Water use efficiency (WUE)
How much carbon a plant fixes per unit of water it transpires. Elevated CO2 raises it sharply, because the plant takes in more carbon while opening its stomata less.
WUE links the two things a leaf trades through the same pores. Measured at the leaf, it is the ratio of net photosynthesis to transpiration. Measured at the crop scale, it is closer to grams of dry matter produced per liter of water applied.
CO2 enrichment improves WUE from both directions at once. Carbon uptake rises while stomata close somewhat, so the numerator grows as the denominator shrinks. In cannabis, Chandra and colleagues recorded a 111 percent gain in WUE at 750 ppm CO2.