Photosynthetic Responses to Phosphorus Nutrition in Eucalyptus grandis Seedlings

Abstract

Eucalyptus grandis seedlings were grown in growth units in which plant roots were suspended in air while continuously being sprayed with nutrient solution (aeroponic system). Phosphorus was added to nutrient solutions in exponentially increasing amounts which determined plant growth rates. Plants were grown at five relative phosphorus addition rates, and photosynthetic performance of leaves was compared across treatments. Carbon assimilation rates ranged from 11.7 μmol m^-2 s^-1 for plants with lowest phosphorus status to 23.1 μmol m^-2 s^-1 for plants with highest phosphorus status. Intercellular partial pressures of CO₂ concomitantly decreased from 260 pbar for plants with lowest to 220 μbar for plants with highest phosphorus status. Leaves in all treatments showed a decrease in assimilation rate at intercellular partial pressures of CO₂ above c. 600 μbar. There was no consistent correlation between the extent of that decrease and the phosphorus status of leaves.

Assimilation rates were correlated with leaf phosphorus content. This relationship was apparent on either a unit leaf area or unit leaf weight basis. Assimilation rates and leaf nitrogen content per unit leaf weight were also correlated. In contrast, there was no correlation between leaf assimilation rate per unit leaf area and nitrogen content per unit leaf area, as nitrogen content per unit area was similar for all phosphorus treatments. The differences between correlations on a weight and area basis were due to differences in specific leaf area in different treatments, with plants with lower phosphorus status having less leaf area per unit leaf weight. The photosynthetic measurements showed that CO₂ assimilation rate, together with relative leaf growth rate, was one of the processes most sensitive to phosphorus nutrition.