ZHANG Xiang,RUl Qiu-zhi,Ll Yuan,CHEN Yuan,CHEN Yuan,ZHANG Xi-ling,CHEN De-hua,SONG Mei-zhen＊

1Institute of Cotton Research,Chinese Academy of Agricultural Sciences/State Key Laboratory of Cotton Biology,Anyang 455000,P.R.China

2Jiangsu Key Laboratory of Crop Genetics and Physiology/Co-Innovation Center for Modern Production Technology of Grain Crops,Yangzhou University,Yangzhou 225009,P.R.China

Abstract The cotton direct seeding after wheat (rape)harvested is under trial and would be the future direction at the Yangtze River Valley region of China.The objective of this study was to quantify the effects of branch and stem architecture on cotton yield and identify the optimal cotton architecture to compensate the yield loss due to the reduction of individual production capacity under high planting density in the direst seeding after wheat harvested cropping system.The characteristics of the stem and branch architecture and the relationships between architecture of the stem and branch with yield formation were studied on eight short season cotton cultivars during 2015 and 2016 cotton growth seasons.Based on the two years results,three cultivars with different architectures of stem and branch were selected to investigate the effect of mepiquat chloride (MC)application on the architecture of the stem and branch,boll retention,and the yield in 2017.Significant differences were observed on plant height,all fruiting nodes to branches ratio (NBR)in the cotton plant,and the curvature of the fruiting branch(CFB)among the studied cultivars.There were three types of stem and fruiting branch structures: Zhong425 with stable and suitable plant height and NBR (about 90 cm and 2.5,respectively),high CFB (more than 10.0),and high boll retention speed and seed cotton yield; Siyang 822 with excessive plant height and NBR,low CFB,and low boll retention speed and seed cotton yield; and other studied cultivars with unstable structure of stem and branch,boll retention speed,and seed cotton yield across years.And MC application could promote the appropriate plant height and NBR and high CFB and thus resulted in high boll retention speed and the yield.The results suggested that the suitable plant height and NBR (about 90 cm and 2.5 respectively),and high CFB (more than 10.0),which was related to both genotype and cultural practice,could promote the higher boll retention speed and seed cotton yield.

Keywords:short season cotton,architecture of the stem and branch,boll retention speed,seed cotton yield,mepiquat chloride

1.lntroduction

The medium-maturing cotton cultivar was planted widely in China,especially in the Yangtze River region.However,short season cotton cultivars are planted extensively now because of wheat (rape)-cotton double cropping pattern is adopted to relieve the competition for limited arable land between grain and cotton production (Donget al.2010; Luet al.2017).Cotton is usually managed with the transplanting pattern which involves lots of agronomic practices,such as seedling nursery,transplanting seedlings to the field,three to four times application of fertilizer and growth regulators,plant topping,and multiple hand harvest due to the long boll-setting period.The labor intensive and costly properties of transplanting (Chenet al.2016),together with the rural labor shortage caused by rural urbanization (Wanget al.2016),resulted in sharply reduced cotton planting area.In order to sustain cotton production,the cotton direct seeding after wheat (rape)harvested is under trial and would be the future direction at the Yangtze River Valley region (Dai and Dong 2014; Duet al.2015)to replace the traditional seedling transplanting (Chenet al.2016).However,the growth duration become markedly shorter because of late sowing date,which resulted in reduction of individual plant productivity.In order to obtained similar lint yield to that of seedling transplanting cotton,high planting density is usually adopted,and increasing boll retention under high planting density is required.Boll retention is related to light utilization,and the light use efficiency has close relationship with plant type.Thus,constructing the plant type,especially the stem and branch structure,with high photosynthetic efficiency of short season cotton is the structural basis for higher boll retention rate.In the plant type parameters,the fruiting nodes to fruiting branch ratio(NBR)reflects vertical and horizontal space utilization and affects light utility.The curvature of the fruiting branch(CFB)reflects bending degree of the fruiting branch,the more the boll loaded in the fruiting branch,the greater the CFB value of the fruiting branch was,therefore,the construction of stem and branch of short season cotton is important for high yield in the wheat (rape)-cotton double cropping pattern.

