Impact of canopy nitrogen profile in wheat on growth

Non-uniform canopy N distributions have been reported to result in substantially higher canopy photosynthesis over short periods of time compared to a uniform distribution. Canopy N distribution profiles were observed in five spring wheat cultivars grown in the field under optimum management conditions at two planting dates. A curvi-linear relationship of maximum leaf photosynthesis (A(m)) with leaf N was established. The impact of canopy N profile on growth was assessed with the crop growth simulation model SUCROS, adjusted for this purpose. The calculated growth with the observed profiles did not exceed the growth calculated with the standard version of SUCROS. A uniform canopy N profile with observed upper leaf N had little effect, because photosynthesis deep in the canopy is almost entirely determined by radiation intensity. A uniform profile based on average canopy leaf N underestimated growth. No major differences in response to canopy N profiles became apparent among cultivars. Increase in biomass production through increased leaf N was found to depend strongly on the curvature of the A(m)-leaf N relationship. Responses appeared particularly sensitive to changes in initial light-use efficiency. It is concluded that determination of total canopy LAI and upper leaf N suffices for growth estimation. (C) 1999 Elsevier Science B.V. All rights reserved.