Hydraulic properties of Eucalyptus grandis in response to nitrate and phosphate deficiency and sudden changes in their availability

Some herbaceous plant species have been shown to dynamically alter the hydraulic properties of their roots in response to sudden changes in the concentrations of mobile nutrients. These hydraulic adaptations effectively allow plants to 'chase' mobile nutrients across the rhizosphere. Trees, on the other hand, could mitigate effects of heterogeneous, dynamic soil environments with extensive root systems as such systems would effectively equalize nutrient availability. In addition, large dendritic root systems would reduce the effectiveness of rapid, localized, physiological hydraulic changes as these local changes might cancel each other at lower-order root junctions. Thus, the aim of this study was to determine if trees (Eucalyptus grandis) employ short-term (minutes to hours), physiological hydraulic changes or rely on long-term (days), growth-based hydraulic acclimations to enhance mobile nutrient uptake. We used two nutrients, nitrogen (N) and phosphorus (P), that are characterized by contrasting soil mobility: N being mobile and P immobile. Transpiration, whole-plant hydraulic resistance (liquid phase), and the hydraulic resistance of single roots of E. grandis plants grown in high and low N combined with high and low P availability were measured. In general, plants grown with high N availability had lower whole-plant hydraulic resistance than plants grown with low N availability. When N or P were in short supply, a sudden addition of N or P did not change either single-root or whole-plant hydraulic resistance at a given leaf water potential. However, addition of N reduced the transpiration rate, thus, enhancing plant water status, suggesting that E. grandis behavior prioritizes water conservation over N uptake in short-term. Prolonged exposure to low nutrient availability resulted in high overall hydraulic resistance further suggesting prioritization of water conservation over N gain.