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 photosynthesis. In this study we report the dynamics of vertical leaf N distribution during vegetative growth of wheat in response to changes in N availability and sowing density. The question of to what extent the observed vertical leaf N distribution maximized canopy photosynthesis was addressed with a leaf layer model of canopy photosynthesis that integrates N-dependent leaf photosynthesis according to the canopy light and leaf N distribution. Plants were grown hydroponically at two amounts of N, supplied in proportion to calculated growth rates. Photosynthesis at light saturation correlated with leaf 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 constant during crop development. At low N supply, the leaf N profiles fluctuated between more uniform and steep distributions. These changes were associated with reduced leaf area expansion and increasing N remobilization from lower leaf layers. The distribution of leaf N with respect to the gradient of absorbed irradiance was close to the theoretical optimum maximizing canopy photosynthesis. Sensitivity analysis of the photosynthesis model suggested that plants maintain an optimal vertical leaf N distribution by balancing the capacity for photosynthesis at high acid low light. (C) 2000 Annals of Botany Company.