Interaction of erect panicle genotype and nitrogen fertilizer application on the source-sink ratio and nitrogen use efficiency in rice

This study was conducted to determine whether the interaction between the erect panicle (EP) genotype and N-fertilizer application affects the source-sink ratio and N use efficiency (NUE) in yield formation processes. For two consecutive years, two EP rice genotypes with ‘Akitakomachi’ (AKI) and ‘Liaojing 5′ (LG5) genetic backgrounds were grown with their respective near-isogenic lines of non-erect panicle (NEP) genotype in Kyoto, Japan (2016 and 2017), under three nitrogen (N) application rates (0, 6, and 20 g m⁻²). The effect of the EP genotype was positive in the yield and harvest index but neutral in biomass at maturity. The effect of the EP genotype was also positive on sink size (spikelet number per meter-square multiplied by single grain weight) but negative for the available carbohydrate per spikelet (source-sink ratio) due to a reduction of nonstructural carbohydrates (NSC) at the heading stage. The biomass produced after the heading stage was not consistently affected by genotype, with its components, cumulative intercepted solar radiation (IR), and radiation use efficiency (RUE) compensating for each other's reduction or increase. The interaction between genotype and N application rate was hardly detected for these variables. The interaction between genotype and genetic background was detected for some items. In the AKI genetic background, the EP genotype had a more positive effect on yield, harvest index (HI), and sink size, and a negative effect on NSC at the heading stage compared to the effect in the LG5 genetic background. The NUE, in terms of the yield per applied N, was not affected by genotype, whereas the NUE components were. Although the EP genotype had no effect on the amount of N in plants per applied N (uptake efficiency), the biomass at maturity per N in plants (physiological efficiency) was lower in the EP genotype than the NEP genotype. No interaction between genotype and N application was detected for either N uptake efficiency or agronomic efficiency. These findings indicated that EP genotypes have a lower source-sink ratio, regardless of the N supply conditions and that the EP genotype does not affect NUE. The two genetic backgrounds differed in the genotype effect on the physiological efficiency of yield, presumably owing to differences in plant height.