Response of Transpiration Characteristics and Water Use Efficiency of Setaria viridis to the Enhancement of Simulated Photosynthetic Radiation and CO2 Enrichment

To determine how transpiration characteristics and water use efficiency of annual €4 plants respond to simulated photosynthetic radiation enhancement and CO2 enrichment, we used LT-6400 Portable Photosynthesis System to examine photosynthesis rate ( Pn ) , transpiration rate ( Tr ) , intercellular CO2 concentration ( Ci) , stomatal conductance ( Gs ) and vapor deficit at the leaf surface ( Vpdl) of Setaria viridis under simulated photosynthetic radiation (SPR) intensities from 0 to 2 400 jumol · m -2 · s-1 and CO2 concentration from 300 to 1 000 μmol · mol -1. Pn, Tr and Gs increased with enhanced SPR intensity. The increase in Pn, Tr and Gs with each unit SPR added was reduced as SPR intensity increased. At last, these physiological parameters tend to dynamic balance. Water use efficiency ( WUE) increased with enhanced SPR, then plateaued as SPR intensity exceeded 1 200 μmol·m-2 ·s-1 . For photosynthesis process demands C02, so Ci intensity enhancement. Vpdl also decreased with SPR intensity increase, then reached dynamic balance as SPR intensity exceeded 600 μmol · m- 2 · s-1. Pn increased with CO2 concentration between 300 and 600μmol · mol -1, and then decreased as C02 concentration increased from 600 to 1 000 μmol· mol-1. Ci, Vpdl and WUE of Setaria viridis rose with the elevation of CO2 concentration. Both Gs and Tr dropped as the CO2 concentration was elevated . We conclude that Pn of annual C4 plants was not sensitive to C02 concentration variation. The response of Tr and WUE to CO2 enrichment was most significant. It is obvious that the function of instantaneous simulated CO2 enrichment to increased photosynthesis rate was gradually diminished. However, it could enhance primary production by improving the use efficiency of existing water.