mmol g^-1). Night respiration of foliage is a greater proportion of daily carbon balance for leaves with low N. When environmental factors constrain carbon assimilation foliage with high N is most affected.