More studies have been reported for cotton plant type of high yield under the influence of cultivar and cultural practices.The plant height,ratio of plant height to nodes,fruiting branch length,internode length and diameter,and leaf angel distribution were evaluated for high yield in different cultivars (Kerby and Buxton 1978; Nutiet al.2006; Deolet al.2011; Caoet al.2011).The regulating impacts of sowing date,planting density,and mepiquat application on plant type were also studied (Cathey and Meredith 1988; Reddyet al.1990; Bozbeket al.2006; Boquet and Clawson 2009;Ruthet al.2013),and the relationships between plant type,light energy utilization,and lint yield were also investigated in production practice (Reihardt and Kuhlemerier 2002;Maroiset al.2004).However,the previous studies focused on medium-maturing cotton cultivar and monocultural and interplanting pattern.The plant habit relating to mechanical harvest had been studied in short season cotton in recent years (Willifordet al.1994; Vanet al.2013),including optimum plant height,node at which the first fruiting branch occurs,number of fruiting branches per plant,main stem diameter,height of the node where the first fruiting branch locates,height of the lowest harvestable boll,and length of fruiting branches (Wanget al.2016).However,less was reported about the relationship of plant type,especial for architecture of stem and branch,with the boll retention for short season cotton.Therefore,the primary objective of the research was to quantify the effect of architecture of the stem and branch on boll retention and yield in short season cotton cultivars,determine the optimum architecture of the stem and branch under the cotton direct seeding after wheat harvested cropping system,and provide cultural practices guidelines on the regulation of the morphology of stem and branch for short season cotton.

2.Materials and methods

2.1.Plant materials and experimental design

The experiments were conducted on the farm of Yangzhou University,China (32°30′N,119°25′E)in 2015-2017.The soil contained 2.05 g kg?1organic matter,and available N,P,and K of 109.3,27.5,and 80.6 mg g kg?1,respectively.Cotton was sown at a row distance of 81 cm on June 12 in 2015,June 13 in 2016,and June 10 in 2017 after wheat harvested.The plant density was 13.5 plant m-2.The plot dimension was 8 m×4.86 m.Fertilizer was applied according to local practices: 195 kg ha?1N,45 kg ha?1P2O5,and 150 kg ha?1K2O.Defoliation theripener (Thidiazuron 450 g ha-1+Ethephon 4 500 mL ha-1)was sprayed on the cotton plant when 30% bolls opened.The Mepiquat chloride(MC)dose was 270 g ha-1.Soil moisture supply and other production measures were conducted according to local farm management practices.

In 2015 and 2016,eight short season cultivars (Han818,Han258,Jinke 707,Shikang 126,Siyang 822,Zhong3013,Zhong425,and Zhong50)from the main cotton breeding organizations of China were selected to assess the relationship between plant habit of stem and fruiting branch and boll retention,the cultivar traits were shown in Table 1.The experiment was arranged in a single factor randomized completed block design with three replications.

In order to verify the conclusions from the experimental results of 2015 and 2016,MC application with different dose were designed to regulate the structure of stem and fruiting branch at a field experiment in 2017,the three cultivars with different plant habits of stem and fruiting branches and boll retention rate were selected.The experiment was arranged in a split plot design with three replications.The main plot treatment was cultivar (Zhong425,Han258,and Siyang 822),and the sub-plot treatment was MC application dose (0,270,and 540 g ha-1,respectively).MC was applied in three growth stages: (1)peak squaring stage (i.e.,12 main stem leaves); (2)peak flowering stage (i.e.,18 main stem leaves); and (3)peak boll developing stage (i.e.,24 main stem leaves).The dose at each stage was tailored to the plant size,and 20,40,and 40% of total dose was sprayed at three stages,respectively.MC was applied in sunny days.

2.2.Measurements

Yield and yield componentsFor each experiment,all open bolls from the central four rows (86 plants for each row)of each plot were manually harvested one time before first frost(Oct.31),and measured for seed cotton yield.

The continuous 20 plants in the central two rows in each plot were counted on Sep.20,and the total number of bolls on a unit ground-area basis was determined.The open bolls from the 20 plants before frost were harvested,and boll weight per each boll (moisture≤11%)was measured.Boll retention speed Effective boll formation under direct seeding after wheat harvested usually lasts from the end of Jul.to Sep.20,and the bolls retained before Aug.31 are considered as high-quality bolls.Therefore,the boll number was recorded on Jul.30,Aug.31,and Sep.20 from 20 plants in the central two rows in each plot,and the boll daily retention speed per hectare was calculated by dividing the number of bolls developed within the period by the number of days.

Structure of stem and fruiting branchTen cotton plants from each plot were sampled for the measurements of stem and fruiting branch structure at boll open stage.The plant height was measured from the cotyledon node to the main stem terminal.The number of fruiting branches and nodes per plant was also counted.The number of fruiting nodes was divided by the number of fruiting branches to obtain the nodes to branches ratio (NBR).The angles between main stem and fruiting branch base (AMF),and the angles between main stem and the connection of the fruiting branch base to the terminal (AML)were measured (Fig.1),AML was divided by AMF to obtain the curvature of the fruiting branch (CFB).

