Photosynthesis at the leaf level represents a finely balanced economic challenge to the plant: the beneficial uptake of CO2 through stomata for photosynthesis is unfortunately coupled to the unavoidable loss of typically 100-fold more water to the atmosphere. Plants must therefore adopt the best strategy, both in short-term behaviour and long-term investment in traits, to maximise reproductive gain and avoid death through desiccation, in order to succeed in a challenging and competitive environment.

Here I present results on two aspects of this economic challenge to the plant: the short-term water-carbon balance in response to dynamic changes in light intensity and the long-term carbon investment in functional traits to support photosynthesis. The first provides key relations between stomatal opening and closing speed, the speed of photosynthetic induction and steady-state water-use strategy across vascular plants in response to dynamic changes in light intensity. The second uses a theoretical evolutionary framework to explain the fundamental coordination of photosynthesis, leaf gas exchange and investment in supplying water to the leaf observed across vascular plants. In doing so, the latter shines light on the relative costs of functional traits required for photosynthesis.