| Atmospheric carbon dioxide(CO2)concentration has increased from 280 μmol·mol-1 before the Industrial Revolution to nearly 420 μmol·mol-1 at present,and is expected to reach 550 μmol·mol-1 by 2050.Compared to enclosed or semi-enclosed chambers,FACE(free air CO2 enrichment)technology,studying crop performance under farmland conditions with free air flow,provides more realistic simulation of crop growth environments with elevated CO2 concentration in the future.Previous FACE studies on rice,an important cereal crop,have showed that elevated CO2 concentration promotes rice growth and enhances grain yield,but deteriorates grain quality in most cases.To date,rice FACE studies have been only conducted in a few growth conditions with relatively few experimental materials,and the interactions between open-air CO2 enrichment and different cultivation/environmental factors such as rice types,transplanting densities and air temperatures remain unclear,especially whether CO2induced changes in rice yield and quality traits differ among grain positions within rice panicles under different conditions.Therefore,a series of multifactor experiments were carried out at a large rice FACE platform to investigate the effects of elevated CO2(+200 μmol·mol-1)on rice grain yield and quality traits at different conditions with the variation among spikelet positions within panicles being focused.The interactions between elevated CO2 concentration and different growth seasons,rice varieties(japonica and indica rice),transplanting densities(16 and 24 hills per m2)and air temperatures(ambient temperature and temperature elevation by approximately 1℃)on rice yield and grain quality traits were also investigated.Furthermore,the meta-analysis method was used to quantitatively evaluate the responses of rice growth,yield and quality traits to elevated CO2 concentration and their influencing factors based on all rice FACE publications.The objective of this study is to clarify the effect of elevated CO2 concentration and its interacti ve factors on grain yield and quality traits of fieldgrown rice,and to elucidate the different responses of rice yield and grain quality traits among spikelet positions,so as to provide a theoretical basis for formulating high-yield and goodquality rice production strategies in future climate change environment.The main results of the study are as follows:When averaged across two consecutive growth seasons,the grain yield of super hybrid rice YLY900 was increased by 15%at elevated CO2 concentration,which is mainly related to significant increases in panicle number(+9%)and spikelet number per panicle(+6%),in comparison with relatively small changes in fully-filled grain percentage and fully-filled grain weight.In 2016(low temperature with less sunshine and more rain)and 2017(high temperature with more sunshine and less rain),the CO2-induced increases in grain yield were 9%and 22%,respectively,mainly caused by interannual differences in the responses of panicle number to elevated CO2:the panicle number was increased by 17%in 2017,but was not changed in 2016.The yield increase of FACE rice was mainly associated with a higher increase in grain yield on secondary branches(+19%),while the increase in grain yield on primary branches was relatively small(+9%).Similar trends were found in the two growth seasons:elevated CO2 concentration significantly increased peak viscosity and breakdown,decreased head rice percentage,weight ratio of green grains in brown rice,setback,protein and S concentrations,but had no changes in grain shape of brown rice and chalkiness traits,Ca,K,Mg,Mn,Zn concentrations of milled rice.The CO2 response of weight ratio of green grains in brown rice varied between grain positions:the decreased weight ratio of green grains of FACE rice mainly resulted from a sharp decrease in weight ratio of green grains of inferior spikelets.Rice grain yield of two varieties WYJ27 and YD6 were increased by 28%on average by elevated CO2 concentration,which was mainly due to a large increase of 27%in panicle number.The CO2-induced increase in grain yield of YD6(+40%)was approximately threefolds that of WYJ27(+13%),and the increase in grain yield on secondary branches(+50%)was much higher than that on primary branches(+24%)for YD6,while similar increases(1015%)were found in grain yield on primary and secondary branches of WYJ27.However,the spikelet and branch formation per panicle and grain filling capacity were less affected by CO2 elevation.On average,FACE treatment deteriorated processing,appearance and nutritional quality of rice grains,as shown by significant decreases in head rice percentage and grain protein,amino acids,Mg and S concentrations,as well as significant increases in chalky grain percentage and chalkiness degree.In contrast,the overall taste of FACE rice was improved because of enhanced gel consistency,peak viscosity and breakdown,but a declined setback.In most cases,the responses of grain quality to elevated CO2 concentration were influenced by rice varieties or grain positions on panicles.For example,under FACE treatment,the increase in chalky grain percentage and the decreases in protein and total amino acid concentrations of WYJ27 were greater than those of YD6 due to different responses of corresponding parameters of superior spikelets between varieties,while no significant changes were detected in those of inferior spikelets for both varieties.When averaged across two transplanting densities,substantially increased panicle number(+22%)of SLY136 at elevated CO2 concentration resulted in a 24%increase in grain yield.The increase of grain yield was higher at high transplanting density(+26%)than at low transplanting density(+21%),and the increase in grain yield of secondary branches(+32%)was much higher than that of primary branches(+11%).Elevated CO2 concentration