The Changes In Accumulation And Translocation Of Stem Non-structural Carbohydrates In Rice Cultivars During The Genetic Improvement And Underlying Physiological Basis

The non-structural carbohydrates(NSC)accumulated in culms and leaf sheaths before heading stage is crucial to yield formation in rice,which is one of the main sources of carbohydrates for grain filling.NSC is often refered to soluble sugars and starch.Rice grain yield was gradually increasing with the genetic improvement of cultivars.However,characteristics of NSC accumulation and translocation and underlying physiological mechanism remain unclear.In this study,twenty-one representative mid-season indica rice cultivars were used,which among cultivars were released and widely cultivated in China over the past nearly 80 years.There were 5,4,6,and 6 rice cultivars included in the period from 1940s-1950 s,1960s-1970 s,1980s-1990 s,and 2000s-2010 s,respectively.According to the breeding ages and the trend of grain yield,the selected cultivars released in different ages were divided into early cultivars and modern cultivars.The objectives of this study were to(1)investigate the characteristics of accumulation and translocation of stem NSC and(2)elucidate the physiological mechanism for the changes in accumulation and translocation of stem NSC.The results of this research may provide insight for the high-yielding and high-efficiency production and genetic improvement.The experiments were carried out in farmer’s fields in Dajin town,Wuxue County,Hubei Province,in 2015 and 2016.The main results of this research are as follows:1.The rice grains yield significantly increased during the genetic improvement,which was mainly due to the increases in total spikelets per m2 and spikelets per panicle.From 1940s-1950 s to 1960s-1970 s,the increase in grains yield was mainly attributed to the increase in harvest index;From 1960s-1970 s to 2000s-2010 s,the simultaneous increase in biomass and harvest index,especially the increase in biomass,resulted in the increase in grain yield.2.The total accumulation of stem NSC at heading stage(TMNSC)was gradually increased with the genetic improvement of rice cultivars.Compared with the early cultivars,the increased TMNSC in modern cultivars was resulted mainly to improve the stem NSC concentration and biomass at heading stage.The increase in TMNSC for early cultivars was resulted mainly to improve the stem NSC concentration at heading stage;however,the increase in TMNSC for modern cultivars was mainly due to the increased biomass.3.Compared with the early cultivars,the increased TMNSC in modern cultivars was due to the decreased activities of acid invertase(INV)and sucrose synthase in the direction of sucrose cleavage(SSc),and the significant increase in activities of sucrose synthase in the synthesis direction(SSs)and sucrose-phosphate synthase(SPS)in leaves.Moreover,the activity of stem adenosine diphosphate glucose pyrophosphorylase(AGPase)in modern cultivars was also significantly higher than that in early cultivars.4.the varieties that were released from 1980s-1990 s had the maximum values of the apparent transferred mass of NSC(ATMNSC),the apparent rate of transferred NSC(ARNSC),and apparent contribution of transferred NSC for grain yield(ACNSC),followed by the varieties that from 2000s-2010 s,1960s-1970 s and 1940s-1950 s.The results showed that ATMNSC,ARNSC,ACNSC in modern cultivars were higher than those in early cultivars.5.Compared with the early cultivars,modern cultivars had greater mass of NSC transferred to grains,which may be attributed to:(1)modern cultivars had higher TMNSC at heading stage,and higher activities of stem SPS during grain filling;(2)modern cultivars had more large and small vascular bundles of peduncle than early cultivars;(3)both INV and AGPase activities in grains and sink capacity were higher in modern cultivars than early cultivars.Therefore,the increased transfer of stem NSC in modern cultivars was due to the improved source-sink-flow characteristics during the genetic improvement of rice cultivars.6.Compared with these under normal nitrogen application(150 kg N ha-1),plants showed the significant increase under TMNSC under low nitrogen(0 kg N ha-1)application,which was attributed mainly to the increase of stem NSC concentration at heading stage.The increasae of stem NSC concentration under low nitrogen application was due to the decreased INV and SSc activities,and the significant increase in activities of SSs and SPS in leaves.7.Compared with normal nitrogen application,there was greater mass of NSC transferred to grains under low nitrogen applicaiton,which may be attributed to:(1)there were higher in TMNSC at heading stage,and higher activities of SPS,α-amylase and β-amylase in stem during grain filling;(2)both SSc and AGPase activities in grains were higher under low nitrogen applicaiton than normal nitrogen applicaiton.