Water use efficiency (WUE)
The amount of carbon gained or biomass produced relative to water used. Its numerical meaning depends on whether it is measured at a leaf, plant, or crop scale.
Water use efficiency is a family of ratios rather than one universal measurement. Leaf-level photosynthetic WUE commonly compares carbon dioxide assimilation with transpiration Transpiration Water moving up through the plant and evaporating out through the stomata. It drives nutrient uptake, and carries calcium, which moves almost entirely by that flow. . Intrinsic WUE compares assimilation with stomatal conductance. Crop WUE may compare harvested biomass with evapotranspiration or irrigation water.
Values from different definitions, timescales, or experiments cannot be compared without checking their units and boundaries. A treatment can improve leaf-level WUE without producing the same proportional change in whole-crop biomass per liter.
Cannabis WUE responds to light, carbon dioxide, humidity, temperature, nutrition, water availability, cultivar, and developmental stage. The mechanism behind a change matters: carbon gain can rise, water loss can fall, or both can change.