title: Least-cost input mixtures of water and nitrogen for photosynthesis.
abstract: In microeconomics, a standard framework is used for determining the optimal input mix for a two-input production process. Here we adapt this framework for understanding the way plants use water and nitrogen (N) in photosynthesis. The least-cost input mixture for generating a given output depends on the relative cost of procuring and using nitrogen versus water. This way of considering the issue integrates concepts such as water-use efficiency and photosynthetic nitrogen-use efficiency into the more inclusive objective of optimizing the input mix for a given situation. We explore the implications of deploying alternative combinations of leaf nitrogen concentration and stomatal conductance to water, focusing on comparing hypothetical species occurring in low- versus high-humidity habitats. We then present data from sites in both the United States and Australia and show that low-rainfall species operate with substantially higher leaf N concentration per unit leaf area. The extra protein reflected in higher leaf N concentration is associated with a greater drawdown of internal CO2, such that low-rainfall species achieve higher photosynthetic rates at a given stomatal conductance. This restraint of transpirational water use apparently counterbalances the multiple costs of deploying high-nitrogen leaves.
authors.affiliation: Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia. iwright@rna.bio.mq.edu.au
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Wright
authors.forename: Peter B
authors.initials: PB
authors.lastname: Reich
authors.forename: Mark
authors.initials: M
authors.lastname: Westoby
mesh_terms: D001315:Australia; D002245:Carbon Dioxide / Q000378:metabolism; D003624:Darkness; D004734:Energy Metabolism; D004777:Environment; D006813:Humidity; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D010788:Photosynthesis; D018526:Plant Transpiration; D010944:Plants / Q000378:metabolism*; D011891:Rain; D014481:United States; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water; D002245:Carbon Dioxide; D009584:Nitrogen