| The global carbon dioxide (CO2) concentration has risen from 280μmol·mol-1 before the in-dustrial revolution to about 400 μmol·mol-1 at present, which is expected to reach 550μmol·mol-1 by the middle of this century. Increasing atmospheric CO2 concentration ([CO2]) promoted rice growth and increased grain yield, but it was not clear whether this effect varied with different environment and cultivation conditions. The spatial advantage of the FACE (Free Air CO2 Enrich-ment) platform provides a unique opportunity for the exploration of this aspect. By using FACE platform, the CO2 concentration in the middle of this century (approximate 550μmol·mol-1) was simulated. Five experiments were conducted in order to:(1) screen sensitive materials and study the interactions between CO2 and cultivars. Four cultivars each of japonica, indica and hybrid rice were grown under two levels of CO2 concentration (ambient and ambient+200 μmol·mol-1) re-spectively. (2) Study the interactions between CO2 and transplanting density:II Y084 were grown under three levels of planting density with 16 (LD),24 (MD),32 (HD) hill-m’2 as low, medium and high density, respectively. (3) Study the interactions between CO2, nitrogen application and transplanting density:Shanyou 63 were grown under two levels of nitrogen application rate (LN, 15; HN,25 g·m-2) and plant density (LD,16; MD,24 hill·m-2). (4) Study the interactions between CO2 and temperature:Ⅱ Y084 were grown under two levels of CO2 concentration in combination with ambient and elevated temperature (Ambient+1℃). (5) Study the interactions between CO2 and source-sink manipulation:Ⅱ Y084 were grown under three levels of source-sink manipula-tion which were achieved through cutting off the whole flag leaf (LC, leaf cutting) or half of the spikelets at heading (SC, spikelets cutting; remove every other primary branch of a panicle) with CK of no leaf and spikelets cutting. In order to explore factors influencing CO2 fertilizer effects and its possible reasons, field experiments were carried out in the past several years to systemati-cally study the effects of elevated [CO2] on growth and yield formation of rice under different conditions (including cultivars, planting density, fertilizer application, temperature and source-sink changes). The main results are as follow:1. Study 1:Averaged over all 12 cultivars, grain yield per unit area increased from 832 to 958 g·m-2, up to 15%, by elevated [CO2]. There were significant differences among cultivars in their responses to elevated [CO2], the maximum yield response was 4 times of the smallest. When analyzed based on different cultivar types, elevated [CO2] increased grain yield of japonica, indica and hybrid rice by 64 (+9%),124 (+15%) and 190 g·m-2 (+20%) respectively, with the hybrids showed the highest response. ANOVA results showed significant interactions between CO2 and cultivar or type. The results of this experiment indicated that the sensitivity of different rice culti-vars to CO2 fertilizer effect was different and the response of hybrid rice was much higher than that of conventional rice cultivars.2. Study 2:Elevated [CO2] had no effect on growth process, but increased final plant height (+6%) of Ⅱ Y084. Elevated [CO2] increased leaf net photosynthetic rate (Pn), intercellular CO2 concentration (Ci), but decreased stomatal conductance (Gs) at each growth stage. The effects on leaf transpiration rate (Tr) varied with growth stages. Elevated [CO2] increased above-ground bi-omass of Ⅱ Y084 at tillering, jointing, heading and maturity by 42%,22%,39% and 33% respec-tively. Elevated [CO2] increased the number and total length of adventitious roots per hill, volume and dry weight of roots per hill significantly at each growth stage (+18-50%). With the higher response of roots than shoots, root to shoot ratio were increased significantly at tillering and joint-ing stage by elevated [CO2]. Elevated [CO2] increased grain yield of Ⅱ Y084 by 304 g·m-2, up to 31%, due to the increase of panicle number per square meter (+20%), spikelet number per panicle (+7%), and spkelet number per square meter (+29%). Analyzed from different planting densities, elevated [CO2] increased grain yield of Ⅱ Y084 under low, medium and high density by 183 (+19%),440 (+45%) and 287 g·m-2 (+30%) respectively. Similarly, biomass were increased by 23%,45% and 30% respectively. Analyzed over growth and yield factors, significant CO2 by planting density interactions were found on above-ground biomass at tillering and maturity stage, panicle dry weight at maturity stage, panicle number per unit area, spkelet number per panicle and per unit area. This results indicated that the appropriate increase in planting density could enhance CO2 fertilizer effect, but too much increase showed opposite trend.3. Study 3:Elevated [CO2] had no effect on plant phenology but significantly increased the final plant height (+7%) of Shanyou 63. Elevated [CO2] increased leaf Pn, Ci by 26%,54% re-spectively, reduced leaf Gs by 22% in early grain filling stage, but had no effect on leaf Tr. Elevated [CO2] increased the above-ground biomass of Shanyou 63 at jointing, heading and maturity stage by 33%,46% and 33% respectively due to the promotion on growth of above-ground organs. El-evated [CO2] increased the ratio of stem or panicle weight to above-ground biomass, while the ratio of leaf to above-ground biomass was decreased. Elevated [CO2] increased the number and total length of adventitious roots per hill, volume and dry weight of roots per hill at jointing and heading stage significantly. As the root dry weight of two stages increased larger than shoots, the root-shoot ratio increased significantly. On average, CO2 increased grain yield per unit area by 297 g·m-2, up to 