Evolution Characteristics Of Grain Yield And Plant Type For Mid-season Rice Cultivars

Rice is one of the most important food crops in China and its grain yield per hectare has been over 6 ton in recent years, which is 65% higher than that of the world, on average. Improvement in rice cultivars plays an important role in the increase of the grain yield. However, little is known about physiological and agronomical features responsible for the improvement. In this study, 16 typical mid-season indica and 11 japonica rice cultivars (including hybrid combinations) applied in the production in Jiangsu Province during the last 50 years were used as materials. All the test cultivars were field grown and the evolution characteristics of grain yield and its components, grain quality, source-sink relationships and plant types were investigated. The main results are as follows:1.Grain yield was progressively increased with the evolution of the cultivars. Increase in grain yield was attributed mainly to the increase in total number of spikelets, and increase in spikelets was resulted mainly from large panicles. Hybrid rice had lower percentage of filled grains, which limited its high-yielding potential. From old cultivars to modern hybrid rice, biomass and harvest index were simultaneously increased, and the biomass, especially after heading, was increased more relative to the increase in harvest index. Grain quality was also improved with the evolution of rice cultivars, especially for mid-season indica cultivars.2.Total leaf area index (LAI), LAI of productive tillers and LAI of top three leaves at the heading stage were significantly increased with the evolution of the cultivars. Increase in spikelets, however, was more than that in leaf area, which resulted in the increase of grain-leaf ratio (the ratio of total spikelet number to leaf area at heading). The net photosynthetic rate (Pn) of modern cultivars was significantly higher than that of old cultivars at heading. LAI, root weight and root activity of modern cultivars were much greater than old cultivars at each growth stage. However, senescence in leaves and roots at the late grain filling stage was earlier for modern cultivars than for old ones, which may limit photosynthetic productivity of modern cultivars.3.Modern cultivars, especially hybrid combinations, had large panicles and its grain-filling process was asynchronous between the superior and inferior grains. Inferior grains had smaller grain filing rate but showed longer active grain filling period, when compared with superior grains. It would be an important approach to increase in grain plumpness and grain weight through accelerating grain-filling rate for inferior grains of hybrid rice.4.Compared with those in old cultivars, modern cultivars had higher amount of nitrogen (N), phosphorus (P) and potassium (K) absorption in plants at each growth stage. The absorption and accumulated of N, P and K in plants for all types of cultivars were mainly in the period from jointing to heading stage, and the proportions of the nutrients absorbed during this period to the total absorption during the whole growth period were increased with the evolution of the cultivars. Old cultivars had higher amount of nutrients in stems and sheaths at maturity than that of modern varieties. Modern cultivars, especially hybrid combinations, exhibited a high translocation percentage of nutrients from vegetative organs to grains during the grain filling period.5.The length, width and area of top three leaves of modern cultivars were much larger than those of old cultivars. Improvement in rice cultivars significantly reduced the leaf angle of the top three leaves on stems. The plant height of hybrid combinations was higher than that of semi-dwarf cultivars. The increase in plant height was mainly contributed to the increase in the upper internodes (1st and 2nd internodes from top). The thickness and weight of internodes, especially the upper two, were much greater for modern cultivars than for early ones. The panicle length was increased with the evolution of cultivars. The increase in spikelet number, however, was more than the increase in the panicle length, leading to an increase in spikelet density (the ratio of spikelet number per panicle to panicle length). The number of primary and secondary branches, especially the secondary branches, was increased greatly with the improvement of rice cultivars. Based on the evolution characteristics of rice cultivars, the indices of plant type or population for high-yielding (>9t ha-1) were suggested, i. e., plant height, 100-110cm; the leaf angle of the top three leaves, <15o; LAI at heading, 7-8; Total spikelet number, 450-500×106 ha-1; spikelet/leaf(cm2), >0.6; Total biomass, > 18t ha-1; root dry weight at heading, > 2.5t ha-1; ratio of root to shoot, >0.25; harvest index, >0.5. The regulation approaches and key cultivation techniques for creating the high-yielding population were discussed.