| The increasing atmospheric carbon dioxide（CO2）is one of the most important climate change factors,which will have a profound impact on rice（Oryza sativa）growth by promoting photosynthesis.Source-sink relationship is one of the hot issues in crop cultivation physiology.However,it is unclear whether regulating source-sink relationship affects the responses of rice yield and grain quality to elevated CO2.Cutting off leaves or removing spikelets are commonly used methods to study the source-sink relationship of crops.The original source-sink relationship of rice could be artificially changed by reducing the source and/or sink at the heading stage and then compared with the control plants in these methods.We grew japonica cultivar Wuyunjing 27（WYJ27）and hybrid cultivar Yongyou 538（YY538）,which are widely used in production,under ambient CO2 concentration（aCO2）and elevated CO2 concentration（eCO2,200 μmol·mol-1 higher than aCO2）in a free air CO2 enrichment（FACE）facility in 2017-2018.At heading stage,five source-sink treatments were set by cutting off the flag leaf（LC1）and the top three leaves（LC3）,removing one branch in every three branches of a panicle（SR 1/3）and one branch in every two branches of a panicle（SR1/2）,in the control（CK）,there was no leaf-cutting or spikelet-removal.In order to investigate the effects of eCO2 and sourcesink treatments on rice yield formation,dry matter production and distribution,photosynthetic capacity of leaves in different positions and quality traits,and the response of rice yield to source-sink treatment and eCO2 was explained from the perspective of photosynthesis.The purpose of this study was to provide a scientific basis for the development of rice production strategies under high CO2 concentration in the future.The main results were as follows:1.Compared with ambient,eCO2 increased the grain yield per unit area of the two varieties by 18.8%**on average,in which WYJ27 and YY538 increased by 15.7%**and 21.8%**,and crops under CK,LC1,LC3,SR1/3,and SR1/2 treatments were significantly increased by 17.0%**,20.5%**,26.1%**,16.7%**and 15.4%**,respectively.For both varieties,the increase rates of leaf-cutting crops were higher than those of the control or spikeletremoval crops,and the LC3 crops had the largest increase,and this was mainly related to the greater increase of grain filling rate and average grain weight of rice by eCO2.In terms of source-sink treatments,compared with control crops,LC1,LC3,SR1/3 and SR1/2 treatments reduced grains yield by 5.7%+,43.1%**,24.9%**and 39.1%**,respectively.For leaf-cutting crops,the decrease of grain yield was mainly related to the decrease of grain filling rate and grain filling weight,while for spikelet-removal crops that was mainly due to the decrease of spikelet per panicle.Further study on the effects of eCO2 and LC3 treatment on grain yield of WYJ27 showed that,compared with ambient,the grain yield of control and LC3 crops increased by 15.9%**and 25.1%**under eCO2;and in terms of different grain positions,eCO2 increased the yield of superior,medium,and inferior spikelet by 15.6%**,20.4%**,22.7%**,respectively.The increase of grain yield of medium and inferior spikelet was higher than that of superior spikelet,which was mainly related to the increase of grain filling rate and average grain weight.Compared with CK,LC3 reduced the yield of superior,medium and inferior spikelet grain yields by 26.7%**,53.2%**and 66.1%**respectively.The decrease of medium and inferior spikelet grain was larger than that of superior grain,which was mainly related to the decrease of grain filling rate of medium and inferior spikelet grain caused by LC3 treatment.2.The eCO2 had no effect on leaf dry weight,but significantly increased dry weight of stem（14.5%）and panicle（18.6%）,resulting in a significantly increase in above-ground biomass（14.1%）,and the above-ground biomass of crops under CK,LC1,LC3,SR1/3 and SR1/2 treatments increased by 12.5%*,15.2%*,18.5%*,13.7%**and 12.2%*,respectively,and.the average increase of leaf-cutting crops was greater than that of control or spikeletremoval crops.Compared with CK,LC1,LC3,SR1/3 and SR1/2 treatments reduced aboveground biomass by 6.7%*,30.4%*,5.1%+,and 6.1%*,respectively.In terms of matter distribution,eCO2 decreased dry weight ratio of leaf by 12.4%**and