As the economy develops and urbanization progresses, the amount of arable land continues to decline. In this context, the cultivation of double-season rice is particularly important for enhancing yield per unit area. However, research on the physiological mechanisms that contribute to high yields in double-season early rice varieties with short growing seasons is still limited. To address this gap, we conducted a field study using two early rice varieties, Zhongzu18 and Yongxian15, to examine their production characteristics, photosynthesis, fluorescence, and energy metabolism. The results indicate that Zhongzu18 has a significantly higher seed-setting rate, grain weight, and total grain yield compared to Yongxian15. Additionally, Zhongzu18 exhibits a higher head rice rate and a lower degree of chalkiness, along with a reduced chalky grain rate. Furthermore, the total dry matter weight and the ratio of panicle weight to total weight for Zhongzu18 were significantly greater than those for Yongxian15. After anthesis, Zhongzu18 also demonstrated a higher leaf net photosynthetic rate and actual fluorescence quantum efficiency compared to Yongxian15. Moreover, the levels of ATP and ATPase, as well as the activities of antioxidant enzymes and the expression of sucrose transport-related genes, were significantly increased in Zhongzu18 plants relative to Yongxian15. We conclude that the enhanced photosynthetic efficiency and energy production in Zhongzu18 lead to more effective assimilation and carbohydrate transport to the grains, resulting in higher grain yields and improved rice quality.

With the progress of society and the improvement of agricultural scientific technology, the single focus on high yield for rice production has gradually shifted to high quality. Coordinated development of grain yield and rice quality has become a core issue for researchers, and the underlying mechanisms remain to be solved. Two varieties, Zhongzheyou1 (ZZY1) and Zhongzheyou8 (ZZY8), were used as study materials under field conditions. The yield of ZZY1 was higher than that of ZZY8, which was mainly characterized by a higher seed-setting rate and grain weight. The rice quality of ZZY8 was better than that of ZZY1, primarily due to lower chalkiness and a higher head rice rate. The total dry matter weight of ZZY1 was lower than that of ZZY8, but the proportion of panicle dry matter weight or nonstructural carbohydrate to the total in the former was higher than that of the latter. The maximum grain-filling rate, average grain-filling rate, and key enzyme activities of ZZY1 were significantly higher than those of ZZY8, while the active grain-filling period was shorter than that of ZZY8. Furthermore, the ATP/ATPase content and energy charge values in the grains of ZZY1 were higher than those of ZZY8 at the early grain-filling stage. Transcriptome analysis showed that carbohydrate and energy metabolism were the main ways affecting the yield and quality of the two varieties. The energy production of ZZY1 was insufficient to simultaneously supply the needs thus leading to the discordant formation in its grain yield and rice quality formation.