Abstract
The vascular bundles of rice panicles serve to connect the source and the sink, as well as serving as a channel for the transportation of materials. In this study, two homozygous japonica rice strains were used as materials. The vascular bundle structures of the branches in different positions within a rice panicle were observed, and their cross-sectional areas were calculated. In addition, the ultrastructure of the central large vascular bundle (LVB) phloem in the rachillae of superior spikelets (SS) and inferior spikelets (IS) was observed during the grain filling period. Moreover, the soluble sugar and protein contents of the SS and IS rachillae were also measured to study whether the differences in the structure of vascular bundles of the branches were related to the plumpness of grain at different positions. The results showed that vascular bundle cross-sectional areas of the basal primary branches were greater than those in the upper primary branches. Moreover, there was little difference in the areas of vascular bundles between the basal secondary branches and upper secondary branches. However, the vascular bundle areas of the IS rachillae were lower than those in the SS rachillae. Therefore, we believe that the poor vascular tissue channel of the IS rachillae could be the limiting factor in IS plumpness. The results also showed that a similar time course in the degradation pattern of some organelles of the sieve elements and companion cells in central LVB was observed in the SS rachillae and IS rachillae during the grain filling period. Compared with the IS rachillae, more abundant mitochondria and plasmodesmata were found in the companion cells of SS rachillae at the beginning of the filling stage, while no significant differences between SS and IS rachillae were identified at the middle and late filling stages, which implies that the SS rachillae were relatively more effective at transportation compared with the IS rachillae at the initial filling stage. Therefore, the undeveloped vascular bundles of the IS rachillae and their poor physiology and lack of ability to transport at the initial filling stages could be the limiting factor in IS plumpness.
Highlights
Rice (Oryza sativa L.) is one of the most important food crops in the world
WJ165 had higher grain length, grain width, and grain weight compared with W1844, whereas the plumpness and the seed setting rate were lower than that of W1844, indicating that WJ165 has larger grains but poorer plumpness compared with W1844
WJ165 demonstrated higher grain length, grain width, and grain weight compared with W1844, whereas the plumpness and the seed setting rate were lower than those of W1844, indicating that WJ165 has larger grains but poorer plumpness compared with W1844 (Table 1)
Summary
Rice (Oryza sativa L.) is one of the most important food crops in the world. Due to the continuing growth of the global population and a decrease in arable land, increasing the rice yield per unit area is the main goal of breeders (Khush, 2005). The sink capacity has been expanded by increasing the number of spikelets per panicle and creating large-panicle rice varieties (Kato et al, 2007). These varieties generally do not produce the yield expected due to the low seed setting rate and grain weight of inferior spikelets (Zhang et al, 2009; Yang and Zhang, 2010). The IS are located on the basal secondary branches, and they flower later, fill more slowly, and produce smaller grains (Mohapatra et al, 1993; Ishimaru et al, 2003). Improving IS grain filling is of substantial significance to ensure high-yield potential for large-panicle rice varieties
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