Physiochemical mechanisms involved in the improvement of grain-filling, rice quality mediated by related enzyme activities in the ratoon cultivation system

The grain quality of rice in the ratoon cultivation system is often higher than in the main crop (late season). To investigate the reason for that difference, a study of physiochemical factors, particularly relevant enzyme activities and their relationships with grain-filling and rice quality, was conducted with two hybrid rice cultivars, indica type "Taifengyou 3301" and indica-japonica type "Yongyou 2640". The results showed that the head rice yield of the superior and inferior grains in ratoon rice were significantly higher than those of their counterpart main crops (late season) to which rice heading time was synchronized. The degree of grain chalkiness and chalky grain percentage of both superior and inferior grains in ratoon rice were much lower than those of the late season main crop of both cultivars over two years of study. The starch RVA (Rapid Viscosity Analyzer) profile parameter analysis indicated that the maximum viscosity and setback viscosity of the superior and inferior grains of ratoon rice were higher than the late season main crop counterparts in both cultivars. We also found that the grain-filling of ratoon rice, especially for inferior grains, initiated earlier and continued longer and steadier than that of the late season main crop at the early and middle stages, which was driven by better translocation of photosynthate and the coordinated activity of enzymes including starch synthase and starch branching enzyme. These findings suggest that lower grain sink capacity and higher photosynthate remobilization ability result in fast and steady grain-filling in ratoon crops. Furthermore, coordinated enzyme activity appears to be involved in the process of starchsynthesis and dry matter translocation that drives the improvement of grain-filling and the grain quality of ratoon rice.