Carbon uptake and water use by plants: is there pre-stomatal control?

Plants take up carbon dioxide for photosynthesis through pores in the epidermes of leaves called stomata. Because the inside of the leaf is more humid than the air, water is lost by evaporation. In mathematical descriptions of this process, used in thousands of papers and applied in models of crop performance and of climate change, it is assumed that the air inside the leaf is saturated with water vapour. Recent physiological evidence casts doubt. We propose new stable isotope and gas exchange techniques that will provide novel windows into leaf functioning and secure vital estimates of gas exchange parameters.