Impact of canopy nitrogen profile in wheat on growth

Non-uniform canopy N distributions have been reported to result in substant ially 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 conditi ons at two planting dates. A curvi-linear relationship of maximum leaf phot osynthesis (A(m)) with leaf N was established. The impact of canopy N profi le on growth was assessed with the crop growth simulation model SUCROS, adj usted for this purpose. The calculated growth with the observed profiles di d not exceed the growth calculated with the standard version of SUCROS. A u niform canopy N profile with observed upper leaf N had little effect, becau se photosynthesis deep in the canopy is almost entirely determined by radia tion intensity. A uniform profile based on average canopy leaf N underestim ated growth. No major differences in response to canopy N profiles became a pparent among cultivars. Increase in biomass production through increased l eaf N was found to depend strongly on the curvature of the A(m)-leaf N rela tionship. Responses appeared particularly sensitive to changes in initial l ight-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.