| The world population and the number of people depending on rice have been on a steep increase not just in Asia but also in Africa and Latin America. In addition, global climate changes.the increases in the frequency and intensity of extreme events like heat spikes, droughts or floods, will negatively affect rice production. All the above scenarios pose a serious challenge for meeting the global demand and supply of rice,under current and future climates. Thus, improving rice productivity under folly flooded non-stress conditions becomes especially important and timely. In this research, we systematically studied the physiological and agronomic responses of different crop management practices, nitrogen management and their interaction on population canopy structure, grain yield, yield components, grain quality, grain filling characteristics, nitrogen use efficiency and radiation use efficiency. Field experiments were conducted continuously during the early season and late season of2009-2011in Liuyang County, Hunan Province, China and during the2011wet season (WS) to2012dry season (DS) at the experimental farm of IRRI, Philippines, respectively. The main results of the study are listed below:1. The results of different crop management practices recoeded significant differenceswith in rice grain yield among the different management practices.The ranking for grain yield among different treatments across different years and seasons was as follows: T4> T5> T3> T2> T1. Grain yield averaged across all three years in T4, T5and T3was32.8%,20.4%,15.7%and37.1%,28.8%,14.8%higher than the normal farmers’practice (T2) in early season rice (ESR) and late season rice (LSR), respectively. Reasons behind the yield increase can be explained by yield components, leaf area index (LAI) and dry matter production. T4, T5and T3produced significantly higher panicles m-2and spikelet number m-2compared to T2during both ESR and LSR, and spikelets panicle-1and the grain-filling percentage were slightly decreased in ESR and especially slightly increased in LSR compared with T2. In addition, LAI averaged across all three years in T4, T5and T3was51.0%,27.8%,16.9%and64.0%,49.2%,25.3%higher than with T2during ESR and LSR, respectively.The total biomass averaged across all three years in T4, T5and T3was30.3%, 18.1%,14.2%and39.9%,32.7%,12.9%higher than with T2in ESR and LSR, respectively. Facilitating better N uptake2. The total nitrogen content and nitrogen use efficiency (NUE) varied significantly among the different management patterns. The total nitrogen content of plants in ESR and LSR was higher by42.1%,23.7%,18.2%and89.1%,75.5%,35.0%in T4, T5and T3than in T2, respectively. Nitrogen agronomic efficiency (AE) in ESR and LSR was increased by45.2%,46.0%,68.2%and127%,127.3%,97.8%and partial factor productivity of applied nitrogen (PFP) in ESR and LSR was increased by9.5%,19.1%,42.5%and16.0%,27.5%,38.5%in T4, T5and T3compared with T2, respectively. Significant increase of the total nitrogen content and NUE was mainly due to the total nitrogen content after heading in ESR and LSR was increased by78.3%,54.8%,68.8%and74.0%,63.4%,31.8%in T4, T5and T3compared with T2, as a result of postponement of the N fertilizer application. On the other hand, combination with shallow wetting and drying and KH2PO4application as foliar spray resulted in the increase of nitrogen translocation, and nitrogen translocation during ESR and LSR by22.0%,10.7%,2.8%and137.3%,104.6%,59.5%in T4, T5and T3compared with T2, respectively.3. Intercepted radiation (IR) and radiation use efficiency (RUE) differed significantly among the different management practices. IR averaged in ESR and LSR of T4, T5and T3was13.4%,9.6%,4.6%and14.1%,11.7%,4.4%higher than that in T2, respectively. RUE averaged in ESR and LSR of T4, T5and T3was15.0%,8.0%,9.4%and16.7%,33.9%,8.4%higher than that in T2, respectively. Both IR and RUE were significantly positively correlated with grain yield (p＜0.001) in both seasons, while r of intercepted radiation in ESR was greater than in LSR, and r of RUE in LSR greater than in ESR. The above results indicated that increasing IR of rice population was the key approach of ESR yield improvement, and improving RUE via optimizing rice population quality was the main approach of LSR yield improvement in double-season rice.4. Nitrogen treatments and cultivars had significant effects on grain filling parameters. GRo increased, T2and Tmax extended and T1decreased with increasing N supply, resulted in lower mean grain rate. On the other hand, S1shortened and S2and S3increased with increasing N supply, resulted in higher grain filling rate at early filling stage and lower grain filling rate at mid and later filling stage. The grain filling characteristics across cultivars was significantly different between dry season (DS) and wet season (WS), differed more than that between different N treatments. The grain filling characteristics had significantly effect on rice grain yield and especially head rice yield. Shortening T1and extending T2to accelerate MGR1and decelerate MGR2and MGR3can improve head rice yield.5. Comparative analysis between the actual observed yield (AY) and the climatic yield potential (PY) derived using ORYZA2000indicated that AY across current elite cultivars supplied with80kg N ha-1were up to22%-33%lower than PY, and AY with120kg N ha-1were12%-21%lower than PY during WS. Dry matter production with80kg N ha-1and120kg N ha-1was16-23%and3%-15%lower than potential dry matter production during DS, respectively. AY across current elite cultivars supplied with adequate N (140and210kg N) was close to100%of the climatic yield potential derived using ORYZA2000during DS Our results demonstrated the immediate urgency in incorporating new and diverse germplasm into ongoing breeding programs targeted toward enhancing yield under fully flooded non-stress conditions. On the other hand, the use of a derived climatic yield potential as an unbiased reference for rice yield potential studies will help account for climatic and edaphic variability across different rice-growing geographies and it allows for cross cutting meta-analysis of genetic yield gains over time.6. Different populations in rice had significant effects on grain yield, climate yield and especially head rice yield. Grain yield was improved with increasing N application, except for some cultivars with lower yield under high N, which indicated response to N supply varied with cultivars, while head rice yield increased with increasing N supply. Even with similar paddy yield, the cultivars differed significantly in head rice recovery, and head rice yield.Based on our results, we redefine rice yield potential as "the climatic yield of a cultivar with superior head rice yield recovery when grown in environments to which it is adapted, with nutrients and water non-limiting and with pests, diseases, weeds, lodging, and other stresses effectively controlled." The inclusion of "climatic" yield would allow establishing a measuring stick for tracking yield gains over time after accounting for edaphic and environmental conditions and "with superior head rice recovery," which determines the actual gain in terms of ecollllnomic units that determine cultivars’marketability, consumer preferences, and wider farmer adoption. |
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