| Global atmospheric and climatic changes as a result of human activities will significantly alter many elements of the future crop production environment. One of these changes is the rapid increases in tropospheric ozone concentration. High tropospheric ozone concentration inhibits rice growth and causes yield reduction, but how this effect changes with different cultivation managements remains unclear, especially under open field conditions. Super rice, as China’s major technological innovation, played important role in achieving the significant yield increase of rice. However, what’s the difference between super rice and other rice types in the response to ozone stress and its possible reasons are unclear? FACE (Free Air gas Concentration Enrichment) experiments were conducted under completely open field conditions, it is considered by many researchers as the best method to assess the actual impact of changing atmospheric components on crop yield. In order to investigate the effects of surface ozone concentration and plant density on growth and development, photosynthesis, dry matter production, nutrient uptake and yield formation of different rice cultivar, and to provide scientific basis for formulating of rice production strategy and cultivation measures that minimize the negative ozone impacts in future high ozone concentration conditions, we conducted a free air ozone concentration (FACE) enrichment experiment. Super hybrid rice cultivar II you084and conventional indica rice cultivar Yangdao6with3spacing levels:low plant density (LD,16hills m-2), medium (MD,24hills m-2) and high plant density (HD,32hills m-2) was grown at current (C-O3) and elevated ozone concentrations (E-O3, current×1.5). Results showed as follows:1. Elevated ozone had no impact on growth stage of Yangdao6, but accelerated heading and maturity of II you084for3and7days, respectively. Elevated ozone reduced plant height of Ⅱ you084by8%at maturity, but had no significant effect on plant height of Yangdao6. Weak interactions were detected between03and cultivar for plant height at maturity.2. Ozone stress had no significant effect on panicle number per unit area, but reduced spikelet number per panicle, filled grain percentage and filled grain weight by9.9%,9.3%and3.4%, respectively. When comparing within different cultivars, the impacts of ozone stress on II you084were bigger than Yangdao6, which were proved by the significant ozone by cultivar interactions for spikelet number per panicle, filled grain percentage, the ratio of spikelet number to stem dry weight and filled grain weight. Significant ozone by cultivar by planting density interactions were detected for spikelet number per panicle:Spikelet number per panicle of Yangdao6was decreased significantly by elevated ozone under low planting densities, but showed small increase under high planting densities; However, for Ⅱ you084, the impacts of elevated ozone on spikelet number per panicle showed no significant differences among different planting densities.3. The impacts of elevated ozone on rice yield varied with different cultivars. Elevated ozone reduced rice yield by235g m-2on average, which is24.5%reduction compared to the control. The yield reductions due to high ozone treatment were142g m-2or15.9%for Yangdao6and279g m-2, or27.3%for II you084, respectively. Week interactions of ozone by cultivar by planting density were detected:The rice yield of Yangdao6decreased with the increase of planting density, on contrast, no significant differences on yield response to elevated ozone between different planting densities were observed for II you084.4. Elevated ozone decreased the leaf area index (LAI) of rice by16.9%,25.8%and9.5%on average at tillering stage, jointing stage and heading stage respectively. The response of II you084to ozone was slightly larger than that of Yangdao6. Ozone stress had no significant effect on net assimilation rate (NAR) of rice from tillering to jointing stage, but reduced it from jointing to heading stage by11.6%on average. The effect of ozone×cultivar, ozone×cultivar×density reached extremely significant and significant level respectively for NAR from jointing to heading stage. Elevated ozone reduced leaf SPAD value of Ⅱ you084by5.9%,10.5%and12.5%, at63,77, and88days after transplanting, respectively. The decrease increased with time went on. 5. Ozone stress decreased leaf net photosynthetic rate (Pn) by17.9%and26.2%at77and88days after transplanting, respectively, and also reduced leaf stomatal conductance (Gs) by17.9%and58.6%at the same period. Elevated ozone had no impact on leaf intercellular CO2concentration (Ci) and leaf transpiration rate (Tr) at77days after transplanting, but decreased it by9.9%and68.4%respectively at88days after transplanting. Ozone stress