Effects Of Nitrogen Application On Lodging Resistance Of Japonica Rice And Its Physiological Mechanism

Lodging resistance of crops is one of important agricultural quantitative traits,whichis related to the cooperation of genetics,environmental factors and management practics.The improvement of lodging resistance of crops can increasing grain yield largely.It is wellknown that panicle nitrogen fertilizer can increase spikelet unmber and thus,incrase grainyield.However,excessive nitrogen(N)application rates are the cause of increase inlodging risks due to weaker solar radiation in population and stem stiffness.To explore thereason for the changes of lodging resistance by increased N fertilizer,two experiments wereconducted in Baolin Farm,Danyang,Jiangsu Province,China,during rice growing seasonfrom late May to late October in 2011-2014.The effects of N application on lodgingresistance of japonica rice and its related physiological mechanism were studied in thispaper.A greater understanding of that information would provide a theoretical basis toincrease lodging resistance combinations with improved grain yield.1.In 2013-2014,Field experiments were conducted by using two japonica riceWuyunjing23(lodging-resistance variety)and W3668(lodging-susceptible variety)withthree panicle N fertilizer rates(0,135 and 270 kg N ha-1)in in Baolin Farm,Danyang,Jiangsu Province,China(32°00?N,119°32? E,7 m altitude).The aims of this researchwere to investigate effect of N fertilizer on:(1)the reason for difference of lodgingresistance in japonica rice population and its related physical mechanism;(2)morphological and anatomy traits,stem components response to panicle N and its relationto lodging resistance;(3)secondary cell wall synthesis in japonica rice as affected bypanicle N fertilizer and its relation to culm strength.2.In 2011-2012,Field experiments were conducted by using japonica riceWuyunjing23 with three N fertilizer rates(0,150 and 300 kg N ha-1)and two ratios ofbasal-tiller to panicle fertilizer(8:2 and 5:5)in Baolin Farm,Danyang,Jiangsu Province,China(32°00?N,119°32?E,7 m altitude).The objectives of this research were to determine the morphology and physiological mechanism of lodging resistance and itsresponse to N application management.The main results were as following:1.Lodging resistance of japonica rice under different panicle N rates and its relatedphysical mechanismGrain yield of Wuyunjing23 was significant higher than that of W3668.However,lodging rate and lodging index of Wuyunjing23 was lowered rapidly comparing withW3668 which owed to larger breaking strength.With increasing panicle N rates,grain yieldof Wuyunjing23 increased by 17.4%owing to higher spikelet per panicle and lower lodgingrate.As for W3668,lodging occurred earlier and heavier which resulted in poor spikeletsfertility and lower 1000-weight,thus,grain yield decreased.With higher panicle N,breaking strength decreased significantly,and lodging rate and lodging index increasedrapidly,whereas bending moment by whole plant varied quite low.In addition,panicle Nhad increased section modulus slightly,but reduced bending stress largely,especially forW3668.Path analysis showed that the size of breaking strength was the key factordetermining lodging index,and bending stress as important first-order trait that influencedbreaking strength.These results suggested that enhancing breaking strength of japonica riceplants could be a good approach increasing lodging resistance and grain yield.Panicle Nhad reduced breaking strength primarily resulting from lower bending stress,thus,lodgingindex increased.2.Morphology and anatomy traits,physiological characteristics of japonica riceresponse to panicle N and its relation to lodging resistance(1)The longer elongated internodes of the basal intemodes were responsible for higherplant height and lodging index,and source of variation as follows:Years>Varieties>N.With increasing panicle N application,culm diameter increased slightly,outer and innerdiameter of the minor axis(a1 and a2)and b2 were increased with significant difference.Inaddition,number of larger and small vascular bundles varied little,but area of larger andsmall vascular bundles and thickness of mechanical tissue decreased remarkably under highN condition.Correlation analysis showed that breaking strength was significantly andpositively correlated with the thickness of mechanical tissue and area of larger and smallvascular bundles,but significantly and negatively correlated with inner diameter of themajor axis(b2).However,the lodging index showed opposite relationship with these traits.After histochemical staining,the vascular bundles structure of Wuyunjing23 varied little,but lignifications of vascular bundles sheaths in W3668 reduced rapidly under high N,which resulted in looser structure.An improvement in the lodging resistance of japonica rice plants could be achieved by suitable plant height with reducing length of lower internodes,decreasing inner culm diameter and thicker mechanical tissue.The increase of panicle N fertilizer increased lodging risks primarily by reducing lignifications of vascular bundle sheath,area of vascular bundles and thickness of mechanical tissue at the lower internodes.