Moderate Water-deficit Treatment Enhances Grain Filling Of Inferior Spikelets By Regulating Stem Translocation Of Non-structural Carbohydrates In Rice (Oryza Sativa. L)

Water is one of the key factors that affect rice yield.Moderate water-deficit treatment can not only save water resources,but also improve yield of rice.During the grain filling,post-anthesis photosynthetic products in functional leaves and non-structural carbohydrates accumulated in stems before heading,which are the main resources for grain filling,are transported to grains in the form of sucrose through vascular bundles.With the development and release of super hybrid rice,the yield of rice has been greatly increased.However,the poor grain filling limits grain yield.It is important for high yield potential and food safety to reveal the mechanisms of assimilates translocation to inferior spikelets.Pot experiments were carried out in the experimental field of Huazhong Agricultural University,Hubei,Wuhan,China,during May to October in 2016 and 2017.The hybrid varieties,Liangyoupeijiu and Shanyou 63,with significant difference in grain filling percentage,were used,Treatments were consisted of two irrigation regimes including moderate water deficit(MD)treatment and well-watered treatment(WW),and MD was applied from 6 d after heading to maturity.In the MD regime,irrigated pots were not irrigated until the soil water potential reached –25 kiloPascal(kPa)at 15–20 cm depth below the soil,and were then irrigated with 2–3 cm water depth in pot.The WW regime involved the mainenance of a continuous flood with 2–3 cm water depth in pot until one week before harvest as recommended farming practices.The objectives were to investigate the effects of moderate water-deficit treatment on translocation of stem nonstructural carbohydrates and its relationship with grain filling of inferior spikelets.The main results are as follows:(1)Moderate water-deficit treatment increased grain filling percentage,1000-grain weight,harvest index and yield,compared with well water treatment.(2)The different treatments had no impact on grain filling of superior spikelets.Average grain filling rate and grain weight of inferior spikelets was significantly enhanced,and active grain filling period was significantly shortened.High grain filling rate accelerated grain filling process of inferior spikelets under moderate water-deficit treatment when compared with well water treatment.(3)The study observed no significant differences in SPAD,photosynthetic rate,and stomatal conductance of leaves between moderate water-deficit treatment and well water ed treatment;however,moderate water-deficit treatment reduced green leaf area.On the other hand,moderate water-deficit treatment had no effect on the numbers and cross section areas of large and small vascular bundles,however,significantly increased the transport rate in large and small vascular bundles via CFDA method.(4)Moderate water-deficit treatment enhanced translocation of stem non-structural carbohydrates to grains.The apparent transferred mass,apparent rate of transferred nonstructural carbohydrates,and apparent contribution of transferred non-structural carbohydrates to grain yield were higher under moderate water-deficit treatment when compared with well water treatment.Moderate water-deficit treatment resulted in higher activities of ?-amylase,?-amylase and sucrose-phosphate synthase in stems,which promoted starch hydrolysis and sucrose synthesis.Therefore,moderate water-deficit treatment accelerated the redistribution of assimilates to grains.(5)The different treatments had no impact on cell wall invertase activity,relative expressions of genes and proteins of cell wall invertase and sucrose transporters in superior spikelets.However,moderate water-deficit treatment significantly enhanced cell wall invertase activity,relative expressions of genes and proteins of cell wall invertase and sucrose transporters in inferior spikelets when compared with well water treatment.The MD significantly increased SE-CC,SE-PC and CC-PC plasmodesmal density of inferior caryopses.The results indicated moderage water-deficit treatment enhanced the phloem unloading of assimilates in inferior spikelets.(6)The study observed the different grain filling characteristics in superior and inferior spikelets of Liangyoupeijiu and Shanyou 63.Superior spikelets started grain filling rapidly and had high grain filling rate at the early stage of grain filling,while inferior spikelets started filling at the middle stage and had lower grain filling rate.Superior spikelets had higher activities of adenosine diphosphate glucose pyrophosphorylase and sucrose synthase in the direction of sucrose cleavage during the early grain filling stage(from 6 to 12 days)than inferior spikelets,and reached the maxium activities in the 12 th day;however,inferior spikelets reached the maxium activities untill the 18 th day.On the other hand,activities of adenosine diphosphate glucose pyrophosphorylase and sucrose synthase in the direction of sucrose cleavage were significantly correlated with grain filling rate,respectively.Together,the results showed that the rapid start of grain filling in superior spikelets was tightly associated with rapid sucrose hydrolysis and starch synthesis.In the study,moderate water-deficit treatment increased activities of adenosine diphosphate glucose pyrophosphorylase and sucrose synthase in the direction of sucrose cleavage in inferior spikelets,and further enhanced grain filling.Summarily,MD enhanced the grain filling in inferior spikelets.The MD had no influence on leaf photosynthetic rate,grain weight and grain filling percentage of superior spikelets,while significantly reduced green leaf area,suggesting that the treatment did not increase assimilation supply for grain filling in inferior spikelets,and the MD had no effect on the numbers and cross section areas of large and small vascular bundles.However,the MD significantly increased the transport rate in phloem via CFDA method,the stem NSC translocation and phloem unloading in inferior spikelets,enhanced grain weight and grain filling percentage in inferior spikelets,which contributes to grain yield.Besides,?-amylase,?-amylase,SPS activities,transport rate in large and small vascular bundles,phloem unloading of assimilates in inferior spikelets,SSc,AGP with NSC,starch remobilization in stems and grain filling of inferior spikelets.These results suggested that improved grain filling of inferior spikelets may be attributed to the enhanced translocation of NSC and phloem unloading in inferior spikelets.