decreased spikelet numbers on primary branches in upper,middle and lower parts of rice panicles by 3%to 6%,but increased spikelet numbers on secondary branches at three parts of rice panicles by 4%to 26%,which resulted in a slight increase in spikelet number per panicle.At heading stage,elevated CO2 concentration reduced dry matter accumulation and nitrogen uptake per stem,enhanced nonstructural carbohydrate concentration in stems,but had little effect on differentiation and degeneration of rice branches and spikelets.For superior,medium and inferior spikelets,the empty grain percentages were all increased,but fully-filled grain weight and average grain weight were decreased at elevated CO2 concentration.At different transplanting densities,FACE treatment had little effect on head rice percentage,grain shape and grain chalkiness traits,but dramatically decreased weight ratio of green grains in brown rice,especially in inferior spikelets.Elevated CO2 concentration increased gel consistency,peak viscosity,hot viscosity,breakdown and final viscosity of milled rice flour,and decreased amylose content,setback and gelatinization temperature,and the RVA parameters were improved more at low transplanting density than at high transplanting density.Relative to superior spikelets,inferior spikelets showed stronger responses to CO2 elevation in peak viscosity,breakdown,setback and gelatinization temperature.Elevated CO2 concentration reduced the concentrations of protein(-11%),amino acids,K and S in milled rice,but increased the concentrations of P,B,Mn and phytic acid,and the percentage decreases in protein,total amino acid and especially non-essential amino acid concentrations were higher in inferior spikelets than in superior spikelets,similar trends being found at two transplanting densities.The grain yield and yield components of NJ9108 were not significantly affected by elevated CO2 concentration,neither at ambient temperature nor at elevated temperature.On average,elevated CO2 concentration increased head rice percentage and amylose content,but decreased chalky grain percentage,setback and peak time.With the increase of CO2 concentration,lower protein(-11%)and S concentrations,but higher K,Mg,P and Mn concentrations were detected in milled rice.On average,elevated temperature caused a slight decrease of 4%in grain yield of NJ9108,mainly due to significant decreases in grain yield of primary branches in upper and middle parts of rice panicles.At elevated temperature,the chalkiness degree,chalkiness area,gelatinization temperature and concentrations of Mg,P,S and Zn in milled rice were increased,but hot viscosity was decreased.In several yield and quality traits,apparent interactive effects between CO2 and temperature were detected:the spikelet number of primary branches in upper parts of rice panicles remained unchanged at elevated CO2 concentration or elevated temperature alone,but was significantly reduced by 8%at simultaneous elevation of CO2 concentration and temperature;elevated CO2 concentration increased peak viscosity(+6%)and breakdown(+10%)but decreased setback(-21%)of superior spikelets at ambient temperature,however,these positive CO2 effects disappeared at elevated temperature;elevated CO2 concentration reduced grain Zn concentration(-11%)at ambient temperature,but did not changed that at elevated temperature.The meta-analysis of data from rice FACE publications,showed that elevated CO2 concentration significantly promoted photosynthesis rate(Asat,+22%)and nitrogen uptake(+7),resulting in enlarged sink size and thus significant increases in final biomass(+17%)and grain yield(+16%),and meanwhile improved rice cooking and eating quality as indicated by higher breakdown(+5%)and overall taste value(+5%)and lower setback(-12%).However,the processing,appearance and nutritional quality of rice were generally deteriorated in high CO2 environments,as reflected by significant decreases in head rice percentage(-8%)and concentrations of protein(-9%),Fe(-5%)and Zn(-4%),as well as significant increases in chalkiness degree(+30%)and molar ratio of phytic acid to Zn(+3%).Compared with conventional japonica rice,hybrid rice benefits more from high CO2 environments,such as larger increases in biomass and grain yield,and relatively smaller reductions in grain protein and S concentrations,which may be linked with the lack of obvious photosynthetic adaptation at later growth stages of hybrid rice.Moderate nitrogen supply promoted rice tillering under FACE conditions,thereby enlarging yield increase,and also alleviated the negative CO2 effects on rice processing and appearance quality.Temperature elevation of 1-2℃ depressed the responses of fully-filled grain percentage and grain weight to FACE treatment,resulting in a weaker yield increase(+17%vs.+10%).Elevated CO2 concentration further decreased the concentrations and bioavailability of several elements in milled rice,in comparison with those in brown rice,indicating that research based on brown rice may underestimate the adverse effects of elevated CO2 concentration on rice nutritional quality.In conclusion,the atmospheric CO2 concentration in the middle of this century(approximately 550 μmol·-mol-1)will promote rice growth,increase sink capacity and grain yield,improve rice palatability,but dilute grain protein and S concentrations.Great variations exist in CO2 effects on rice yield,growth and grain quality,which are influenced by rice varieties,transplanting densities,air temperatures and grain positions with various degrees.Combined with results from the meta-analysis,selecting heat-resistance and large-sink rice,supplemented with moderate increases in transplanting density and nitrogen supply,is expected to further increase rice yield and improve rice quality in the future. |
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