Effects of paclobutrazol application on plant architecture,lodging resistance,photosynthetic characteristics,and peanut yield at different single-seed precise sowing densities

Jihao Zhao,Huajiang Lai,Chen Bi,Mengjie Zhao,Yanling Liu,Xiangdong Li,Dongqing Yang

State Key Laboratory of Crop Biology,Agronomy College of Shandong Agricultural University,Tai’an 271018,Shandong,China

Keywords:Sowing density Paclobutrazol application Lodging resistance Photosynthetic characteristics Peanut yield

ABSTRACT The key to high-yielding peanut cultivation is the optimization of agricultural production practices.Regulating single-seed precise sowing(SSPS)density and paclobutrazol(Pbz)application concentration are effective practices that increase peanut yield by improving plant architecture,lodging resistance,and photosynthetic characteristics.Therefore,we conducted a two-factor field optimization experiment for the sowing density(D1:1.95×105 plants ha?1,D2:2.40×105 plants ha?1,D3:2.85×105 plants ha?1,and D4:3.30×105 plants ha?1)and Pbz application concentration(P0:0 mg L?1 and P1:100 mg L?1).The objective was to optimize agricultural production practices and provide a theoretical basis for highyielding peanut cultivation by evaluating the effects of sowing density and Pbz application on plant architecture,lodging resistance,photosynthetic characteristics,and yield.The results showed that at the same Pbz application concentration,increasing sowing density increased lodging percentage and reduced leaf photosynthetic capacity.At the same sowing density,Pbz application reduced lodging percentage by decreasing plant height(PH),improving lignin biosynthesis-related enzyme activities,and enhancing stem puncture strength(SPS)and breaking strength(SBS).The paclobutrazol-induced alterations in plant architecture and lodging resistance improved light transmission at the middle and bottom leaf strata,resulting in the increase in relative chlorophyll content and net photosynthetic rate(Pn)of leaves.Furthermore,D3P1 treatment had the highest peanut yield among all treatments.In summary,the production strategy combining the sowing density of 2.85×105 plants ha?1 with the application of 100 mg L?1 Pbz was found to be the optimal agricultural production practice for giving full play to production potential and achieving higher peanut yield.

1.Introduction

Peanut(Arachis hypogaea L.)is one of the most widely consumed crops because of its high edible oil and crude protein content[1].China is the largest peanut producer globally,and its peanut production levels have increased substantially over the past few decades[2].Traditionally,peanut has been grown through the double-seed and multi-seed modes,but they have been found to limit the improvement of peanut production potential.These traditional sowing modes are characterized by a high seeding rate per unit area[3,4],which not only enhances competitive shading within the leaf canopy structure,but the resulting competition also leads to prominent contradictions between individuals and populations[5].Thereby increasing the risk of lodging,decreasing the photosynthetic capacity,and limiting the improvement of peanut yield.In recent years,the single-seed precise sowing(SSPS)mode has been proposed in peanut production in China[6].

The SSPS mode has solved the problems associated with double-and multi-seed sowing modes.It enables peanut plants to be distributed evenly,reduces the competition among individuals,and attains the full production potential of the individual plant[6].A previous study has reported that the SSPS mode reduced roots competition and improved roots function[7].The photosynthetic pigment content and photosynthesis of peanut plants grown in the SSPS mode were markedly higher than those grown via the conventional double-seed sowing mode[8].In addition,the SSPS mode improved the distribution and transfer rate of nutrients to the pods during pod formation,resulting in an increase in pod weight[9].For these reasons,the SSPS mode has been recognized as a new way to increase the economic coefficient(economic yield/biological yield)while preserving the total biomass stable.Although the SSPS mode can allow the individual plant to attain its full production potential[3],a sufficiently large number of plants must be available to produce a higher population yield.Increasing sowing density in the SSPS mode is thus essential for peanut production.

