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

We compared the effects of nitrogen fertilization on shoot hydraulic archit ecture and leaf photosynthetic properties of Eucalyptus grandis Hill ex Mai den trees in Hawaii. It was hypothesized that water transport capacity woul d adjust to nutrient availability, with leaf specific hydraulic conductivit y (k(l)) increasing in fertilized trees in coordination with higher photosy nthetic 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 k(l) 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 cm(2) and total leaf area increase d from 15 to 40 m(2) between the lowest and highest rates of fertilizer add ition. When trees were compared on the basis of leaf nitrogen per unit area (N-area), light-saturated rates of photosynthesis on an area and mass basi s and the maximum rate of electron transport all increased from 50% to more than 100% as N-area increased from 0.8 to 2.1 g m(-2). Branch specific hyd raulic conductivity (k(s)) and k(l) increased with height in the crown. How ever, there was no change in branch k(l) or the ratio of leaf area to sapwo od area of the whole shoot in response to fertilization, and k(s) and densi ty of the sapwood were unrelated to leaf N-area. In contrast to photosynthe sis, stomatal conductance did not respond to fertilization, leading to decr eased internal carbon dioxide partial pressure (p(i)/p(a)) in fertilized pl ants and similar leaf water potentials in all plants. Consistent with the b ehavior of p(i)/p(a) carbon isotope discrimination decreased by 2 parts per thousand with increasing leaf N-area, supporting the conclusion that intri nsic water-use efficiency was enhanced by fertilization. Increased growth i n response to fertilization involved adjustment at the leaf level rather th an 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 photosynt hesis and stomatal conductance are partially constrained by the hydraulic a rchitecture of the tree.