Benefits of CO2 enrichment on crop plants are modified by soil water status

Three species, wheat, maize and cotton, were grown in pots and subjected to high (85-100% field capacity, theta (F)), medium (65-85% theta (F)) and low (45-65% theta (F)) soil moisture treatments and high (700 mul l(-1)) and low (350 mul l(-1)) CO2 concentrations. Biomass production, photosynthesis, evapotranspiration and crop water use efficiency were investigated. Results showed that the daily photosynthesis rate was increased more in wheat and cotton at high [CO2] than in maize. In addition, differences were more substantial at low soil water treatment than at high soil water treatment. The daily leaf transpiration was reduced significantly in the three crops at the high CO2 concentration. The decrease at low soil water was smaller than at high soil water. Crop biomass production responses showed a pattern similar to photosynthesis, but the CO2-induced increase was more pronounced in root production than shoot production under all soil water treatments. Low soil water treatment led to more root biomass under high [CO2] than high soil water treatment. CO2 enrichment caused a higher leaf water use efficiency (WUE) of three crops and the increase was more significant in low than in high soil water treatment. Crop community WUE was also increased by CO2 enrichment, but the increase in wheat and cotton was much greater than in maize. We conclude that at least in the short-term, C-3 plants such as wheat and cotton may benefit from CO2 enrichment especially under water shortage condition.