Responses of Photosynthetic Activity in Flag Leaves and Spikes as well as Seed Development of Wheat (Triticum aestivum L.) to Artificial Shading

The accumulation of photoassimilates in the sinks during the grain filling stage is affected by the conditions of the various source organs. This study was conducted to investigate changes in various source and sink organs when the flag leaves and spikes were shaded from heading to harvest in wheat. Shading the flag leaves increased chlorophyll content and chlorophyll fluorescence in the uppermost leaves by 34.9% and 0.3% in 2022 and 75.3% and 3.3% in 2023, respectively, maintaining a relatively high photosynthetic rate from heading to the mid-grain filling stage. However, shading the spikes had a more substantial negative impact on spike growth than the flag leaf shading. On the other hand, the uppermost leaves continued to serve as a source more actively even when the flag leaves were shaded, implying a compensating effect. At 35 days after treatment (DAT), the relative water content (RWC) of the spike in the spike shading (SS) treatment was 19.4% and 49.7% higher than that of the control in 2022 and 2023, respectively. However, grain weight in the SS treatment decreased by 39.7% in 2022 and 5.3% in 2023 compared with the control. In the flag leaf shading (FS) treatment, grain weight declined by 3.5% and 6.2% in 2022 and 2023, respectively. These results indicate that the reduction in grain weight due to shading was less pronounced in the SS treatment than in the FS and combined flag leaf and spike shading (FSS) treatments. The results suggest that spikes play a buffering role when assimilate-transport functions decline in the source organs. Our results provide a better understanding of the architectural properties, including flag leaf, spike, and the uppermost leaf, for photosynthetic contribution to grain filling in wheat. Also, identifying target characteristics for improving photosynthetic source organs will be valuable for developing wheat varieties with high yield stability.