(2)Compared with W3668,Biomass production of Wuyunjing23 increased significantly,but lodging index reduced rapidly.The culm and leaf sheath dry weight per cm of Wuyunjing23 was significantly higher than that of W3668,and stronger leaf sheath reinforced breaking strength of basal internodes largely;in addition,non-structural carbohydrate(NSC)of Wuyunjing23 was lowered significantly,but structural carbohydrate(e.g.cellulose and lignin)increased highly and thus,bending stress and breaking strength enhanced.With increasing panicle N fertilizer rates,stem dry weight per plant at heading stage and dry weight per cm of basal internodes culm and leaf sheath reduced significantly,thus,stem stiffness lowered.In addition,high N had decreased structural carbohydrate contents rapidly,the cellulose content varied little,but lignin content reduced largely.The lignin contents in basal culm and leaf sheath of Wuyunjing23 and W3668 were reduced by 32.3%,33,1%and 41.3%,38.9%,respectively.Correlation analysis showed that cellulose and lignin contents in basal culm and leaf sheath were significantly and positively correlated with bending stress but negatively correlated with lodging index.These results suggested that dry weight per cm of basal culm and leaf sheath and structural carbohydrate play a vital role for improving stem strength in japonica rice.Panicle N rate decreased lodging resistance primarily due to poor stem stiffness,by reducing structural carbohydrate content,especially lignin contents in lower internode stem.(3)With increasing panicle N fertilizer,section modulus of culms in japonica rice increased slightly,but bending stress reduced significantly which contributed to lower breaking strength.With higher panicle N rates,secondary cell wall of sclerenchyma cells in culms were reduced.Comparing cell wall components,cellulose content reduced slightly but with higher accumulate rate,the lignin content decreased significantly and with lower accumulate rate.After histochemical staining,compared with low N treatments,high N application did not alter the pattern of cellulose deposition in the sclerenchyma cells of secondary cell wall and vascular bundle region,but decreased lignin deposition rapidly in those cell walls,which caused the defect synthesis of secondary cell walls,practically for W3668.In addition,Expression of genes involved in lignin biosynthesis,OsPAL,OsCoMT,Os4CL3,OsCCR,OsCAD2 and OsCAD7 were down-regulated under higher nitrogen condition during culm growth early stage whereas no differences were obtained during culm growth late stage.Expression of genes involved in cellulose biosynthesis,OsCesA4 and OsCesA7 showed down-regulation,but OsCesA9 was increased.These results suggest that genes involved in lignin biosynthesis are down-regulated by high N,which cause defect lignin deposition in secondary cell walls and collapsed mechanical tissue structure and,subsequently,make these internodes have weak mechanical strength and poor lodging resistance.3.Approach of N application management for improving lodging resistance in japonica rice and its related physiological mechanismWith increasing N fertilizer rates,lodging index of Wuyunjing23 increased significantly,and increase panicle fertilizer ratio had reduced lodging index with no significant difference.The size of breaking strength was the key factor determining lodging index.With higher N rates,plant height and gravity center height increased rapidly,culm diameter and culm wall thickness decreased slightly,and dry weight per cm of basal culm and leaf sheath,non-structural carbohydrate(NSC)cellulose and lignin contents reduced significantly.In addition,culm diameter,culm wall thickness and dry weight per cm of culm and leaf sheath increased with higher panicle fertilizer ratio,especially under middle N rate.Spikelets per panicle also increased and thus,elevated grain yield.It is a good ways to increase lodging resistance and grain yield in japonica rice by decreasing total N application rates and increasing N ratio at panicle initiation stage.