The corn plant (Zea mays) has a C4 type photosynthesis. This means that the first glucide formed has four carbon atoms. As for all C4 type plants, corn has a higher photosynthesis effectiveness and water efficiency than C3 type plants. In non limiting growing conditions (water, nutrients, light), maize or sorghum produces 40 kg of DM/mm/ha whereas wheat or ray-grass only produces 25 kg DM/mm/ha and alfalfa 20 kg DM/mm/ha (Barrière, 2001).

On other terms, grain maize and forage maize need less water to produce as much biomass as alfalfa or wheat (in non limiting conditions) because they have an excellent physiological water use efficiency.

But when a stress occurs, maize looses this outstanding efficiency because of a weak root system (compared to other C4 plants such as sorghum). If the physiological water use efficiency remains the same, the global water use efficiency under water stress drops