Roles of Carbon Gain and Allocation in Growth at Different Nitrogen Nutrition in Eucalyptus camaldulensis and Eucalyptus globulus Seedlings

Abstract

Potted Eucalyptus camaldulensis and E. globulus seedlings were grown in sand with added mineral nutrients. Mineral nitrogen was added, either (1) only a small initial quantity (N₁) or (2) regularly (N₂). Other nutrients were supplied regularly to both treatments. Biomass increment and nutrient concentrations of organs and gas exchange of leaves were measured.

Growth, foliage area, and foliar N concentration changed little in N₁ seedlings, all increased exponentially in N₂ seedlings. Partitioning of dry matter and nitrogen was unaffected by ontogeny or treatment in E. globulus, but changed with time in E. camaldulensis. Within a treatment biomass increase was similar for both species, but foliar biomass and area, total foliar N, and response of carbon assimilation to foliar N were greater in E. globulus. Effects of these on carbon assimilation were partly offset by higher foliar N concentrations in E. camaldulensis. Comparative growth rates of the species were not related to leaf area.

Carbon assimilation and diffusive conductance were linearly related with different relationships for the two treatments. Intercellular partial pressures of CO₂ were constant at c. 300 and 220 μPa Pa^-1 in N₁ and N₂ treatments of both species. This indicates a controlling mechanism that maintained intercellular CO₂ constant, but was affected by treatment.

Instantaneous potential assimilatory transpiration efficiency [(rate of carbon assimilation)/(rate of transpiration)] and instantaneous potential assimilatory nitrogen use efficiency [(rate of carbon assimilation)/(N content)] of leaves (at saturating light) were similar in the two species and showed treatment effects apparently not caused by foliar N concentration. These efficiencies were generally high compared to values published for woody plants.

A simple model illustrates the importance in predicting growth of accurately knowing: foliage mass, partitioning of biomass to foliage, and rates of carbon assimilation. Effects of small differences in carbon assimilation on biomass increment are shown to be potentially large because they can cause accumulation of large differences in foliar mass.