Simulated Response of Tobacco Crops to Irradiance and Defoliation

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Abstract
Simulated rates of CO2 exchange of tobacco crops in response to incident irradiance and to defoliation were compared with measured values from the field. Comparison of daily trends in net rate of CO2 exchange and its change with upward defoliation showed that rates were largely dependent on the interaction of solar position and input with features of the architecture of the crop. These included plant extent and the distribution of sunlit leaf area with respect to the leaf-beam angle. The model satisfactorily simulated net rate of CO2 exchange of a range of different intensities of downward defoliation. However, comparisons of estimated and observed responses indicated differences in the structure and/or photosynthetic characteristics of a medium intensity of defoliation as compared with a non-defoliated control or a crop which experienced the most severe defoliation.