Leaf Senescence And Yield Formation Of Field-grown Short-season Cotton Without Plastic Mulching

Cotton(Gossypium hirsutum L.)is a staple cash crop and raw material for textile industry in China,which is greatly correlated to the national economy and the people's livelihood.However,in recent years,due to the increasingly prominent competition for land between grain crops and cotton,as well as the increasing labor and material inputs and low economic benefits,the cotton planting area had been significantly reduced,and the cotton planting had been continuously transferred to medium and low fertility fields such as saline soil,dry land,and sandy soil.In the medium and low fertility fields,full-season cotton varieties are traditionally planted in mid-April at a medium plant density under plastic mulching.It also implements fine pruning and multiple harvesting by hands.Although the yield of cotton per unit area is relatively high,this traditional cotton planting mode requires a lot of material and labor inputs and is thus high costly and labor-intensive.Increased plastic film residue in the soil has also caused widespread environmental concern.Short-season cotton is characterized by a shorter period of growth and development,and relatively concentrated flowering and boll-setting,and it thus sows late but matures early.It is known that planting short-season cotton under plastic mulching in low and medium fertility cotton fields not only obtain a better harvest,but it is also easy to realize concentrated maturity for mechanical harvest,but residual plastic film pollution still exists.Short-season cotton without mulching is a fundamental approach to solve the residual pollution.So far,however,it is unclear about the feasibility of short season cotton without mulching in low and medium fertility cotton fields,as well as the growth and development,and yield formation in the Yellow River valley.Therefore,a full-season cotton cultivar“K836”and a short-season cotton cultivar“Lumian 532”were used in this study.A five-year field study was consecutively conducted in 2017-2021,at the Shandong Cotton Research Center Experimental Station(experimental field with medium lower soil fertility)in Linqing,Shandong,China.The field experiments were arranged in a randomized complete block design with four replications each year.The experiment consisted of four treatments,including the early planting of full-season cotton with(FCM)and without plastic mulching(FC),and the late planting of short-season cotton with(SCM)and without plastic mulching(SC).The main objectives of this study were to determine the effects of different cultivation modes on cotton yield,input-output scenarios,earliness and related physiological and ecological indexes,with focuses on yield components and distribution,photoassimilate partitioning,leaf senescence,root characteristics,endogenous hormone content and related gene expression of short season cotton with and without mulching,to reveal the regulatory effect and mechanism of non-mulching cultivation on leaf senescence and yield formation of short-season cotton.This study will provide a theoretical basis for reducing inputs and increasing economic profits of field-grown short-season cotton without plastic mulching,and provide a solution for residual film pollution in cotton field.The main results and conclusions are as follows:1.FC was 16.2 and 14.7%lower than FCM in seedcotton yield and boll density,respectively.SC produced a comparable seedcotton yield to SCM,while there were no significant differences in average boll weight or boll density between SC and SCM.SC was17.5,5.5,12.9,and 17.3%lower than FCM in seedcotton yield,boll density,boll weight,and total output values,respectively.SC produced 15.0%higher net benefits than FCM as its inputs was 28.1%lower than that of FCM.Therefore,non-mulching significantly reduced the yield of full-season cotton,but had no effect on the short season cotton yield.The yield of short-season cotton without mulching is lower than that of mulched full-season cotton,but the net benefit was higher due to less input.2.The contents of GA3,DHZR,IPA,IAA and CTKs in the main stem growth tips of short-season cotton reached the peak when the second true leaf fully expended,while they reached the peak at the fourth or fifth true leaf stage.Non-mulching greatly delayed the flowering date of FC compared with that of FCM,but did not affect that of short-season cotton.The harvested bolls in number from flowers that bloomed within the first four weeks after initial blooming accounted for 79.2%of the total in FCM,while they accounted for 92.8%of the total in SC.About 74.6%cotton bolls of FCM were distributed in 1-8 fruit branches,and 62.4%in 1-2 fruit nodes,while 91.7%cotton bolls of SC were distributed in 1-8 fruit branches and 97.6%in 1-2 fruit nodes.More than 80%of bolls in SC opened within 42 days after initial boll opening,while only 66.3%of bolls opened in FCM.It is concluded that the content of endogenous hormones related to bud differentiation of short-season cotton reached the peak at 2-leaf stage regardless of mulching or not,while that of full-season cotton reached the peak at 4-leaf or 5-leaf stage.Earlier flower bud differentiation was one of the important mechanisms of early maturation of short season cotton at late sowing.Short-season cotton without plastic mulching had higher flowering rate,and faster and concentrated boll setting and boll opening than full-season cotton with