Light intensityThe light intensities were measured at peak boll period (about Aug.10)with illuminometer (HT-1300,Hongcheng Technology Co.,Ltd.,China).In the morning(09:00)and afternoon (16:00),the radiation intensities in the middle section of the bottom branches of the cotton plant canopy were recorded at east,south,west,and north positions in the bottom.The light intensity at the base of the canopy was averaged over the four positions in the morning and afternoon.

2.3.Statistical analysis

Analysis of variance (ANOVA)was conducted for each experiment on all the characteristics measured.Analysis of quadratic regression for relationship between the plant height,NBR,CFB,and retained boll speed,seed cotton yield were conducted with PROC ANOVA in SAS 6.Multiple comparisons were evaluated by Duncan's multiple range tests atP≤0.05.

Table 1 Characteristics of the eight short season cotton cultivars studied

3.Results

3.1.The yield and boll retention speed of eight short season cotton cultivars in the cotton direct seeding after wheat harvested cropping system

Significant differences in seed cotton yield were observed in the eight studied cultivars (Table 2).Both Zhong425 and Zhong50 had high seed cotton yields during 2015 and 2016 cotton growth seasons,and the yields were more than 3 800 kg ha-1.Low yields were detected in Shikang 126 and Siyang 822,and the yields were lower than 3 300 kg ha-1.The yields of other cultivars varied with years,indicating greater environmental effects on the yield of these cultivars.The results of boll number,including bolls per plant and per hectare,were consistent with the yield.The cultivar with high yield had more bolls for individual plant and unit area.The boll weight varied among cultivars,Zhong3013 and Han258 had higher boll weight during both years,which were more than 4.4 g in 2015,and more than 3.9 g in 2016,and low boll weights were observed in Han818 and Jinke 707,with the boll weights of less than 3.9 g in 2015,and less than 3.7 g in 2016.

Fig.1 Diagram of measured angles between main stem and fruiting branch base (AMF)and angles between main stem and the connection of the fruiting branch base to the terminal (AML).

The boll retention speed at the duration of the effective boll formation period varied among studied cultivars (Fig.2).Highest boll retention speed was observed in Zhong425 in both years,which was more than 20 000)bolls per hectare per day.Cultivars Zhong50 and Han258 also had relatively high retention speed,but the values were unstable during the two cotton growth seasons.Cultivars Shikang 126 and Siyang 822 had low and stable retention speed in both years,with the values of 14 000 bolls per hectare per day.Greater fluctuation of the boll retention speed was observed at the two cotton growth seasons for other cultivars.The results suggest that short season cultivar with high boll retention speed during the effective boll formation period had high seed yield.

3.2.Structure of the stem and fruit branch of eight short season cotton cultivars in the cotton direct seeding after wheat harvested cropping system

Different plant heights were observed among the eight short season cotton cultivars (Figs.3 and 4).During the two cotton growth seasons,Shikang 126 and Siyang 822 had high plant height,and the heights were more than 100 cm.Jinke 707 exhibited the lowest plant height,and the plant height was 79.1 and 81.5 cm in 2015 and 2016,respectively.The stable heights were also detected in Zhong425 and Zhong50 during both years,and the values were about 90 cm.The plant height of Han258,Han818,and Zhong3013 changed greatly during the two studied years,which showed that growth environment in different years had important influence on the plant heights of these cultivars,and growth environment in 2016 was more favorable for the increase of the plant height.

Table 2 The yield and yield components of eight short season cotton cultivars

Fig.2 The boll retention speed of eight short season cultivars(Han818,Han258,Jinke 707,Shikang 126,Siyang 822,Zhong3013,Zhong425,and Zhong50)in 2015 and 2016.Uppercase letters indicate differences among the cultivars in 2015,and lowercase letters show differences in 2016.Differences between cultivars labeled with the same letters within the same year are not statistically significant at P=0.05.

The relationships between boll retention speed,seed cotton yield,and plant height may be quantified by quadratic regression equations (Figs.5 and 6).Based on the equations,the maximum boll retention speed were 19.1 to 22.0 thousand bolls per hectare per day at plant height of 90.9 to 94.0 cm,and the maximum seed cotton yields were also obtained at plant heights,of 87.0 to 89.0 cm in 2015 and 2016,respectively.The results showed that the optimum plant height was around 90 cm and would promote boll retention and high yield.