decreased actual photochemical efficiency (Yield), initial fluorescence in the dark (Fo) and photochemical quenching (qP) by11.7%,5.5%and6.6%, respectively, and increased non-photochemical quenching (qN) by12.3%at88days after transplanting.6. Ozone stress decreased dry matter accumulation of rice by21.5%on average at plant maturity. Ozone stress decreased dry matter production of rice by6.1%,21.0%and32.0%on average form tillering to jointing stage, jointing to heading stage and heading to plant maturity, respectively. Elevated ozone increased dry matter distribution in leaves of rice by3.0%and5.9%on average at jointing stage and heading stage, respectively. Ozone stress decreased dry matter distribution in stems of rice by2.8%,2.3%and6.3%on average at jointing stage, heading stage and plant maturity, respectively. Elevated ozone decreased harvest index (HI) of rice by4.5%on average. The analysis of variance showed that significant interactions were detected of ozone by cultivar for dry matter production form heading to maturity stage and harvest index. The response of II you084to ozone was larger than that of Yangdao6. Week interactions were also detected of ozone by planting density for dry matter accumulation at maturity stage.7. Elevated ozone reduced soluble carbohydrate concentration in stem of Ⅱ-you084by22%,21%and32%, decreased starch concentration by27%,17%and36%, decreased non-structural carbohydrate concentration by24%、19%and33%, at jointing, heading and plant maturity, respectively. Elevated ozone decreased soluble carbohydrate accumulation by28%,35%and44%, decreased starch accumulation by34%、35%and367%, decreased non-structural carbohydrate accumulation by31%、35%and45%, at jointing, heading and plant maturity, respectively.8. Elevated ozone increased plant nitrogen (N) concentration by6.2%,9.2%and13.4%at joniting stage, heading stage and plant maturity, respectively. Elevated ozone had no clear impact on plant nitrogen content of Yangdao6, but decreased it of Ⅱ you084by8.3%,4.9%,4.7%and19.2%at tillering stage, jointing stage, heading stage and plant maturity, respectively. Elevated ozone decreased nitrogen use efficiency for biomass by6.2%,9.2%and12.7%on average at jointing stage, heading stage and plant maturity, respectively. Elevated ozone decreased nitrogen use efficiency for grain yield of rice by10.1%, but increased nitrogen harvest index by3.4%on average. The effect of ozone stress on nitrogen distribution of rice was irregular.9. Elevated ozone increased plant phosphorus (P) concentration by16.5%at tillering stage, but had no effects on heading stage. The plant P concentration was decreased by16.0%and5.6%due to ozone stress at joniting stage and plant maturity, respectively. Elevated ozone had no clear impact on plant phosphorus content at tillering stage, but decreased it by21.7%,13.2%and24.4%at jointing stage, heading stage and plant maturity, respectively. Elevated ozone increased phosphorus use efficiency for grain yield of rice by5.8%, and phosphorus harvest index by9.0%on average. The effect of ozone stress on nitrogen distribution of rice and phosphorus use efficiency for biomass production was irregular.10. Elevated ozone increased plant Potassium (K) concentration by4.2%,9.8%and7.7%at tillering stage, heading stage and plant maturity, respectively, but had no clear impact at jointing stage. Elevated ozone had no clear impact on plant K content of Yangdao6at tillering stage, jointing stage and heading stage, but decreased it by9.1%at plant maturity, and also decreased it of Ⅱ you084by9.8%,8.3%,8.0%and19.7%at tillering stage, jointing stage, heading stage and plant maturity, respectively. Elevated ozone decreased Potassium use efficiency for biomass by4.2%,9.5%and6.9%on average at tillering stage, heading stage and plant maturity, respectively, but had no effects at jointing stage. Elevated ozone decreased Potassium use efficiency for grain yield of rice by4.3%, but had no effects on Potassium harvest index. The effect of ozone stress on Potassium distribution of rice was irregular.In summary, tropospheric ozone concentration projected for this mid-century (25%higher than the current ozone concentrations) inhibited leaf photosynthesis of rice at middle and late developing stages, reduced plant growth, thereby decreased rice yield production significantly. In general, the regulation effects through selecting tolerant cultivars were significantly greater than changing planting densities. Therefore, the selection of high-yielding rice varieties combined with relatively high planting density will be beneficial to rice production, because this strategy can minimize the ozone damage to rice plants, thus reduce yield loss in the future high ozone conditions. |
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