mulching,which was another important mechanism of late sowing and early maturity.3.The number of early-and mid-season bolls of SC was less than that of SCM,but the number of late-season green bolls increased by 67.2%,the rotten boll rate decreased by 36.0%,the boll weight of mid-season bolls and 5-8 fruit branch bolls decreased by 5.7 and 9.2%,respectively,and the boll weight of late-season bolls and of in the 9th and the upper fruit branches increased by 8.1 and 9.9%,respectively.SC was 26.7 and 65.7%higher in chlorophyll(Chl)content and Net photosynthetic rate(Pn)rate,but 20.9%lower in Malondialdehyde(MDA)concentration than SCM in the late season.The canopy photosynthetic rate(Net CO₂exchange capacity of the ecosystem,NEE)of SC was 13.0%lower than that of SCM at the beginning of flowering,the two were the same at the boll setting stage,but increased by 3.1 folds at the boll opening stage.SC increased photoassimilates partitioning to the vegetative organs by 9.0%,decreased the partitioning to the reproductive organs by 28.3%compared with SCM at the initial flowering stage.However,SC was 9.2%lower in photoassimilate partitioning to the vegetative organs,but12.3%higher in photoassimilate partitioning to the reproductive organs than SCM at the initial boll-opening stage.It is thus concluded that non-mulching had no significant effect on the yield in short-season cotton compared with plastic mulching,but changed the temporal and spatial distribution of cotton bolls and corresponding boll weight,which was possibly attributed to the delayed leaf senescence and enhanced photoassimilate partitioning to bolls in late season under non-mulching.4.The boll load per leaf area of SC was 32.3%lower at 14 days after flowering,and the bleeding sap content in the boll-opening stage was 78.3%higher than that of SCM.The root length density,root surface area density and root volume density of 0-20 cm soil layer SC were 61.5,58.9,and 55.8%lower than those of SCM in the boll setting stage,respectively,the soil layer of 20-40 cm increased by 52.2,42.7,and 39.1%,respectively.In the late-season leaves,the key genes of chlorophyll synthesis Gh LHCB,CTKs synthesis related genes Gh IPT1 and Gh IPT2,and ABA degradation related genes Gh CYPA707A1,Gh CYP707A2,and CYP707A4 in SC were significantly up-regulated,the expression of ABA synthesis related genes Gh NCED2,Gh NCED3,Gh NCED5,and Gh NCED9,and JA synthesis related genes Gh AOC3,Gh LOX3 and Gh AOS6 were significantly down regulated,compared with SCM.SC decreased the contents of ABA,Methyl jasmonate(JA-Me)and salicylic acid(SA)by 25.2,12.3,and 43.3%,and increased CTKs(ZR+DHZR)content by 48.3%at the initial boll-opening stage compared with SCM.To better understand the molecular mechanisms of SC and SCM responses to late-season senescence,a comparative transcriptome analysis of cotton main-stem functional leaves was performed at the early boll opening stage.A total of 995differently expressed genes(DEGs)were identified,with 630 DEGs up-regulated and 365DEGs downregulated.Go enrichment analysis showed that DEGs were mainly enriched in biological processes.KEGG enrichment indicated that the DEGs were involved in plant pathogen interaction,protein processing in endoplasmic reticulum,plant hormone signal transduction,and starch and sucrose metabolic pathways.And transcription factor families,including C2H2,WRKY,MYB,HSF,AP2/B3-Like,NAC and PLATZ,were responsive to the leaf senescence.The 16 DEGs were validated by q RT-PCR analysis to confirm the high-throughput sequencing results,the results showed that the q RT-PCR data were basically consistent with the results of high-throughput transcriptome results,indicating that the transcriptome sequencing data were reliable.It was thus speculated that the difference in leaf senescence between SC and SCM was also attributed to the differential expression of these senescence related genes.Thus,the delayed leaf senescence of short-season cotton without mulching in the initial boll-opening stage was attributed to the reduced sink to source ratio,the deeper roots distribution,the altered content of related hormones and the differential expression of senescence-related genes.In summary,short-season cotton had earlier flower bud differentiation,faster flowering and boll setting,more concentrated boll opening,and the resulting better earliness than full-season cotton regardless of plastic mulching or not.Compared with that of full-season cotton under mulching,the yield of short-season cotton without plastic mulching was relatively reduced,but the cost was greatly reduced and the net benefit was thus significantly increased.Planting short-season cotton without plastic mulching had a promising application prospect to replace the traditional full-season cotton under mulching in fields with medium or low fertility in the Yellow River Valley.The leaf senescence of short-season cotton without mulching was delayed in late season,which promoted the photosynthetic production and photoassimilate partitioning to cotton bolls leading to more and larger late growth-season bolls,which not only made up for fewer early-and mid-season bolls,but also reduced the number of rotten bolls.Therefore,the final yield between SC and SCM was comparable.The delayed leaf senescence of short-season cotton without mulching was attributed to the lower sink source ratio and deeper root distribution.Senescence related genes and endogenous hormones were also involved in the regulation of leaf senescence.