NBR varied among the short season cultivars during two years (Fig.7).The highest NBR was observed in Siyang 822,with the values of more than 3.0.Zhong425 and Zhong50 had stable and appropriate NBR,and the values were between 2.23 to 2.68.Large variation of NBR with years was observed in other cultivars.The values of Jinke 707 was 1.83 in 2015,but 2.81 in 2016,which suggested that growth environment of different years impacted NBR of short season cotton.

Analysis of quadratic regression indicated that the maximum boll retention speed and seed cotton yield could be obtained at appropriate NBR (Figs.8 and 9).The maximum retention speed was 18.4 to 22.4 thousand bolls per hectare per day at NBR of 2.51 and 2.71,the maximum seed cotton yields were 3 598.9 to 4 167.0 kg per hectare at NBR of 2.43 and 2.65 in 2015 and 2016,respectively.

Differences in CFB were observed at different sections of cotton plants for the studied cultivars (Table 3).The fruiting branch at base of the plant had the highest CFB,and the upper fruiting branch had the lowest CFB.Significance differences of CFB were also detected among the cultivars.Zhong425 had the highest CFB,and the values were more than 10.0 at the base,9.5 at the middle section,and 6.0 at the upper section.Siyang 822 and Shikang 126 exhibited the lowest CFB in both years in the three sections of the cotton plant,with the values of less than 8.0 at the base,6.0 at the middle section,and 4.0 at the upper section.

There were significant positive correlation between boll number per plant,boll retention speed,seed cotton yield and CFB,except for boll retention speed with CFB at the base and middle sections in 2017 with the increasing CFB at base,the higher boll number per plant was detected.Increase of CFB at the upper of the cotton plant contributed to the bolstered boll retention speed.With the increased CFB at every section of the cotton plant,higher seed cotton yield was observed.

3.3.MC application on the structure of stem and fruit branch and yield formation

Fig.3 The plant type of Jinke 707 (A),Zhong50 (B),and Zhong425 (C).

Fig.4 The plant heights of eight short season cotton cultivars(Han818,Han258,Jinke 707,Shikang 126,Siyang 822,Zhong3013,Zhong425,and Zhong50)in 2015 and 2016.Uppercase letters indicate differences among the cultivars in 2015,and lowercase letters show differences in 2016.Differences between cultivars labeled with the same letters within the same year are not statistically significant at P=0.05.

Fig.5 The relationship between plant height and boll retention speed of eight short season cotton cultivars in 2015 (A)and 2016 (B).

In order to verify whether the yield and boll formation could be improved by adjusting the stem and branch structure to the optimum value,MC with different levels was applied on the three short season cultivars (Table 4).The results showed that MC application markedly affected the structure of stem and fruiting branch.The boll retention speed,boll number,and yield all reached the highest values for Zhong425 when plant height,NBR,and CFB were adjusted to the optimum range under 270 g ha-1MC.Similar results were also observed for Han258.However,the highest boll retention speed,boll number,and yield were detected for Siyang 822 under 540 g ha-1MC,due to the fact that the plant height,NBR,and CFB were close to the optimum values under 540 g ha-1MC.The results further proved that the regulation of optimum stem and branch structure(plant height,NBR,and CFB)by cultural practice would benefit the yield formation for short season cotton cultivars with different genetic traits.

4.Discussion

4.1.Constructing suitable structure of stem and branch would benefit the yield formation for short season cotton

Fig.6 The relationship between plant height and seed cotton yield of eight short season cotton cultivars in 2015 (A)and 2016 (B).

Fig.7 Fruiting nodes to branches ratio (NBR)of eight cultivars(Han818,Han258,Jinke 707,Shikang 126,Siyang 822,Zhong3013,Zhong425,and Zhong50)of short season cotton in 2015 and 2016.Uppercase letters indicate differences among the cultivars in 2015,and lowercase letters show differences in 2016.Differences between cultivars labeled with the same letters within the same year are not statistically significant at P=0.05.

Fig.8 The relationship between nodes to branches ratio (NBR)and boll retention speed of eight short season cotton cultivars in 2015 (A)and 2016 (B).

