It is revealed that the maximum Pn, E and WUE of the two concerned grasses appears at both dawn and dusk time, comparing to a slow variation at other time due to the co-action of sandy environment factors and the inter-factors of the grass. %A. halodendron% has a higher daily average Pn and WUE but lower daily average E than %A. frigida.% It suggests that %A. halodendron% has a relatively higher primary productivity and water saving capability in aridity situation. At natural conditions, %A. frigida% has mu...