An empirical model that uses light attenuation and plant nitrogen status to predict within-canopy N distribution and upscale photosynthesis from leaf to whole canopy

Modelling the spatial and temporal distribution of leaf nitrogen is central to specify photosynthetic parameters and simulate canopy photosynthesis. Leaf photosynthetic parameters depend both on local light availability and whole plant N status. The interaction between these two levels of integration has generally been modelled by assuming an optimal canopy functioning, which is not supported by experiments. During this study we examined how a set of empirical relationships with measurable parameters could be used instead to predict photosynthesis at the leaf and whole canopy levels. The distribution of leaf nitrogen per unit area (Na) within the canopy was related to leaf light irradiance and to the Nitrogen Nutrition Index (NNI), a whole plant variable accounting for plant N status. Na was then used to determine the photosynthetic parameters of a leaf gas exchange model. The model was assessed on alfalfa canopies under contrasting N nutrition and with N2-fixing and non-fixing plants. Three experiments were carried out to parameterise the relationships between Na, leaf irradiance, NNI, and photosynthetic parameters. An additional independent dataset was used for model evaluation. The N distribution model showed it was able to predict leaf nitrogen on the set of leaves tested. Na at the top of the canopy appeared to be related linearly to the NNI whereas the coefficient accounting for nitrogen allocation remained constant. Photosynthetic parameters were related linearly to Na irrespective of N nutrition and the N acquisition mode. Daily patterns of gas exchange were simulated accurately at the leaf scale. When integrated at the whole canopy scale, the model predicted that raising N availability above an NNI of 1 did not result in increased net photosynthesis. Overall, the model proposed offered a solution for a dynamic coupling of leaf photosynthesis and canopy N distribution without requiring any optimal functioning hypothesis.