As an important agricultural production practice,the main significance of increasing sowing density is the resultant increase in yield per unit area[10,11].Increasing peanut sowing density can improve peanut yield by increasing the peanut population sink[10,12]and intercepted photosynthetic active radiation[13,14].However,a large increase in sowing density may promote excessive vegetative growth,increase plant height(PH)and height of center of gravity(HCG),and reduce the quality and quantity of light penetration into the canopy[15,16],which can subsequently lead to the decrease in population lodging resistance and photosynthetic capacity[17].As a result,selecting a suitable sowing density to optimize plant architecture,enhance lodging resistance,and improve the use of light energy are crucial for high-yielding peanut cultivation.Paclobutrazol(Pbz),an important plant growth regulator,has been extensively used in agricultural production for regulating plant growth and development[18].The application of Pbzinhibited the increase in wheat PH,thereby reducing the lodging percentage[18].Kuai et al.[19]found that Pbzapplication decreased PH and increased rhizome thickness.Zhang et al.[20]reported that peanut plants treated with Pbzshowed increased leaf chlorophyll content and increased pod yield.Therefore,the application of Pbzplayed an important role in increasing crop yield.

Although studies like these have reported that the application of Pbzcould increase peanut yield,few studies have focused on how the combination of sowing density and Pbzapplication may produce higher peanut yield by coordinating the growth of individuals and populations and giving full play to production potential.An understanding of the response of peanut growth to the effects of sowing density and Pbzapplication is necessary to further attain the full production potential and improve peanut yield.The objectives of our study were to measure and assess the effects of sowing densities and Pbzapplication on plant architecture,lodging resistance,photosynthetic characteristics,and peanut yield.We hoped that our study could provide a theoretical basis for high-yielding peanut cultivation by optimizing sowing density and Pbzapplication concentration to create greater production potential and achieve higher peanut yield.

2.Material and methods

2.1.Experimental site

Field experiments were conducted at the experimental station of Shandong Agricultural University,Tai’an,China(36°09?N,117°09?E,and 128 m above sea level)in the 2018 and 2019 growing seasons.This region has a warm and semi-humid continental monsoon climate,with an annual average total solar irradiance of 5.08×103MJ m-2.Weather statistics during the peanut growth cycle are shown in Fig.S1.The soil in the study area is classified as Eutric Cambisol according to the World Reference Base for Soil Resources.The basic soil characteristics(0–20 cm)of the experimental site were shown in Table S1.

2.2.Experimental design

The peanut cultivar used in the present study was Shanhua 108(SH108,a Virginia type with an erected phenotype).It was sown on May 9,2018 and May 10,2019,and harvested on September 9,2018 and September 8,2019.The experimental design was a split-plot design with three replicates.The main plots included four different sowing densities:1.95×105plants ha-1(D1),2.40×105plants ha-1(D2),2.85×105plants ha-1(D3),and 3.30×105plants ha-1(D4).The applied concentration of Pbzwas the subplots:0 mg L-1(P0)and 100 mg L-1(P1).Thus,the study comprised eight treatments:D1P0,D1P1,D2P0,D2P1,D3P0,D3P1,D4P0,and D4P1.Based on the results of our pre-experimental and previous research[21],the application concentration of Pbzwas determined to be 100 mg L-1.At the continuation of flowering stage[22],100 mg L-1Pbzsolution(distilled water as control)was sprayed onto the whole plant at a rate of 1000 L ha-1with an electric sprayer.Peanut plants were sprayed at 4:00–5:00 PM on a sunny day.Each experimental plot(5 m×2 m)was ridgeplanted with six rows(ridge width 1.80 m and furrow width 0.20 m).The row spacing on the same ridge was 30 cm,and the intra-row spacing of the plants was 17,13.5,11.5,or 10 cm depending on the sowing density.To each experimental plot was applied 600 kg ha-1compound fertilizer(N 15%,phosphorus oxide(P2O5)15%,and potassium oxide(K2O)15%)as basal fertilizer.

2.3.Sampling and determination

2.3.1.Plants sampling

Ten uniform and representative peanut plants were sampled from each treatment at the flower-pegging stage,pod-setting stage,and pod-filling stage[22],respectively.Five plant samples were used to measure the plant architecture parameters.The main stems of the other five plant samples were immediately frozen in liquid nitrogen for at least 30 min and stored at–80°C for the measurement of lignin content and lignin biosynthesis-related enzyme activity.

2.3.2.Plant architecture

A vernier caliper was used to measure the diameter of the base of the main stem(SD).PH,HCG,and first lateral branch length(FLBL)were measured using a steel ruler.Specific leaf weight(SLW)is defined as the dry weight per unit leaf area.Specific stem weight(SSD)refers to the dry weight of stem per unit height.Five uniform main stems were used to determine SSD at the base of each treatment plant main stem.

