Least-Cost Input Mixtures of Water and Nitrogen for Photosynthesis

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.