Relationships between hydraulic architecture and leaf photosynthetic capacity in nitrogen-fertilized Eucalyptus grandis trees

Abstract

We compared the effects of nitrogen fertilization on shoot hydraulic architecture and leaf photosynthetic properties of Eucalyptus grandis Hill ex Maiden trees in Hawaii. It was hypothesized that water transport capacity would adjust to nutrient availability, with leaf specific hydraulic conductivity (kl) increasing in fertilized trees in coordination with higher photosynthetic capacity per unit leaf area. Trees were grown from seedlings in the field for 10 months at four rates of nitrogen (N) fertilization between 0 and 336 kg ha-1. Leaf water potentials, photosynthetic capacity and kl were measured before whole shoots were harvested to determine total growth, leaf area and sapwood density. Mean tree height increased from 4 to 5.3 m, stem basal area increased from 27 to 67 cm2 and total leaf area increased from 15 to 40 m2 between the lowest and highest rates of fertilizer addition. When trees were compared on the basis of leaf nitrogen per unit area (Narea), light-saturated rates of photosynthesis on an area and mass basis and the maximum rate of electron transport all increased from 50% to more than 100% as Narea increased from 0.8 to 2.1 g m-2. Branch specific hydraulic conductivity (ks) and kl increased with height in the crown. However, there was no change in branch kl or the ratio of leaf area to sapwood area of the whole shoot in response to fertilization, and ks and density of the sapwood were unrelated to leaf Narea. In contrast to photosynthesis, stomatal conductance did not respond to fertilization, leading to decreased internal carbon dioxide partial pressure (pi/pa) in fertilized plants and similar leaf water potentials in all plants. Consistent with the behavior of pi/pa, carbon isotope discrimination decreased by 2‰ with increasing leaf Narea, supporting the conclusion that intrinsic water-use efficiency was enhanced by fertilization. Increased growth in response to fertilization involved adjustment at the leaf level rather than a change in the balance between water transport capacity and leaf area. It is proposed that, when there are changes in leaf properties without any external change in water availability or evaporative demand, leaf photosynthesis and stomatal conductance are partially constrained by the hydraulic architecture of the tree.