| Source and sink is the core concept of crop physiology and a key factor in determining crop yield.Recent studies have found that in addition to leaves,non-foliar organs such as bracts and internodes can provide assimilates for grain filling,and are also important source organs.Carbon isotope composition(δ13C)of a plant organ is an inherent signature reflecting its physiological property,and thus is used as an integrative index in crop breeding.It is also a non-intrusive method for quantifying the relative contribution of different source organs to grain filling in cereals.It provides a new method for the research on the relationship between source and sink.Using the samples collected from two-year field and pot experiments with two nitrogen(N)fertilization treatments(N5-5:base/topdressing,5/5;N10-0:base/topdressing,10/0),we investigated the temporal and spatial variations of δ13C in source organs of leaf,sheath,internode,and bracts,and in sink organ grain.The relative contribution of these organs to grain filling was assessed by comparing the δ13C in its natural abundance in the water-soluble fractions of the sheath,internode,and bracts with the δ13C in mature grains.It is crucial for the comprehensive disclosure of non-foliar organs and is also the theoretical basis of sourcesink relationship.The main results are as follows:1.The spatial and temporal characteristics of plant dry matter distribution during the reproductive growth period were clarified.By analyzing the changes of dry matter accumulation of different organs in rice during different development stages,the spatial and temporal characteristics of plant dry matter distribution were clarified.The results showed that at whole plant level,DM accumulation increased progressively after 21 DBA.For individual organs,DM of leaves peaked at anthesis,then decreased slowly to 50 DAA,and increased slightly until maturity.DM of sheath had a similar changing pattern with that of leaves,but it decreased more sharply at 20 DAA.Obviously,the variation of DM was largest for the internode,increasing until 10 DAA,and then decreasing to its lowest point between 20 and 30 DAA,then increased.Therefore,the changing pattern of the vegetative organs depended mainly on that of the internode.The reproductive organs of the grain and bracts showed a similar pattern in terms of DM accumulation,increasing progressively since the beginning of grain filling and finally plateauing at maturity.Bracts of superior and inferior spikelets basically stopped DM accumulation at 20 DAA.By contrast,grains of superior and inferior spikelets completed the filling process at 30 and 40 DAA,respectively.N fertilization mode had significant influence on DM accumulation.The N5-5 treatment that applied N topdressing at panicle initiation resulted in heavier grains,thus increasing grain yield per pot.2.Temporal and spatial variations of carbon isotope signature of different organs are revealed.By analyzing the changes of different organ δ13C in rice during different development stages,the temporal and spatial variations of carbon isotope signature of different organs were clarified.The results showed that there was a constitutive difference inδ13C among different organs,showing a consisitant trend in the order of leaf(-27.84 ‰)<grain(-27.82 ‰)<sheath(-27.24 ‰)<bracts(-26.81 ‰)<internode(-25.67 ‰).For different positions of individual organs within the plant,δ13C of the leaf and sheath presented a diminishing trend from the top(flag leaf and its sheath)to the bottom(the last leaf in reverse order and its sheath).No obvious pattern was found for the internode.For temporal variations,δ13C of the leaf and sheath had a peak(the most negative)at 10 days after anthesis(DAA),whereas that of the internode rose gradually after 21 DBA and reached its maximum at 20 DAA,10 days later than that of leaf and sheath.This time point of 20 DAA corresponds to the time when DM of the internode was at the lowest value,implying the substantial translocation of assimilates in the internode to the growing grains.δ13C of the bracts showed a marked increase at the time point of anthesis,implying a transformation from sink to source organ.N topdressing lowered δ13C of the leaf,sheath,internode,bracts and grain.3.The relative contribution of different source organs to rice grain filling was estimated.By comparing the δ13C in its natural abundance in the water-soluble fractions of the sheath,internode,and bracts with the δ13C in mature grains,the relative contribution of these organs to grain filling was assessed.The results showed that only the δ13C of the internode and bracts at 10 DAA and 20 DAA agreed with the assumption of the estimation method used in the current study.Thus we calculated the relative contribution of internode,and bracts,with leaf as the reference.The internode accounted for as high as 32.64%and 42.56%at 10 DAA and 20 DAA,respectively.Meanwhile,bracts(52.67%)presented a larger contribution than the internode(37.6%),with superior bracts(54.56%)being higher than inferior bracts(50.77%).In addition,N topdressing reduced the contribution of the internode and bracts.It should be pointed out that the value of bracts was lower in 2019 than that in 2018,and this explain the difference in water status as indicated by δ13CG between the two experimental years.Combined with the above results,this study revealed the temporal and spatial variations of carbon isotope signature of different organs in rice,and analyzed the relative contribution of different source organs to grain filling.The study found that the contributions of bracts and internode to grain filling were 52.67%and 37.6%.Combining the spatial and temporal characteristics of plant dry matter accumulation a nd distribution in the reproductive growth period,this study revealed the key role of bracts and internode in the formation of rice yield,clearly proved the actual significance of non-foliar organs of the internode and bracts for rice yield formation,thus extending our basic knowledge of source and sink relations. |
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