Water-use efficiency (WUEDM) is directly related to radiation-use efficiency (RUE) and inversely related to crop conductance (g(c)). We propose that reduced WUEDM caused by shortage of nitrogen results from a reduction in RUE proportionally greater than the fall in conductance. This hypothesis was tested in irrigated wheat crops grown with contrasting nitrogen supply; treatments were 0, 80 and 120 kg N ha(-1) in 1998 and 0, 80, 120 and 160 kg N ha(-1) in 1999. We measured shoot dry matter, yield, intercepted solar radiation and soil water balance components. From these measurements, we derived actual evapotranspiration (ET), soil evaporation and transpiration, WUEDM (slope of the regression between dry matter and ET), WUEY (ratio between grain yield and ET), RUE (slope of the regression between dry matter and intercepted radiation), and g(c) (slope of the regression between transpiration and intercepted radiation). Yield increased from 2.3 in unfertilised to an average 4.7 t ha(-1) in fertilised crops, seasonal ET from 311 to 387 mm, WUEDM from 23 to 37 kg ha(-1) mm(-1), WUEY from 7.6 to 12.4 kg ha(-1) mm(-1), RUE from 0.85 to 1.07 g MJ(-1), while the fraction of ET accounted for soil evaporation decreased from 0.20 to 0.11. In agreement with our hypothesis, RUE accounted for 60% of the variation in WUEDM, whereas crop conductance was largely unaffected by nitrogen supply. A greater fraction of evapotranspiration lost as soil evaporation also contributed to the lower WUEDM of unfertilised crops. (C) 2001 Published by Elsevier Science B.V.
