Introduction

Rice (Oryza sativa L.) is a staple food for more than one-third of the global population1,2. Rice grain yield is required to increase by more than 1% annually to meet the expected increase in demand as a result of continued population growth and decrease in available paddy area3,4,5. Breeding of high-yielding cultivars and determination of optimal agronomic practices for different cultivars are required to realize the required improvement in rice yield6. Actual grain yield is the product of the harvest index (HI) and biomass. The HI of many elite rice cultivars has already attained an extremely high value (above 0.5), thus further improvement of the yield potential is reliant on an increase in biomass7. This realization is already reflected in current rice breeding programs, in which genotypes with a long growth duration, characterized by a longer grain-filling period, are selected for greater biomass production and correspondingly higher grain yield at harvest, as exemplified by ‘Y-Liang-You 900’8,9,10. ‘Y-Liang-You 900’ differs from many rice cultivars, which typically have a growth duration of 110–120 days, in having a growth duration of more than 140 days11. This comparatively long growth duration necessitates development of novel field practices to realize the yield potential of such high-yielding rice lines in the field.

A number of field practices are now routinely used in modern rice cultivation, including promotion of germination in a seedbed, organic fertilizer application before transplanting, water management during the tillering stage, topdressing during the young panicle differentiation stage, and alternate wetting and drying during the grain-filling stage10,12. Among these practices, different topdressing schedules can differentially influence photosynthetic efficiency13 and panicle differentiation, thereby resulting in differences in panicle number per plant and number of seeds per panicle14,15,16, and the functional longevity of leaves during the grain-filling stage17,18,19.

The process of young panicle differentiation of rice can be divided into eight stages30,31. Preheading organic carbon storage in the stem and sheath can potentially contribute to as much as 30% of the grain yield in early-maturing rice cultivars32,33. The majority of these conclusions were drawn based on studies using rice cultivars with a shorter growth duration. In the present study, grain yield in YLY900 and YLY6 was not significantly correlated with either RRVO or CRVO (Fig. 4), which indicated that preheading reserves make a relatively minor contribution to grain yield of rice cultivars with a long grain-filling period. Instead, grain yield of YLY900 and YLY6 showed a significant positive correlation with the leaf SPAD value28. Interestingly, the Pearson correlation coefficients were increasingly higher and more strongly significant with progression of the developmental stage after heading (r = 0.2166 ns, r = 0.4219*, r = 0.5640** at the heading stage, 15 days after heading, and 30 days after heading, respectively; * p < 0.05, ** p < 0.01, ns = nonsignificant; Fig. 5). Consistent with previous observation that topdressing at the initial stage of flag-leaf extension resulted in the highest grain yield, the maximal significant correlation coefficient of 0.87 was observed at 15 days after heading (Fig. 6). These results suggested that rice cultivars with a long growth duration show novel characteristics, i.e., grain yield is strongly dependent on canopy photosynthetic CO2 uptake during the grain-filling stage, rather than organic carbon stored in the stem and sheath. Previous studies have estimated that during the grain-filling stage, canopy photosynthetic CO2 uptake may contribute 60–100% of the final grain yield of rice30,31 and more than 40% of canopy photosynthesis can be contributed by the flag leaves34. In the current study, on average, the contribution of canopy photosynthetic CO2 uptake to total grain yield formation in YLY900 and YLY6 was higher than 78% and 87%, respectively (Table 3), which was much higher than the percentages reported previously for other rice cultivars35.

Figure 4
figure 4

Relationship between grain yield and remobilization rate of reserves in vegetative organs (RRVO; A) and contribution of reserved organic matter in vegetative organs to grain yield (CRVO; B) in the 2 years, two cultivars, and seven treatments.

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Figure 5
figure 5

Correlation analysis between grain yield and net photosynthetic rate (Pn) in the two years, two cultivars, and seven treatments. (A) Heading stage; (B) after heading 15 days; (C) after heading 30 days; (D) mean of three stages. * and ** indicates significant correlation between yield and Pn at P < 0.05 and 0.01, respectively.

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Figure 6
figure 6

Correlation analysis between grain yield and leaf SPAD value in the two years, two cultivars, and seven treatments. (A) Heading stage; (B) after heading 15 days; (C) after heading 30 days; (D) mean of three stages. * and ** indicates significant correlation between yield and SPAD at P < 0.05 and 0.01, respectively.

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Conclusion

Compared with rice cultivars with a relatively shorter growth duration, topdressing must be applied at an appropriate stage of young panicle differentiation to realize the enhanced grain-yield potential of cultivars with a longer growth duration. Specifically, for the two cultivars tested in this study, topdressing at the initial stage of flag-leaf extension results in greater enhancement of grain yield, which is primarily contributed by the improved physiological activity of the flag leaves at the middle and advanced grain-filling stages. In contrast to earlier-maturing rice cultivars, in which remobilization of stored carbon in the stem and sheath contributes as much as 30–40% of the final grain yield, in YLY900 and YLY6 the photosynthetic CO2 uptake rate during the grain-filling stage contributed to more than 78%–87% of the final grain yield. The information provided in this study can be used to improve topdressing application for rice cultivars with a long growth duration.