Plant competition for light analyzed with a multispecies canopy model : II. Influence of photosynthetic characteristics on mixtures of wheat and wild oat.

The importance of photosynthetic characteristics such as quantum efficiency or carboxylation efficiency for carbon gain of plants competing for light in dense stands is dependent on several environmental factors and structural features of the canopy. A quantitative analysis of photosynthesis of competing plants in mixed stands of wheat and wild oat (Avena fatua L.), a common weed of wheat, involved measuring photosynthetic parameters of individual leaves at different heights in the canopy throughout the growing season. This information combined with detailed assessments of canopy structure was used with a multispecies canopy model to evaluate the importance of different photosynthetic characteristics for carbon gain in this canopy environment. Independent photosynthesis data sets were used to validate predictions of the model. Carboxylation efficiency (CE) and CO2-and light-saturated photosynthetic capacity (AML) were highly correlated and decreased with depth in the canopy for both species. Quantum efficiency (α) did not tend to decrease with depth in the canopy. Sensitivity analyses with the model for whole-plant carbon gain of each species over entire day periods were conducted. These showed that changes in CE and AML had an influence similar to that of changes in α on carbon gain for both species. This was not necessarily expected from single-leaf photosynthetic behavior in response to changes in CE, AML and α. The influence of α is more pronounced in the lower, more shaded portions of the canopy than are changes in CE and AML. Appreciable differences between the species were apparent for carbon gain under different weather conditions. The differences between the species in carbon gain when in competition for light were associated more with structural features rather than with photosynthetic characteristics.