increased that of spikelet by 2.9%**,while it had no effect on that of stem.The dry weight ratio of leaves under CK,LC1,LC3,SR1/3 and SR1/2 treatments decreased by 11.2%**,12.8%**,11.7%**,14.6%**and 11.2%**,respectively,while the dry weight ratio of panicle and stem was not affected by eCO2.Compared with CK,the LC1,LC3,SR1/3 and SR1/2 treatments increased dry weight ratio of stem by 6.5%,25.6%,31.9%,and 54.0%,and decreased that of spikelet by 1.5%**,5.3%**,23.0%**,and 37.5%**,respectively;LC1 and LC3 treatments reduced the dry weight ratio of leaves by 10.3%**and 43.3%**,while SR1/3 and SR1/2 treatments increased it by 15.6%**and 20.8%**,respectively.Variety interactions were observed on dry weight ratio of stem or panicle between CO2 and LC3 or SR1/3 or SR1/2,and many significant interactive effects on dry weight or dry weight ratio of rice organs were observed between source-sink treatment and variety.3.Compared with ambient,eCO2 significantly increased net photosynthetic rate（Pn）and water use efficiency by 9.3%**and 19.4%**,and decreased stomatal conductance（Gs）,transpiration rate（Tr）and stomatal limit decreased by 11.3%**,7.0%*and 17.5%**of the flag leaves in CK,SR1/3 and SR1/2 crops at filling stage on average.The eCO2 increased Pn of the second leaves（CK and LC1 crops）and fourth leaves（CK and LC3 crops）by 5.4%and 10.1%on average,respectively,with no significant effect on CK,while the second leaf of LC1 crops increased by 10.7%*and the fourth leaf of LC3 crops increased by 18.9%**on average.In addition,Pn of remaining leaves of LC1 and LC3 crops significantly increased at the later stage of filling under eCO2,with no effect on Gs and Tr.There were different degrees of interactions between CO2 and source-sink treatments on Pn of the second and fourth leaves.Compared with CK,SR1/3 and SR1/2 treatments reduced Pn of the flag leaves by 5.7%**and 8.5%**,respectively.On the contrary,the leaf-cutting treatment significantly increased the Pn of the remaining leaves during the whole grain filling stage,in which LC1 increased the Pn of the second leaves by 15.7%**,LC3 increased the Pn of the fourth leaves by 30.0%**on average.The leaf-cutting treatments significantly increased the Gs of remaining leaves at the filling stage（the second leaves of LC1 crops and the fourth leaves of LC3 crops increased by 26.1%*and 58.1%**on average）,while spikelet-removal treatments significantly decreased the Gs of the flag leaves at the middle or late filling stage by more than 10%.The former was beneficial for rice to use the increased CO2 concentration,while the latter was contrary.4.Compared with ambient,the eCO2 increased non-structural carbohydrate（NSC）concentration in leaf and stem by 9.2%**and 19.1%**,respectively,during the filling stage,with the increase of leaf mainly in the late filling stage,but significantly increased NSC in stem during the whole filling stage.Compared with CK,LC1 and LC3 treatments reduced NSC concentration by 11.4%**and 24.8%**,while SR1/3 and SR1/2 treatments increased it by 108.6%**and 197.5%**,respectively;the leaf NSC concentrations increased by 5.7%*,8.6%**and 18.6%**under LC1,SR1/3 and SR1/2 treatments,respectively,while decreased under LC3 treatment.In terms of nitrogen concentration,eCO2 significantly decreased leaf nitrogen concentration（-2.9%）at filling stage,and leaf-cutting treatments tended to increase stem nitrogen concentration,while spikelet-removal treatments tended to reverse.The spikelet-removal treatments after anthesis significantly increased th e NSC concentration in stem during filling stage,which may have feedback inhibition on leaf photosynthesis and weaken leaf response to eCO2.In addition,the decrease of leaf nitrogen concentration at late filling stage may also be one of the reasons for inhibiting photosynthetic response.On the contrary,the decrease of NSC and the increase of nitrogen concentration by leaf-cutting treatments during filling stage may enhance the photosynthetic response of remaining leaves to eCO2,and weaken or even eliminate the photosynthetic adaptation.5.Compared with ambient,eCO2 reduced the milled rice percentage and the head rice percentage of the two varieties by 1.3%*and 1.5%*on average,and increased the chalkiness grain percentage and chalkiness degree of milled rice by 21.9%**and 