Seedlings of Eucalyptus grandis were grown at five different rates of nitrogen supply. Once steady-state growth rates were established, a detailed set of CO 2 and water vapour exchange measurements were made to investigate the effects of leaf nitrogen content ( N ), as determined by nitrogen supply rate, on leaf structural, photosynthetic, respiratory and stomatal properties. Gas exchange data were used to parametrize the Farquhar-von Caemmerer photosynthesis model. Leaf mass per area (LMA) was negatively correlated to N . A positive correlation was observed between both day ( R d ) and night respiration ( R n ) and N when they were expressed on a leaf mass basis, but no correlation was found on a leaf area basis. An R d / R n ratio of 0·59 indicated a significant inhibition of dark respiration by light. The maximum net CO 2 assimilation rate at ambient CO 2 concentration ( A max ), the maximum rate of potential electron transport ( J max ) and the maximum rate of carboxylation ( V cmax ) significantly increased with N , particularly when expressed on a mass basis. Although the maximum stomatal conductance to CO 2 ( g scmax ) was positively correlated with A max , there was no relationship between g scmax and N . Leaf N content influenced the allocation of nitrogen to photosynthetic processes, resulting in a decrease of the J max / V cmax ratio with increasing N . It was concluded that leaf nitrogen concentration is a major determinant of photosynthetic capacity in Eucalyptus grandis seedlings and, to a lesser extent, of leaf respiration and nitrogen partitioning among photosynthetic processes, but not of stomatal conductance.

Eucalyptus grandis trees were grown in plantations with and without added fertiliser to examine the effects of plant nutrition on photosynthesis and growth. Leaves were sampled from known locations within canopies of selected trees and leaf N and P concentrations were measured. Contour maps of N and P distributions were then produced for crowns of trees aged between 6 and 16 months. Gas exchange measurements on sample leaves were used to estimate parameters of a model of C photosynthesis as a function of leaf N and P contentrations. Linear relationships were obtained between model parameters and leaf N concentration, but P appeared to be present in excess, since no correlation was found with P contentration. Photosynthetic light response curves were calculated for model leaves with differing N concentrations. The curves show that optimal concentrations of N in leaves depend on mean levels of irradiance during growth.

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Seedlings of Eucalyptus grandis were grown using an aeroponic culture technique in which continuously circulating solution was sprayed over seedling roots. Nutrient solutions, made up so that nitrogen was the element limiting growth, were added hourly at relative addition rates between 0.04 and 0.12 d-1. This technique enabled stable seedling nitrogen concentrations and relative growth rates to be maintained during experimental periods of 40-80 days in most treatments. Once steady state growth was established, allocation of dry matter to stems compared with foliage increased with ontogeny and the allometric relationship was not influenced by plant nitrogen status or genotype. In contast, the intercept of the allometric relationship between root mass and foliage mass was dependent on plant nitrogen concentration, but the slope was equal to unity at all nitrogen concentrations. Our data indicate that nitrogen-induced changes in allocation of dry matter between foliage and root components, together with changes in specific leaf area, had a larger influence on seedling growth rate than did changes in net rate of carbon gain per unit leaf area.