Mechanism And Regulation Of Spikelet Differentiation And Degeneration In Rice Varieties With Different Panicle Sizes

High spikelet numbers per panicle and larger panicle sizes are essential characteristics in modern high-yielding rice varieties.However,the physiological mechanism of spikelet formation in rice varieties with large panicle sizes is still unclear.It is of great significance for high-yielding rice cultivation and breeding to clarify the characteristics and physiological mechanisms of spikelet differentiation and degeneration in rice varieties with different panicle sizes.In this study,rice varieties with different panicle sizes were used as materials to analyze the characteristics of spikelet differentiation and degeneration,and their relationship with grain yield was investigated.The effects of panicle nitrogen fertilizer application rates,grain fertilizer application rates,alternate wetting and moderate soil drying(AWMD),and straw returning and nitrogen application ratios on spikelet differentiation,degeneration,and grain yield in rice were also studied.The main results are as follows:1.Regardless of the cultivation conditions,which included manual field transplanting,machine transplanting,and pot cultivation,the number of spikelets per panicle had a significant or extremely significant positive correlation with grain yield.The direct path coefficient of the spikelet number per panicle to rice yield was the largest,indicating that increasing the spikelet number per panicle played a crucial role in improving rice yield.The large number of differentiated spikelets and the low rate of degeneration of secondary spikelets were the main reasons for the large number of total spikelets and high yield in rice varieties with a large panicle size.2.Shoot dry weight,leaf area,plant nitrogen(N)uptake,non-structural carbohydrate(NSC)accumulation,and leaf photosynthetic rate in the main stem from the branch differentiation stage(BDS)to the spikelet differentiation stage(SDS)were positively correlated with the number of differentiated secondary branches and secondary spikelets per panicle(r=0.796-0.997**).Additionally,these factors were significantly negatively correlated with the expression of OsCKX2 in young panicles from BDS to SDS.On the other hand,plant N uptake,NSC accumulation in stems,leaf photosynthetic rate,and the expression of OsCBSDUF1 at pollen mother cell meiosis(PMC)were negatively correlated with the number of degenerated secondary branches and secondary spikelets per panicle(r=-0.979**--0.741).3.High zeatin(Z)and zeatin riboside(ZR)contents in young panicles and roots during BDS-SDS,as well as large amounts of NSC accumulation in stems and root activity(including root dry weight,root oxidation activity,root bleeding rate,root total absorbing surface area,and root active absorbing surface area)from PMC to pollen filling(PF),were vital physiological factors in the formation of large panicles in rice.Improvements in the above indexes during the panicle differentiation.stage were likely to contribute to the formation of large panicles and the increase in rice yield.4.Under the same amount of base-tiller fertilizer(162 kg N ha-1),the spikelet number per panicle gradually increased while the seed-setting rate and 1000-grain weight gradually decreased with increasing panicle nitrogen fertilizer application rate(PNR)in the range of 0-216 kg ha-1.The higher the PNR,the more obvious the decrease in seed-setting rate and 1000-grain weight.The appropriate PNR varies with panicle size,and rice varieties with small panicle sizes had a greater yield-increasing effect from PNR.The application of panicle nitrogen fertilizer greatly increased the number of differentiated and surviving secondary spikelets in rice varieties with small panicle size,which was the main reason for the higher yield-increasing effect compared to rice varieties with medium and large panicle sizes.The appropriate PNR was beneficial for maintaining a high effective leaf area ratio,grain-leaf ratio,NSC translocation amount,sugar-spikelet ratio,root oxidation activity,activity root of spikelet,and Z+ZR content in grains and roots after heading,under the premise of a higher total number of spikelets.This helped to maintain a high seed-setting rate and grain weight,ultimately increasing the grain yield.5.Under the condition of the same amount of base fertilizer,tiller fertilizer,and spikelet-promoting fertilizer,in the grain fertilizer application rate(GFAR)range of 0-216 kg ha-1,the seed-setting rate,1000-grain weight,and grain yield increased first and then decreased with the increase of GFAR.The appropriate GFAR varies with panicle sizes,and rice varieties with small panicle size had greater yield-increasing effects of GFAR.Post-anthesis dry matter accumulation and apparent transferred mass of NSC in stems from heading to maturity were significantly positively correlated with rice yield.The contribution of dry matter accumulation from heading to maturity to grain yield,NSC remobilization,and NSC contribution to grain yield of large-panicle varieties were 74.4-82.8%,48.3-61.2%,and 11.9-13.4%,respectively,which were significantly higher than those of small-panicle and medium-panicle varieties.Under the appropriate GFAR,root oxidation activity,Z+ZR contents in roots and leaves,and leaf photosynthetic rate were increased at 0-20 days after anthesis,which in turn increased post-anthesis dry matter accumulation and rice yield.The excessive GFAR will inhibit the α-amylase and β-amylase activity at 30-50 days after anthesis and reduce the apparent transferred mass of NSC,hence not being conducive to increased grain yield.6.Compared to the continuously flooded regime,the grain yields of Huaidao 5 and Yongyou 2640 were significantly higher under the AWMD regime during the panicle differentiation stage,with an increase of 11.5%and 7.3%,respectively.The main reason for the large increase in spikelet number per panicle and grain yield in small-panicle varieties under the AWMD regime was the high number of differentiated and low number of degenerated secondary branches and secondary spikelets per panicle.Shoot dry weight,leaf area,plant nitrogen uptake,non-structural carbohydrate accumulation,and leaf photosynthetic rate in the main stem during BDS-PMC increased significantly,which was a major physiological mechanism for the high number of differentiated and low number of degenerated secondary branches and secondary spikelets in small-panicle varieties under the AWMD regime.7.Increasing the proportion of base fertilizer application under straw returning was beneficial for increasing the spikelet number per panicle and grain yield,with a greater increase observed in small-panicle varieties.Increasing the proportion of base fertilizer application under straw returning significantly increased the differentiated number of secondary branches and secondary spikelets in the middle and lower parts of the rice panicle.At the same time,the degenerated number and degeneration rate of secondary branches and secondary spikelets in the lower part of the rice panicle were also increased.However,the increase in differentiated number was significantly higher than that of degenerated number and degeneration rate.This was the main reason for the increase in spikelet number per panicle and grain yield in small-panicle varieties.The aforementioned results have elucidated the characteristics of spikelet differentiation and degeneration in high-yield and large-panicle rice varieties.The physiological and molecular mechanisms governing rice spikelet differentiation and degeneration were explored from the perspectives of root and shoot morphology and physiology.Furthermore,cultivation measures were proposed to promote the formation of large panicles and increase rice yield through water and nutrient management,including panicle nitrogen fertilizer application rate,grain fertilizer application rate,alternate wetting and drying,straw returning,and nitrogen application ratio.These findings hold significant theoretical and practical value for high-yield and high-efficiency rice cultivation and variety screening.