Canopy water use efficiency of winter wheat in the North China Plain

Canopy water use efficiency (W), the ratio of crop productivity to evapotranspiration (ET), is critical in determining the production and water use for winter wheat (Triticum aestivum L.) in the North China Plain, where winter wheat is a major crop and rainfall is scarce and variable. With the eddy covariance (EC) technique, we estimated canopy W of winter wheat at gross primary productivity (W-G) and net ecosystem productivity (W-N) levels from revival to maturing in three seasons of 2002/2003, 2003/2004 and 2004/2005 at Yucheng Agro-ecosystem Station. Meanwhile we also measured the biomass-based water use efficiency (W-B). Our results indicate that W-G, W-N and W-B showed the similar seasonal variation. Before jointing (revival-jointing), W-G, W-N and W-B were obviously lower with the values of 2.09-3.54 g C kg(-1), -0.71 to 0.06 g C kg-1 and 1.37-4.03 g kg-1, respectively. After jointing (jointing-heading), the winter wheat began to grow vigorously, and WG, WN and WB significantly increased to 5.266.78 g C kg-1, 1.47-1.86 g C kg-1 and 6.41-7.03 g kg-1, respectively. The maximums of W-G, W-N and W-B occurred around the stage of heading. Thereafter, W-G, W-N and W-B began to decrease. During the observed periods, three levels of productivity: GPP, NEP and above-ground biomass (AGB) all had fairly linear relationships with ET. The slopes of GPP-ET, NEP-ET and AGB-ET were 4.67-6.12 9 C kg-1, 1.50-2.08 g C kg-1 and 6.87-11.02 g kg-1, respectively. Generally, photosynthetically active radiation (PAR) and daytime vapor pressure deficit (D) had negative effects on W-G, W-N and W-B except for on some cloudy days with low PAR and D. In many cases, W-G, W-N and W-B showed the similar patterns. While there were still some obvious differences between them besides in magnitude, such as their significantly different responses to PAR and D on cloudy and moist days.