Heterogeneity And Regulations Of Rice Tillers Yield By Nitrogen Nutrition

Rice(Oryza sativa L.)is one of the major food crops in China,which plays a very important role in food security.Rice yield is considered to be positively related with panicle number per unit land area.Therefore,increasing the effective tiller number is an efficient approach to enhance the rice yield.Application of nitrogen(N)fertilizersignificantly promotes the initiation and development of tiller bud,and increasing the rice yields.However,due to the difference of node and emergence time,there is significant heterogeneity in tiller agronomic characters.Typically,late emerging tillers contribute less to grain yield formation.It is of great significance to further improve the rice yield by demonstrating and releasing the limiting factors of late emerging tillers.In the present study,field experiments were conducted to study the effects of N application on the yield and yield components in different rice varieties.The relationship between N nutrition and heterogeneity in rice tillers yield was clarified.Revealing the causes of low production of inferior tillers and the physiological mechanisms of exogenous hormones might regulate the growth traits of inferior tillers grown under different N application conditions.The main results are listed as follows:(1)Application of N significantly increased the productive tillers(panicles),further increasing rice yields.The relationship between nitrogen rate and the number of spikes per unit area conformed to the quadratic function(y=-0.000x2+0.444x+164.2).The panicles per unit area were increased significantly with increasing N rates under low N fertilizer supplied;after N fertilizer reached a certain level,the number of panicles increased slowly with increasing N rates.Compared with no-N treatment,the panicle number per unit area were increased by an average of 30.2% after N application,and the yield increased by an average of 31.3%。When the amount of N fertilizer was 0–165 kg N/ha,the panicle number per unit area of rice increased by an average of 25.0%;under165-247.5 kg N/ha,the number of panicles increased slowly and no significant differences were observed among these excess nitrogen conditions.The highest growth rate of panicle number was 34.5%,but the lowest was only 24.6%,which indicated that the tiller ability of different rice varieties was significantly different from that of nitrogen fertilizer.(2)All tillers within the same N treatments were classified as three types(superior,medium and inferior)based on their production.With the increase of N application rate,the tiller productivity of single superior tiller was improved,but the quantitative proportion of the population was decreased;tiller productivity of single inferior tiller isreduced,but their quantitative proportion was increased.Under no-N treatment,superior tillers with the highest quantitative proportion and yield contribution,accounting for 40%,55% of the population,respectively;inferior tillers with the lowest quantitative proportion and yield contribution,accounting for 20%,10% of the population.Under excessive nitrogen application,the proportion of superior tillers decreased to ~20%,but their yield contribution accounted for 40%.With the increase of nitrogen application rate,the number of grains per panicle,grain filling percentage and grain weight of the superior tillers were increased or unchanged,while the panicle traits of inferior tillers were gradually deteriorated.(3)Tiller heterogeneity was presented as Gini coefficient(G),which was enhanced with increasing N levels.Under no-N levels,the G value of the two years was 0.125;at excessive N application levels,the mean of two-year Gini coefficient was 0.250,which was two times greater than the former.With the increase in tiller order,the production of productive tillers were decreased.The early emerging tillers possessed the characteristics of long growth period,strong intercepted photosynthetic active radiation,and high C/N ratio in straw.From the N0 to N165,the increase grain yield of superior tiller was the main reason for the increase rice yields;from the N165 to N247.5,the increase of the number of inferior tillers was the key factor to improve the rice yields.When the N application rate beyond 165 kg N/ha,the first four superior tillers were maintained at about 2.5 g,and without increasing the genotype of rice,the increase of total yield needed to depend on yield potential of inferior tillers.(4)Premature senescence,unfavorable light energy and N allocation limited the yield potential of inferior tillers.A rapid malondialdehyde accumulation and delayed emergence determined the short growth duration of inferior tillers,which further caused higher retention of N in straw and less was transported to the grains.Only a small amount of light energy was used for the photochemical reaction of light system II(PSII),resulting in insufficient synthesis of assimilation products.Excess light would be distributed to the non-regulated energy dissipation,which is easily damaged by the photosynthetic system.The low sucrose synthase activity and the number of grains per panicle made the capacity and activity of the inferior tillers far lower than the main stems and superior tillers.Which cannot attract enough assimilation products,and it is difficult to obtain high yields.(5)Application of exogenous hormones improved the growth traits of inferior tiller,and further regulated rice yields.Application of the appropriate nitrogen rates,and addition of exogenous cytokinin(6-benzyladenine;6-BA)promoted the increase innumber of grains per panicle and the grain yields.The results of two years field experiments indicated that the highest increases of inferior tillers were 34.4% and 17.2%,respectively.Under no-N or excessive N levels,application of 6-BA promoted the spike differentiation while reduce the grain filling percentage or grain weight,thus the increase effect of population yield was not obvious.Activities of peroxidase(POD),catalase(CAT)and ascorbate peroxidase(APX)obviously strengthened by the application of BA.Application of 6-BA also significantly inhibited the accumulation of malondialdehyde(MDA)and hydrogen peroxide(H2O2).The addition of exogenous 6-BA improved the distribution of light energy in the flag leaf,which reduced the non-regulated energy dissipation ratio and decreasing the risk of oxygen free radical.6-BA treatments stimulated more light energy into photochemical reaction,which showed the high net photosynthetic rate.Exogenous 6-BA by delaying the premature senescence of inferior tillers,increasing the sink capacity,promote the distribution of light energy,then improving the growth characteristics of inferior tillers,which conducive to increase rice yields.Low concentration of endogenous GA1+3 translated to the stunted growth of inferior tillers,which ultimately had a reduced capacity to harvest sunlight.Foliar GA3 spray enhanced the intercepted photosynthetically active radiation(IPAR)and Su Sase activity in inferior tillers considerably;however,dry matter and N accumulation provided adequate resources to grains only when the tillers obtained sufficient N.Similar to the6-BA physiological function,GA3-treated inferior tillers received high-intensity light,and additional energy was allocated to the PSII.A good synergistic effect between N and GA3 was found in our study,the increase of different nitrogen application rates was4.9%–36.1% and 8.6%–28.6%,respectively.In summary,exogenous cytokinin and gibberellin alleviated the premature senescence and light constraints in inferior tillers,respectively.Whilst,improving the light distribution and stimulating the spike differentiation to increase the sink capacity,which significantly improved the panicle traits of inferior tillers.Furthemore,application of exogenous hormones need to be in the appropriate nitrogen supply conditions to play the best physiological function.