Diurnal and Seasonal Variations of Photosynthetic Energy Conversion Efficiency of Field Grown Wheat

Improving canopy photosynthetic light use efficiency and energy conversion efficiency (epsilon(c)) is a major option to increase crop yield potential. However, so far, the diurnal and seasonal variations of canopy light use efficiency (LUE) and epsilon(c) are largely unknown due to the lack of an efficient method to estimate epsilon(c) in a high temporal resolution. Here we quantified the dynamic changes of crop canopy LUE and epsilon(c) during a day and a growing season with the canopy gas exchange method. A response curve of whole-plant carbon dioxide (CO2) flux to incident photosynthetically active radiation (PAR) was further used to calculate epsilon(c) and LUE at a high temporal resolution. Results show that the LUE of two wheat cultivars with different canopy architectures at five stages varies between 0.01 to about 0.05 mol CO2 mol(-1) photon, with the LUE being higher under medium PAR. Throughout the growing season, the epsilon(c) varies from 0.5 to 3.7% (11-80% of the maximal epsilon(c) for C-3 plants) with incident PAR identified as a major factor controlling variation of epsilon(c). The estimated average epsilon(c) from tillering to grain filling stages was about 2.17%, i.e., 47.2% of the theoretical maximal. The estimated season-averaged radiation use efficiency (RUE) was 1.5-1.7 g MJ(-1), which was similar to the estimated RUE based on biomass harvesting. The large variations of LUE and epsilon(c) imply a great opportunity to improve canopy photosynthesis for greater wheat biomass and yield potential.