Research progresses and prospects of sink-source structures and optimal regulation of high-yield varieties of hybrid rice in China

Exploring sink-source structures of high-yield rice varieties is generally significant for breeding practices. High- yield cultivation mode of large-scale rice production was determined based on sink-source characteristics of high-yield rice varieties, key indicators of ecological conditions, soil fertility of planted areas and high-yield cultivation techniques. As a result, it was not only saved manual labor and financial resources needed for the study of cultivation techniques of new varieties, but also accelerated the processes of demonstration and promotion of new varieties. In view of the reported data and research results of decades (over 20 years) of studies, sink-source structures of hybrid rice for high-yield varieties and the research progress on optimal control were summarized in this paper. The main results included: (1) High-yield rice varieties with 160–220 grains per panicle not only had coordinated contradictions between sinks and sources, but also improved photosynthetic efficiency. For big-panicle rice varieties, special attention was needed for the proper fertilizer application to increase grain filling and full use of the photosynthetic function of lower leaves in order to improve seed setting rate and 1000-grain weight. (2) A high-yield cultivation strategy of hybrid mid-rice with sparse cultivation was adopted with increased nitrogen application and photosynthetic source replenished so as to maintain normal grain filling under high grain-leaf ratio conditions. Increase in grain yield of upland rice seedlings over wet rice seedlings was significantly positively correlated with altitude, while it was significantly negatively correlated with yield of wet rice seedlings. Optimized application of nitrogen in hybrid rice was significantly influenced by ecological site conditions and soil nutrient states. Yield-increasing effects of the postponed nitrogen application and optimized application of nitrogen were strongly negatively correlated with soil fertility. Increase in grain yield was due to increased effective panicle number and 1000-grain weight. Highly significant negative correlations were noted between efficient nitrogen application (Y, kghm^-2) for grain filling and SPAD value (X) of the first leaf from top at full panicle stage, Y = 30.798X + 1 340.9 and R² = 0.911 4. A management technique of high-yield, water-saving rice was developed. (3) Transplanting density had significant negative correlation separately with grains number per panicle and applied N amount for varieties. There was significant effect of intensive cultivation system on yield increase, compared with traditional cultivation. A negative correlation was also noted between increased yield and grains number per panicle among varieties. Hybrid rice with less than 170 grains per panicle in traditional cultivation was suitable for intensive cultivation. Grain yield differences were caused by 2 different fertilization modes: nitrogen fertilizer shifted from basal tillering to panicle initiation (PBSP) stage and heavy basal N plus top-dressing at early tillering stage. Negative correlation was also observed between grain yield increase due to postponed nitrogen application difference (Y) and grains number per panicle (X), i.e., Y = 2 607.9 11.02X (R² = 0.630 8). Hybrid rice cultivars with grains number per panicle less than 237 were suitable for PBSP fertilization. This research focused mainly on the theory and technique of optimal control of hybrid rice and then pointed out future research directions. The future researches should focuse on sink-source structures of hybrid rice varieties adaptable to mechanical transplanting and harvesting, early diagnosis and optimal fertilization techniques needed for high yields. Also early detection and prevention mechanisms of lodging and cultivation technique of hybrid rice were needed for reduced nitrogen use, increased yield and high-production efficiency.