Effect Of Low Light At Jointing Stage On Lodging Resistance Of Japonica Rice Stem And Its Physiological Mechanism

Rice is a major food around the world,and the high and stable production of rice is of great significance to Chinese food security.However,the rice stem lodging occurs from time to time in production,which has become an important limiting factor for the improvement of rice yield and quality.In recent years,due to climate change,extreme weather has increased,which may be rainy for a long time,during the jointing stage.In addition,rice breeding and cultivation mainly focus on large panicle varieties,which leads to an increase in the risk of rice lodging.Therefore,it has been great importance for us to figure out the mechanism of low light at jointing stage of the lodging resistance of japonica rice.In this experiment,two japonica rice varieties with different lodging abilities(Ningjing8,lodging-resistant type;W3668,lodging-sensitive type)were used as materials.The joints were set with normal light and low treatments(black shading nets were used for shading,and the shading degree was 70%).The physiological mechanism was studied by comparative analysis of yield and its constituent factors,field apparent lodging rate,mechanical properties,morphological characteristics,endogenous hormone content,carbohydrate content in the second internode,and the expression dynamics of key genes for structural carbohydrate synthesis.In addition,RNA-seq analysis was performed on the clum of Ningjing 8 treated with normal light and weak light at the jointing stage.The main findings are listed as follows:1.Low light at the jointing stage affects the mechanical characteristics of japonica rice culmLow light at the jointing stage significantly reduced the yield,spikelets per panicles and and 1000-grain weight.The yield of Ningjing 8 was higher than that of W3668,and its lodging resistance was higher than that of W3668.The low light treatment at the jointing stage in 2020 caused lodging at the mature stage of W3668.None of the rest of the field fell down.The low light at the jointing stage affected the mechanical characteristics of the clum,leading to an increase in the lodging index of japonica rice.The lodging indexes of W3668 and Ningjing 8 at the jointing stage of low light treatment were 105% and 76.4%,respectively,which increased 19.2% and 27.9% compared with normal light.Among them,the bending moment and the breading strength significantly reduced,and the reduction coefficient of the breading strength is higher than the bending moment.In addition,the low light at the jointing stage caused the length of the long and short axis of the cross section of the stem decreasing,which leads to a decrease in the section modulus and bending stress.The flexural resistance decreased significantly during the two years.Varieties and treatments had an impact on plant height.The plant height of Ningjing 8 under the jointing stage with low light treatment increased by 8.7% compared with normal light,while the plant height of W3668 at the jointing stage with low light treatment decreased by 4.9%.The mechanical characteristics of the stalk indicate that increasing the mechanical strength of the stalk is an effective way to coordinate high yield and lodging resistance.The low light at the jointing stage mainly reduce the mechanical strength of the culm and increases the risk of lodging by reducing the bending moment of the stalk.2.Low light at the jointing stage affects the morphological and structural characteristics of japonica rice culmThe clum morphology and structure changed under the treatment of low light at the jointing stage.The length of the second internode of W3668 and Ningjing 8 both increased after 10 days.The internode elongation of the second internode is mainly concentrated in the early stage.The internode length is basically fixed at 25 days after jointing.The internode length of W3668 jointing stage under low light treatment only increased by 0.5% compared with normal light at 45 days after jointing.However,Ningjing 8 The internode length increased by 32.8% under low light treatment.Compared with normal light,the bacal clum thickness decreased significantly under the treatment of low light at the jointing stage,but the wall thickness did not decrease significantly.The stem thickness and wall thickness of Ningjing 8 is better than W3668.It is found that the low light at the jointing stage significantly reduces the thickness of the thick-walled tissues,resulting in loose cell arrangement,which reduces the supporting ability of the stalk.The thick-walled composite cells of Ningjing 8 stalk were arranged more closely under normal light,which increases the mechanical strength of the stalk.3.Low light at the jointing stage affects the material content of japonica rice stalksThe dry matter accumulation of the second internode significantly decreased after 25 days under low light,reducing the stalk plumpness.The dry matter accumulation of Ningjing8 was higher than that of W3668,but the lodging index was lower.Because the per leanth of sheath increase,which enhanced the support for the stalk.After returning to normal light,the assimilates in the stalk were transferred to the grains,and the base dry matter weight decreased after heading.The base dry matter weight under low light treatment was higher than normal light.The content of soluble sugar and starch decreased under low light treatment.After returning to normal light,the soluble sugar content under low light treatment was higher than normal light after heading,but the starch content was lower than normal light.As a result,the non-structural carbohydrate content was decreased.Structural carbohydrate is a key factor that affects the mechanical strength of japonica rice.The lignin and cellulose content of cell wall structural substances decreases under low light at the jointing stage.What’s more,the mechanical strength of the stalk decreases.After the low light treatment removing,the lignin and cellulose content can not return to normal light level.4.Low light at the jointing stage affects the gene expression of culmRNA-seq analysis of the stems showed that low light treatment resulted in the downregulation of the expression of lignin,cellulose and starch synthesis genes.8914 different genes were identified from 19,366 genes through RNA-seq analysis.The research of KEGG pathway enrichment show that the down-regulation carbohydrate synthesis is the main reason that affects the mechanical strength of clum.Low light treatment inhibited the expression of the key enzyme(phenylalanine ammonia-lyase)gene expression of lignin synthesis.The activity of most laccase and peroxidase genes involved in the polymerization of lignin monomer decreased at 5 days.In the metabolism of starch and sucrose,low light treatment inhibited the expression of putative 1,4-alpha-glucan branching enzyme and cellulose synthase,resulting in a decrease in starch and cellulose content.Low light affected the expression of downstream cellulose synthase by down-regulating the upstream MYB transcription factor.Fluorescence quantitative analysis showed that low light treatment at the jointing stage down-regulated genes related to rice cellulose and lignin synthesis and increased the risk of lodging.In summary,after 15 days of low light treatment at the jointing stage,the length of the second internode increased,and the content of lignin and cellulose decreased,and the expression of lignin-related genes decreased,and the thickness of the thick-walled tissue under the epidermis of the internode decreased,which caused the strength of the clum decreasing.The lodging resistance increased.