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

Abstract

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 W G, W N and W B significantly increased to 5.26–6.78 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 aboveground biomass (AGB) all had fairly linear relationships with ET. The slopes of GPP–ET, NEP–ET and AGB–ET were 4.67–6.12 g 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.