36%. It mainly due to the significant increase of spikelet number per square meter (+29%), which associated with increase of panicle number per square meter (+8%) and spikelet number per panicle (+19%). Panicle number increase mainly due to maximum tiller number in-crease, and the panicle size increase is mainly associated with higher single stem weight. Analyzed over different levels of nitrogen and planting density, elevated [CO2] increased grain yield by 346 (+43%),369 (+46%),269 (+34%) and 207 g·m-2(+23%) for LNLD-, LNMD-, HNLD-and HNMD-crops, respectively. Similarly, above-ground biomass were increased 28%,30%(p=0.18), 41% and 33% respectively. Analyzed over growth and yield factors, there were weak CO2 by N interactions on grain yield per unit area (p=0.17), CO2 by D on grain yield per plant (p=0.13) and root dry weight per hill (p=0.06). This results showed that grain yield of hybrid rice Shanyou 63 was drastically increased by elevated [CO2] due to the increase of sink capacity, appropriate ad-justing the transplanting density and nitrogen application levels can change CO2 fertilizer effect to a certain extent.4. Study 4:The maturity stage of ⅡY084 was reached by 3 days later in elevated [CO2] than ambient. Elevated [CO2] increased the final plant height and stem dry weight by 6% and 49% respectively. Elevated [CO2] increased the above-ground biomass (+48%) whereas no effect on distribution of dry matter among different plant organs. Elevated [CO2] increased the root-shoot ratio and all adventitious root parameters per hill, but showed weak effect on single adventitious root parameters at heading and the 20 d after heading stage. The root biomass increase was related to the number of adventitious root rather than growth of single adventitious root. Elevated [CO2] increased grain yield from 810 to 1204 g·m-2, up to 49%, which mainly result from significant increases of the panicle number (+13%), total spikelet number (+13%), full-filled grain percentage (+36%), full-filled grain weight (+5%), and average grain mass (+26%). Elevated [CO2] induced enhancement on rice seed-set capacity was associated with the increase of stem weight (+57%) and NSC content (+304%) per spikelet. Elevated [CO2] increased grain yield by 373 (+39%) and 413 g·m-2 (+64%) under ambient temperature (AT) and high temperature (ET), respectively. Con-sistent with the grain yield, elevated [CO2] increased biomass by 39% and 60% under AT and ET conditions. ANOVA results showed that CO2×temperature interactions on final plant height, full-filled grain percentage, unfulfilled grain percentage, unfertile grain percentage and average grain mass closed to or reached 0.1 significant level. High temperature decreased grain yield by 24%, which mainly related with decreases of panicle number (-7%), total spikelet number (-14%), full-filled grain weight (-3%), average grain mass (-18%) and final biomass (-20%). This results showed that in high temperature season, elevated temperature caused growth inhibition and pro-duction decline of hybrid rice, elevated [CO2] can alleviate the high temperature stress on rice to a certain extent.5. Study 5:Elevated [CO2] increased final biomass of Shanyou 63 and IIY084 by 36% and 41% respectively, which associated with significant increase of dry weight of each above-ground organ, But elevated [CO2] had no significant effect on dry matter distribution of two cultivars. Elevated [CO2] increased grain yield of Shanyou 63 and Ⅱ Y084 by 234 (+36%) and 281 g·m-2 (+38%) respectively, which due to the substantial increase of panicle and spikelet number per unit area and also associated with the enhancement of seed-set capability. Elevated [CO2] increased spikelet number per unit area, fertilized grain percentage, full-filled grain percentage, full-filled grain weight and average grain mass of Shanyou 63 by 25%,6.5%,6.5%,1.3% and 6.6%, respec-tively, and the corresponding parameters of ⅡY084 were increased by 14%,5.9%,10.9%,6.6% and 13.3%. Elevated [CO2] increased grain yield of Shanyou 63 by 32%,55% and 25% for CK-, LC-and SC-crops respectively, while Ⅱ Y084 by 40%,62% for CK-and LC-crops respectively, but no effect for SC-crops. Elevated [CO2] increased biomass of Shanyou 63 by 39%,43% and 28% for CK-, LC-and SC-crops respectively, while Ⅱ Y084 by 41% ,56% and 27% respectively; two cultivars showed the same order in the response to elevated [CO2], which is LC-> CK-> SC-crops; Fertilized grain percentage, full-filled grain percentage, full-filled grain weight and average grain mass also showed similar trends. Significant CO2 by LC interactions were found on fertilized grain percentage, full-filled grain percentage and average grain mass of Shanyou 63, Significant CO2 by SC interactions were found on fertilized grain percentage, full-filled grain percentage and average grain mass of II Y084. This results indicated that elevated [CO2] could increase grain yield of rice, due to the stronger seed-set capacity, and increase sink-source ratio (such as LC) is beneficial for maximizing the CO2 fertilizer effect on hybrid rice, but reduce the source-sink ratio (such as SC) had opposite effects.To sum up, atmospheric CO2 concentration in the middle of this century (approximately 200 μmol·mol-1 higher than the current) enhanced productivity of hybrid rice. Changing cultivar, trans-planting density, nitrogen application, temperature and source-sink ratio can modify CO2 fertilizer effects. In order to ensure food security under the climate change scenarios in future, selecting or breeding heat-resistant hybrid rice cultivars with high seed-set capacity (Super rice), combined with appropriate amount of nitrogen and transplanting density will be helpful in maximizing the CO2 fertilizer effect on rice grain yield. |