28.7%**,respectively,the increase rate of YY538 was larger than that of WYJ27,and those of crops under spikeletremoval treatments were larger than those under leaf-cutting treatments.The eCO2 increased the maximum viscosity and breakdown value by 2.4%**and 4.8%**,respectively,and decreased the setback value by 341.8%**on average.These were consistent with the determination results of cooked rice（eating value+4.6%**,appearance+3.3%**,stickiness+2.8%**,balance+3.3%**,and hardness-3.4%**）.The eCO2 significantly decreased the concentrations of protein（-2.6%**）,（-1.6%**）and Cu（-12.4%**）,and significantly increased K（+2.2%*）concentration,while there were no significant changes on Ca,Mg,P,B,Fe,Mn,Zn and phytic acid concentrations.Compared with CK,leaf-cutting treatments had little effect on processing quality,while.spikelet-removal treatments-significantly increased both milled rice percentage and the head rice percentage;LC1 and LC3 increased the chalkiness grain percentage by 12.1%**and 14.5%**,and increased chalkiness degree by 22.7%**and 35.7%**,respectively,while SR1/3 and SR1/2 decreased the chalkiness grain percentage by 31.9%**and 60.4%**,and decreased chalkiness degree by 44.4%**and 73.9%**,respectively.Spikeletremoval treatments significantly reduced the amylose content of milled rice,while the effects of leaf cutting varied with varieties.Leaf-cutting and spikelet-removal treatments reduce both breakdown and setback values of YY538,while no effect was observed in that of WYJ27.Both leaf-cutting and spikelet-removal treatments increase the taste values of WYJ27,while no significant changes were observed on YY538.In most cases,the concentrations of protein and elements were increased by leaf-cutting and spikelet-removal treatments.Most interactions among CO2,varieties and source-sink treatments had no significant effect on rice quality indexes,but there were many interactions between source-sink treatments and varieties.6.Compared with ambient,the concentrations of soluble sugar,starch and NSC in rice straw at maturity increased by 11.1%**,4.1%**and 7.2%**,respectively,with a larger increase in stem and a decrease in leaves.The concentrations of crude protein（-5.3%**）,S（-5.9%*）,Cu（-7.8%+）,Mn（-14.8%**）decreased and P（+5.8%**）increased under eCO2,while Ca,K,Mg,Fe,Zn and B was not affected.Compared with CK,LC1 and LC3 treatments reduced the concentration of NSC by 24.6%**and 53.0%**,SR1/2 and SR1/3 increased it by 74.0%**and 100.9%**;LC3 reduced the absorption of crude protein by 29.8%**,SR1/3 and SR1/2 increased it by 25.9%**and 41.7%**,respectively.The concentrations of mineral elements in rice straw were increased by leaf-cutting treatments,while those of Ca,K,B and Zn were decreased by more than 10%under spikelet-removal treatments.There was no significant interaction between CO2 and variety or source-sink treatment on straw feeding quality,while significant interactions between source-sink treatments and varieties were observed.In conclusion,increasing atmospheric CO2 concentration by 200 μmol·mol-1 significantly increased rice yield,but decreased processing,appearance and nutritional quality of rice（protein,S and Cu concentrations were particularly noteworthy）,while improved the taste quality;straw feeding quality was slightly decreased.Both leaf-cutting and spikelet-removal treatments reduced grain yield,but had little effect on processing and eating quality,and increased protein and element concentrations,while had different effects on chalkiness of milled rice（leaf-cutting treatments increased chalkiness,while spikelet-removal treatments decreased it）.Artificial changes in source-sink relationship（leaf-cutting or spikelet-removal）did not significantly affect the response of rice quality to eCO2,however,reducing source-sink ratio enhanced the response of photosynthetic capacity of remaining leaves and increased yield,while spikelet-removal showed the opposite.The fertilizer effect of eCO2 can be further utilized by reducing the source-sink ratio of rice via artificial measures such as reducing the source-sink ratio and expanding the sink（e.g.controlling ineffective tillering,breeding varieties with large sink capacity and increasing flower fertilizer application）in the future. |
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