Light condition during grain-filling stage of main crop strongly influences ratooning ability of low-stubble ratoon rice

Compared with high-stubble ratoon rice (RR), low-stubble RR is superior in yield potential, grain quality, and economic benefit. However, the unstable ratooning ability limits the grain yield of low-stubble RR production. Light condition during the grain-filling stage of main crop (GFMC) may be important for rice ratooning. To elucidate the role of light condition during GFMC in affecting ratooning ability, the key response periods, and their underlying mechanisms, field experiments were conducted using two indica cultivars in 2021 and 2022. To create varied light conditions at the canopy base during GFMC, two planting density treatments combining three nitrogen (N) treatments were established in 2021, and three density treatments combining two N treatments and four shading treatments were established in 2022 for the main crop. Light intensity (LI), light quality as reflected by the ratio of red light/far red light (R/FR), and light transmission ratio (LTR) at the canopy base during GFMC, and ratooning ability were dramatically altered by N fertilization but not by planting density. With increased N application, LTR, root bleeding rate, and maximum ratooning rate significantly decreased in 2021. In 2022, low N rate increased LI, R/FR, and maximum ratooning rate by 155.7–241.4%, 47.4–65.3%, and 15.6–27.5%, respectively, but reduced missing hill percentage (proportion of hills without regenerated tillers to the total number of hills) by 30.0–62.1% compared with high N rate. The missing hill percentage was negatively correlated with the indices of light condition, while the maximum ratooning rate was positively correlated with them for both cultivars. Root activity and the ratios of abscisic acid (ABA) to cytokinins (CTK), indole-3-acetic acid (IAA), and IAA ​+ ​CTK could explain the effect of light condition during GFMC on ratooning ability. Shading experiment confirmed the effect of light condition on ratooning ability and further revealed that only shading during middle and late GFMC affected ratooning ability. These findings provide new insights into the regulation of ratooning ability, which are useful for developing management practices to increase the grain yield and yield stability of low-stubble RR.