Effects Of Late Sowing With Reduced Nitrogen On Cotton Growth And Yield In Central South Hebei Province

Light and Simplified Cultivation of cotton is the development trend of cotton planting in China.It is of great significance to study the relationship between sowing date and nitrogen on the growth physiology and yield of cotton.The experiment was conducted in 2019 and 2020,using transgenic insect resistant cotton Nongda 601 as experimental material,adopting split plot design,the main plot was sowing date(conventional sowing date(S1):around April 22;Delayed sowing date(S2):around May 5),the sub plot was nitrogen application rate(no nitrogen(N0):0 kg/hm2;nitrogen reduction(N1):144 kg/hm2 in 2019 and 180 kg/hm2 in 2020;conventional nitrogen application(N2):240 kg/hm2),the effects of nitrogen reduction by late sowing on Photosynthetic fluorescence parameters,dry matter accumulation and distribution and yield of cotton were studied.In order to provide a theoretical basis for cotton production in Hebei Province,the growth characteristics and yield changes of cotton under the condition of late sowing and nitrogen reduction were studied.The main results are as follows:1.The effect of late sowing and nitrogen reduction on the growth pattern of cotton.After the delay of sowing,the growth process accelerated,the whole growth period of cotton,except flowering and catting,the time experienced was shorter and the emergence speed was accelerated,which was shortened by 4 days at most than the conventional sowing period.After the delayed sowing period,the plant height and stem thickness of cotton increased,the average plant height of the three regions increased by 3.06 cm and the stem thickness increased by 0.22 mm.With the increase of nitrogen application,the plant height and stem diameter of cotton increased,NO was significantly lower than N1 and N2(P<0.05),and the average plant height and stem thickness in the three fields were 15.56 cm,17.24 cm and 2.44 mm and 2.92 mm respectively.Leaf area index(LAI)first rises and then decreases in the growth period of cotton.Except for S1N0 and S2N0 of Qingyuan in 2020,LAI reaches the maximum value(about 4.0)in bolling stage.LAI increased after the delay of sowing time,but decreased after the application of nitrogen fertilizer,but it did not reach the significant level.At NO level,LAI decreased significantly due to insufficient soil nitrogen supply(P<0.05).The changes of LAI directly affect the light transmittance(CLP)of cotton population,CLP is in the form of " V" in the growth period of cotton,contrary to the trend of LAI.Because of the decrease of LAI,CLP of S1N0 and S2N0 is significantly higher in boll and flocculent stage than that of the other treatments.2.Effects of nitrogen reduction by late sowing on Photosynthetic Characteristics of cotton.The net photosynthetic rate(Pn),actual photochemical efficiency(φPSII)and chlorophyll content(Chla+b)of cotton main stem functional leaves increased at first and then decreased during the growth period.After the sowing date was delayed,Chla+b and Pn increased.The average value of Pnin S2 was 6.2%and 3.9%higher than that in S1 in two years.The chlorophyll fluorescence parameter((pPSⅡ)and maximum photochemical efficiency(Fv/Fm)did not change significantly with sowing date.Nitrogen reduction resulted in the decrease of Chla+b,Pn,φPSII and Fv/Fm at boll filling and boll opening stages,but the difference was not significant(P>0.05);at NO level,Chla+b decreased significantly(P<0.05),resulting in the decrease of chlorophyll tissue involved in photosynthesis,and then Pn decreased.Compared with N1 and N2,the average value of Pn decreased by 53.3%and 56.1%respectively in two years and three places.At the same time,(pPSII and Fv/Fm were restricted by nitrogen,and cotton suffered from stress,the value also decreased significantly(P<0.05).The interaction of sowing date and nitrogen had no significant effect on Chla+b,Pn,φPSⅡ and Fv/Fm(P>0.05).3.The effect of late sowing and nitrogen reduction on the accumulation and distribution of dry matter in cotton.After the delayed sowing period,the quality of single plant dry matter decreased in the early stage of cotton growth(bud filling period and initial flowering period),and the quality of single plant dry matter increased in the later stage of cotton growth(flowering period,boll filling period and boll stage)than that of conventional sowing period,and increased with the increase of nitrogen application.In the cotton flogging period in two years and three years,the quality of single plant dry matter of S1N0 and S2N0 was lower than that of the other treatment 30～40g,which was poor the difference was significant(P<0.05).After the delayed sowing period,the distribution ratio of reproductive organs decreased in boll stage and flocculent stage of cotton than that of conventional sowing period.With the decrease of nitrogen application amount,the ratio of dry matter distribution of reproductive organs increased,and the ratio of dry matter distribution of reproductive organs decreased was higher than that of conventional fertilization,which increased by 7.1%at most,at least 0.1%,but did not reach a significant level(P>0.05).The quality of stem per plant and the ratio of distribution of reproductive organs did not significantly affect the sowing time and nitrogen interaction(P>0.05).4.Effects of nitrogen reduction by late sowing on cotton yield and quality.After delaying the sowing date,the number of Fuqian peach decreased,while the number of Fuqian peach and Qiutao peach increased,and the difference of Qiutao peach with the sowing date was significant;the proportion of Fuqian peach and Qiutao peach increased compared with the conventional sowing date,and the proportion of Fuqian peach and Qiutao peach also increased with the increase of nitrogen application rate.The seed cotton yield of S2 decreased by 0.6%and 1%respectively compared with the average value of S1.The micronaire value,breaking strength,uniformity and elongation did not change significantly with sowing date.With the increase of nitrogen application rate,boll number per unit area increased,lint percentage increased,and boll weight per boll did not change significantly,the yield of NO seed cotton decreased significantly(P<0.05),and the average value in two years and three places was 11.8%lower than that in N 1 and N2 Under the interaction effect of sowing date and nitrogen,single boll weight,lint percentage and bolls per unit area did not reach significant level(P>0.05),and seed cotton yield in Cangzhou and Weixian in 2020 reached significant level(P<0.05).In conclusion,the increase of LAI,net photosynthetic rate,chlorophyll content and other indicators delayed the senescence of cotton after delaying the sowing date,but the distribution of dry matter to vegetative organs increased compared with the conventional sowing date,resulting in the decline of yield At the same time,the decline of boll weight and lint percentage was not conducive to the increase of yield Although LAL,chlorophyll content,Pn,Fv/Fm and φPSⅡ were lower than those of conventional nitrogen application level,the difference was not significant,which led to higher dry matter accumulation of cotton under nitrogen reduction level.Meanwhile,the increase of boll weight and lint percentage under nitrogen reduction level was also the guarantee for its yield to maintain at a higher level.The results showed that the seed cotton yield of conventional sowing date and conventional nitrogen application rate(S1N2)was the highest,but the difference between the seed cotton yield of conventional sowing date and reduced nitrogen application rate(S1N1)and delayed sowing date and reduced nitrogen application rate(S2N1)was not significant.The results showed that it was feasible to extend late sowing to reduce nitrogen in the middle and south of Hebei Province.