Short season cotton featured by early flowering,grouped fruiting,low physiological shedding,as well as clearly defined cut-out is ideal for mechanized cropping systems in Yangtze River region in China (Donget al.1994,2010;Seklokaet al.2007).High plant density is usually adopted to compensate yield loss due to the late planting,however,the light interception and the radiation use efficiency reduced under high plant density (Kerbyet al.1990).The light energy utilization was related to the plant type,and the plant type also affected the growth and yield.Suitable plant type,including plant height,length of main stem and fruit branch,number of fruiting node,leaf angle,etc.,were studied for high yield (Wellset al.1986; Reta-Sánchez and Fowler 2002; Ruthet al.2013).Ruthet al.(2013)reported that shorter bush type variety might have higher growth plasticity compensating for stand reductions.MC application can modify plant type and regulate photosynthesis (Maoet al.2015; Tunget al.2018).Our present study further showed that the structure of stem and fruiting branch affected boll retention speed and seed cotton yield.The suitable plant height and NBR (about 90 cm and 2.5,respectively),and high CFB (more than 10.0)would benefit the boll formation and contributed to the increase of the yield for short season cultivars.Three types of stem and fruiting branch structures were observed in our present study: the first type was Zhong425,which had stable and suitable plant height and NBR (about 90 cm and 2.5 respectively),high CFB (more than 10.0),and high boll retention speed and seed cotton yield; the second type was Siyang 822,which had excessive plant height and NBR,low CFB,and low boll retention speed and seed cotton yield; the third type was other cultivars,their structure of the stem and branch varied at different years.The conclusions were further confirmed by MC application in our present study.The sunlight intensity of peak boll period at base of the plant also indicated that the structure of stem and fruiting branch influenced the light distribution.The light intensity was 1 060 lx for Zhong425,but only 620 lx for Siyang 822 at base of the cotton plant,the low light intensity resulted in reduction of the retained boll number.Our results suggested that regulating the architecture of stem and fruiting branch is a potential way to promote boll formation and high yield.

Fig.9 The relationship between nodes to branches ratio (NBR)and seed cotton yield of eight short season cotton cultivars in 2015 (A)and 2016 (B).

Table 3 The curvature of the fruiting branch (CFB)at different sections of cotton plant and correlation coefficient between CFB and boll per plant,boll retention speed,seed cotton yield

Table 4 Effect of mepiquat chloride (MC)application on the structure of stem and branch and yield formation

4.2.Formation of structure of stem and branch was related to cultivar and cultural practices

Variations of plant height and mode of branching established two plant types among cotton cultivars: columnar and bush types (Ruthet al.2013).Columnar types are phenotypes with more compact canopy,dominant main stem,taller stature,and shorter fruiting branches (Fenget al.2016).Bush types have longer fruiting branches,shorter stature,wider canopies,and more vegetative branches (Trebuilet al.1993),and the architecture impacted lint yield(Heitholt 1994).Our experimental results indicated short season cotton cultivars with different structures of stem and fruiting branch exhibited different boll retention speeds and seed cotton yields.In our present study,Zhong425 had stable and suitable plant height,NBR,high CFB,and thus exhibited high boll retention speed and seed yield correspondingly,opposite results were observed in Siyang 822,which exhibited low boll retention speed and seed cotton yield under excessive plant height and NBR,and low CFB.The results suggested that the structure of stem and fruiting branch was related to cultivar for short season cotton.In addition,both the structure of stem and fruiting branch and the yield formation were impacted by MC application.The three cultivars with suitable plant height and NBR,high CFB under the appropriate MC dose had high boll retention speed and seed cotton yield.Zhong425 and Han258 under the MC dose of 270 g ha-1,Siyang 822 under the MC dose of 540 g ha-1had both suitable architectures of stem and branch and high yield.Thus,the structure of stem and fruiting branch was related to cultural practices for short season cotton,which was consistent with previous research results (Bozbeket al.2006; Boquet and Clawson 2009; Tunget al.2018).Therefore,appropriate cultivar with optimized management practice could built suitable structure of stem and fruiting branch which would contribute to the short season cotton production in direct seeding after wheat harvested cropping system.

5.Conclusion

Our results showed that the stem and fruiting branch structure influenced yield formation among short season cotton cultivars.The suitable plant height and NBR (about 90 cm and 2.5,respectively),and high CFB (more than 10.0)would resulted in higher boll retention speed and seed cotton yield.MC application could regulate the structure of the stem and branch,which resulted in improved boll formation and the yield.The optimum architecture of stem and branch was related to both varieties and cultural practices,and building suitable architecture would promote the high yield formation for short season cotton in direct seeding after wheat harvested cropping system.The results suggested that it is important to regulate the structure of stem and branch to the optimum values by breeding and culture practices in short season cotton.

Acknowledgements

The study are funded by the National Key Research and Development Program of China (2018YFD0100400 and 2017YFD0201300),the Engineering Science and Technology Innovation Fund of Chinese Academy of Agricultural Sciences (2016PCTS-1),the National Natural Science Foundation of China (31671613),and the Priority Academic Program Development of Jiangsu Higher Education Institutions,China (PAPD).