Background: The differing floret opening times between subsp. indica and subsp. japonica in rice limit the potential for increased hybrid seed production. Objectives: To elucidate the physiological basis underlying the differences in floret opening time between indica and japonica rice. Materials: A comparative analysis involved nine indica and ten japonica rice varieties. Methods: Using paraffin sectioning, transcriptome sequencing, RT-PCR, and endogenous substance quantification, we investigated the structural variations in floral organs, the differences in the initiation timing of floret opening regulatory pathways, and endogenous regulators. Results: The results indicated insignificant differences in lemma thickness, lodicule thickness, lodicule area, and the coupling-lodicule length between indica and japonica rice. However, japonica rice exhibited larger lodicule-lemma gaps and more vascular bundles compared to indica rice. Within the 9:00 a.m. to 10:00 a.m. interval, the expression of OsAOS1 in alpha-linolenic acid metabolism and OsISA3 in starch and sucrose metabolism notably increased in indica rice, with no significant change in japonica rice. Additionally, the endogenous JA and alpha-amylase surged more significantly in indica rice than in japonica rice. The increase in soluble carbohydrate in indica rice is greater than in japonica rice, but the difference is not significant. Conclusions: These findings suggest that in the process of the floret opening, the alpha-linolenic acid metabolism and starch and sucrose metabolism are initiated earlier in indica rice, accompanied by a more pronounced elevation in endogenous JA and alpha-amylase. Furthermore, the smaller lodicule-lemma gap in indica rice contributes to earlier floret opening compared to japonica rice.
