Shading tolerance in rice is related to better light harvesting and use efficiency and grain filling rate during grain filling period

Low radiation caused by industrial development and environment change has become a limitation in crop production in China. Field experiments were conducted to investigate the effect of shading stress on photosynthetic and chlorophyll fluorescence characters, as well as grain yield and yield components of rice from 2010 to 2012, in Sichuan, China. Rice cv. IIyou 498, Gangyou 188, Dexiang 4103, Gangyou 527, and Chuanxiang 9838 were selected and shaded during grain filling period, which resulted in 53% shading environment (achieved with white cloth above the rice canopy). Shading increased the flag leaf chlorophyll (Chl) content and maximum efficiency of PSII photochemistry under dark-adaption (Fv/Fm), but decreased the net photosynthetic rate, electron transport rate (ETR), saturation irradiance (PARsat), and maximum electron transport (Jmax), which resulted in a sharply falling grain yield mainly due to reduced spikelet filling and grain weight. The shading tolerances were different in different rice varieties. Maximum net photosynthetic rate (Pmax), light saturation point (LSP), quantum yield of PSII (ΦPSII), and photochemical quenching (qP) of Gangyou 188 were reduced under shading. However, IIyou 498 had its adaptive capacity to shading enhanced by increasing the Chl content to improve light-harvesting potential as well as increased Pmax, LSP, Fv/Fm, and ΦPSII. In addition, there was reduced non-photochemical quenching (NPQ), while maintaining a more stable ETR, PARsat, and Jmax to enhance light-use efficiency and reduce the dissipation of light energy, which relieved damage to spikelet filling and resulted in a smaller reduction in grain yield of IIyou 498. Therefore, the improvement in light-harvesting and -use capability and increased spikelet filling under shading stress might be important characteristics for plant breeders. For example, IIyou 498 can be selected as a shade-tolerant variety.