Dynamics of vertical leaf nitrogen distribution in a vegetative wheat canopy. Impact on canopy photosynthesis

The development of vertical canopy gradients of leaf N has been regarded as an adaptation to the light gradient that helps to maximize canopy photosyn thesis. In this study we report the dynamics of vertical leaf N distributio n during vegetative growth of wheat in response to changes in N availabilit y and sowing density. The question of to what extent the observed vertical leaf N distribution maximized canopy photosynthesis was addressed with a le af layer model of canopy photosynthesis that integrates N-dependent leaf ph otosynthesis according to the canopy light and leaf N distribution. Plants were grown hydroponically at two amounts of N, supplied in proportion to ca lculated growth rates. Photosynthesis at light saturation correlated with l eaf N. The vertical leaf N distribution was associated with the gradient of absorbed light. The leaf N profile changed during crop development and was responsive to N availability. At high N supply, the leaf N profiles were c onstant during crop development. At low N supply, the leaf N profiles fluct uated between more uniform and steep distributions. These changes were asso ciated with reduced leaf area expansion and increasing N remobilization fro m lower leaf layers. The distribution of leaf N with respect to the gradien t of absorbed irradiance was close to the theoretical optimum maximizing ca nopy photosynthesis. Sensitivity analysis of the photosynthesis model sugge sted that plants maintain an optimal vertical leaf N distribution by balanc ing the capacity for photosynthesis at high acid low light. (C) 2000 Annals of Botany Company.