Effects Of Sowing Date And Nitrogen Application Rate On Growth And Nitrogen Use Characteristics Of Film-free Cotton And Optimization Combination Screening

In order to clarify the effects of sowing date and nitrogen application rate on the growth physiology,soil nitrogen dynamics and utilization,yield composition and quality of film-free cotton,‘Zhong 619’was used as the test material,and the two factor D-saturation optimal regression design of sowing date and nitrogen application rate was adopted.The sowing date range was set from April 17 to May 6.According to the code value,it was April 17（expressed by S1）,April 25（expressed by S2）and April 30（represented by S3）and May 6（represented by S4）;The range of pure nitrogen application is set to 75～525 kg/hm².Calculated according to the code value,they are N1:75 kg/hm²,N2:270 kg/hm²,N3:389 kg/hm²,N4:525kg/hm²respectively.During application,they are converted into urea according to the effective components.The effects of different sowing dates and nitrogen application rates on agronomic characters,dry matter accumulation characteristics,photosynthetic physiological indexes,spatial distribution of cotton bolls,nutrient absorption and yield composition of film-free cotton were studied.The conclusions are as follows:（1）The plant height of film-free cotton increased rapidly from initial bud to full flower,and the final plant height decreased with the delay of sowing date and increased with the increase of nitrogen application.The stem diameter,the number of leaves,the number of internodes,the number of fruit branches and the node of the first fruit branch decreased with the delay of sowing date,and the height of the first fruit branch increased with the delay of sowing date;The dry matter accumulation per plant of film-free cotton showed a slow fast slow"S"curve.With the delay of sowing date and the decrease of nitrogen application rate,the dry matter accumulation of each treatment gradually decreased,and the dry matter accumulation of reproductive organs and its proportion in the total dry matter mass first increased and then decreased.Except for bud stage,the accumulation of reproductive organs in S2 treatment（medium and early sowing stage）and N2 treatment（medium nitrogen treatment）was the largest in other stages,the accumulation of reproductive organs in flocculation stage was much higher than that of vegetative organs,and the accumulation of reproductive organs in S2N2 treatment was the largest,up to 10069.85 kg/hm²,The minimum treatment of S4N1 is 5 036.31 kg/hm².（2）The full flowering stage is the maximum stage of LAI and photosynthetic function of film-free cotton.The highest LAI of S1N4 treatment was 5.01,and the lowest of S4N1 was 3.86.Under different sowing dates,S1>S2>S3>S4,and under different nitrogen application rates,N2>N4>N1>N3.On the whole,the earlier the sowing date and the more nitrogen application,the greater the LAI of film free cotton population.In the later stage,with the delay of sowing date,the nitrogen fertilizer increased and the LAI decreased.The SPAD value of film-free cotton leaves increased with the increase of nitrogen application and decreased with the delay of sowing date.Appropriate nitrogen application rate（N2treatment）and reasonable sowing date（S2 treatment）are conducive to leaf chlorophyll synthesis and enhance leaf function.（3）With the advancement of growth process,P_n,T_rand G_sof film-free cotton leaves showed a parabola trend.The flowering and Bolling stage was the period with the greatest photosynthetic function of leaves,and the photosynthetic difference of each treatment reached a significant level.P_n,T_r,G_sand C_iof S1N4 were higher,up to 23.22μmol CO₂/（m²·s）,14.23 mmol H₂O/（m²·s）,0.86 mol/（m²·s）and 284.94μmol CO₂/（m²·s）。The change trend of C_iwas opposite to that of P_nand G_s,and the indexes of S4N1 were the lowest,indicating that too late sowing date and too low nitrogen application were not conducive to the improvement of leaf photosynthetic function.The effects of sowing date and nitrogen application rate on leaf growth and photosynthetic indexes were interactive,and the effect of sowing date was higher than that of nitrogen application rate.（4）With the advance of growth period,the total nitrogen content of film-free cotton plants decreased first and then increased.The average leaf nitrogen content of each treatment was as follows:S4N3>S1N4>S2N2>S3N4>S1N1>S4N1,stem nitrogen content was as follows:S4N3>S3N4>S2N2>S1N4>S4N1>S1N1,The nitrogen content of reproductive organs is as follows:S1N4>S3N4>S2N2>S4N3>S4N1>S1N1.The amount of nitrogen transfer from vegetative organs to cotton boll was different in each treatment.On the whole,the amount of nitrogen transfer from organs in S2 treatment（medium sowing）was the largest.The average nitrogen transfer from leaves,stems and sheaths,flower buds to cotton boll were 61.89 kg/hm²,65.88 kg/hm²and 50.96 kg/hm²respectively.Under the same soil depth,the content of total nitrogen and alkali hydrolyzable nitrogen in 0～20 cm soil layer is the largest in S2N2 and the smallest in S1N1;S2N2 is the largest and S4N1 is the smallest in 20～40 cm;For 40～60 cm,S2N2 is the largest and S1N1 is the smallest.（5）Increasing the nitrogen application rate or delaying the sowing date,the bolls and inner bolls of middle and lower fruit branches increased,and the bolls and outer bolls of upper fruit branches decreased;The effect model of sowing date and nitrogen application rate with yield as the objective function was established.It was found that the nitrogen application rate was 270 kg/hm²and the highest yield was 5858.74 kg/hm²on April 25.At this time,the number of plants harvested,the number of bolls per plant and the quality of bolls per plant of film-free cotton reached 188 889 plants/hm²,5.75 pieces/plant and 5.39 g respectively.Through the multi-function set frequency analysis of the simulation scheme,10 combination schemes of sowing date and nitrogen application rate with a unit yield of 5 300 kg/hm²were selected,and the corresponding growth indexes were put forward,which can be used as a reference for production practice.