The Change Of Plant Type And Nitrogen Use Efficiency During Rice Cultivar Improvement

Rice is a most important food crop in China and its grain yield per hectare has been over six 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. During the process of cultivar improvement, rice agronomical features and nitrogen use efficiency both changed. In this dissertation, we use different early-season inbred indica rice, japonica mid-season rice and hybrid combinations applied in the production in Zhejiang Province during the past several decades to study the change of plant type and nitrogen use efficency during rice cultivar improvement.The main contents of this dissertation are as follows:1. Evolution characteristics of rice agnomical featuresAll the tested cultivars were grown in the field and the evolution characteristics of grain yield and its components were investigated. Results showed that grain yield of early-season inbred indica rice were progressively increased with the evolution of the cultivars. Increase in grain yield was attributed mainly to the increase in total number of spikelets, which resulted mainly from large panicles. There were no significant difference in the effective panicles, seed setting rate and 1000 grain weight of rice cultivar which released at different stage. In hybrid combinations, because of the decrease of effective panicle per plant, the grain yield and biomass per plant of hybrid combinations decreased with the evolution of the cultivars, in spite of the plant height, 1000 grain weight and grains per panice increased slightly. In japonica rice, yield increased greatly in 1960 s', the cultivars released at this stage could be divided as compatible panicle-kernel type and multi-panicle type, while after 1970s', the yield of japonica rice increased slowly, most of the varieties released at this stage were compatible panicle-kernel type. In plant morphological characters, compared to the former cultivars (released before 1985), the latter cultivars (released in 1985-1995) had short flag leaf length, small leaf original angle, longer third leaf length and wider flag and third leaf width; with the improvement of japonica cultivar improvement, the plant height, internode length became shorter, while stem density, sheath weight and grain density per panicle became larger.2. Response of grain yield to amount of nitrogen applicationNitrogen is the most important nutrition in rice production, and its function is affected by cultivar, fertilizer application and soil condition. In resent years, paddy field in Yangzi River has a trend of becoming fertile. In order to study the cultivar difference in nitrogen requirement, we planted inbred japonica rice cultivars and hybrid indica rice cultivars in different nitrogen application levels .The results showed that rice yield was significantly increased by nitrogen, most japonica cultivars released before 1985 reached their highest yield and highest effective panicles when nitrogen was applied at 150 kg·hm-2; while to cultivars released after 1985, there were no significant difference in grain yield wether nitrogen was applied 150 kg hm-2 or 255 kg hm-2. The grain yield of hybrid combinations improved with the imcrease of nitrogen application, and the yield increase range of combinations released recently (after 1995) was larger than that of cultivar released before 1995.3. The change of nitrogen use efficiency (NUE)The change of nitrogen use efficiency was studied by planting inbred japonica rice cultivars and hybrid indica combinations in different nitrogen application levels. The results showed that: In japonica rice, nitrogen agronomic efficiency (AE), partialfactor Productivity of applied N (PFP), and nitrogen physilogical efficiency (PE) increased with the evolution of the cultivars in all nitrogen application levels, the change of nitrogen uptake efficiency (RE) was affected by nitrogen application amount, when nitrogen was applied 75 and 150 kg hm-2, nitrogen uptake efficiency (RE) first decreased, then increased with the evolution of the cultivars, while when nitrogen was applied 255kg hm-2, RE of the latter cultivars (released after 1985) was higher than that of former cultivars (released before 1985). The change of NUE in indica hybrid combinations also affected by nitrogen application amount, when nitrogen was applied 75 kg hm-2, AE, PFP, RE and PE all decreased, while when nitrogen was applied 255 kg hm-2, there was no significant difference in NUE. Correlation analysis between NUE and nitrogen accumulation characteristics indicated that there were no significant correlations between NUE and plant nitrogen accumulation in japonica cultivars, while in hybrid rice, AE, PFP significantly positive corrected with plant nitrogen accumulation content, grain yield and biomass.