2.3.3.Lignin biosynthesis-related enzyme activity

Main stem samples(0.5 g)were ground to powder in liquid nitrogen,and extracted with phosphate buffer(pH 7.0).The activities of phenylalanine ammonia-lyase(EC 4.3.1.24,PAL),cinnamoyl-CoA reductase(EC 1.2.1.44,CCR),cinnamyl-alcohol dehydrogenase(EC 1.1.1.195,CAD),and peroxidase(EC 1.11.1.7,POD)were measured using enzyme activity assay kits(Caobenyuan Biotechnology Co.,Ltd.,Nanjing,Jiangsu,China).Each treatment consisted of three biological replicates.

2.3.4.Stem lignin content

Stem lignin content was determined by a modified method described by Zheng et al.[23].Main stem samples(0.1 g)were ground to a powder in liquid nitrogen,washed five times with 95% ethanol to remove soluble metabolites,washed with acetone,and dried in an oven at 50°C.Dried samples were transferred to a 10-mL centrifuge tube,2.5 mL acetyl bromide and acetic acid solution(v/v,1:4)was added,and the mixture was incubated at 70 °C for 30 min.After cooling to room temperature,0.9 mL of 2 mol L-1NaOH was added to the tube to terminate the reaction.The solution was mixed and 0.1 mL of 7.5 mol L-1hydrochloride was added.Finally,acetic acid(4 mL)was added to the tube and the absorbance of the solution was determined at 280 nm using a spectrophotometer.Lignin content was expressed as OD280mL-1g-1fresh weight(FW).Each treatment consisted of three biological replicates.

2.3.5.Main stem puncture strength,stem breaking strength,and lodging percentage

Main stem puncture strength(SPS)and main stem breaking strength(SBS)were measured with a Digital Force Tester(YYD–1,Zhejiang Top Instrument,Hangzhou,Zhejiang,China).The sampled main stem was placed on the plate or the groove of the support pillars to measure SPS and SBS.A uniform force was applied to the internode and increased steadily until the node broke.The values of the force at the moment of breaking were recorded as the SBS.A 0.01 cm2test probe was vertically inserted into the internode,and the displayed values of the force were recorded as the SPS.The percentage of lodging under natural conditions is the proportion of the total number of lodging plants in a plot among all plants in the same plot.

2.3.6.Canopy light transmission ratio

The light transmission ratio(LTR)of canopy layers was measured by the intercepted photosynthetically active radiation(IPAR)and total photosynthetically active radiation(TPAR).IPAR at the middle leaf strata and bottom leaf strata and TPAR above the canopy were measured with a canopy analyzer(SunScan Canopy Analysis System,Delta–T Devices Ltd.,Cambridge,UK)between 10:00 and 11:00 AM on a sunny day.Three independent measurements were performed at each canopy layer within each treatment.The LTR was calculated as follows:

LTR=IPAR/TPAR×100%2.3.7.Relative chlorophyll content and net photosynthetic rate

A hand-held soil–plant analysis development chlorophyll meter(SPAD–502)(Minolta,Osaka,Japan)was used to determine the relative chlorophyll content(SPAD values).The net photosynthetic rate(Pn)was measured using a Li–6400XT portable photosynthesis device(Li–COR,Lincoln,NE,USA).SPAD values and Pnof the third leaves from the main stem of each treatment plant were measured between 9:00 and 11:00 AM on a sunny day.

2.3.8.Peanut yield and yield components

At maturity,an area of 2.70 m2(1.50 m×1.80 m)was marked out in each experimental plot,and all peanut plants in this area were harvested manually to measure peanut yield.The number of all peanut plants in each area was recorded and used to determine the number of plants per unit area.All harvested pods from the peanut plants were air-dried and weighed to obtain the number of pods per kilogram.Fifteen representative plants were sampled from each area to record the number of pods per plant.

2.4.Statistical analyses

DPS v7.05(Hangzhou RuiFeng Information Technology Co.,Ltd.)was used for performing the analysis of the significant differences among means by the LSD method at a significance level of 0.05.Graphs were plotted using SigmaPlot 14.0(Systat Software,Inc.,Richmond,CA,USA)and Origin 9.1(OriginLab Corporation,Northampton,MA,USA).Analysis of variance(ANOVA)was performed for plant architecture parameters using SPSS 23.0(IBM Corporation,Armonk,NY,USA).

3.Results

3.1.Plant architecture parameters

Peanut plants’architecture parameters under all treatments are shown in Table 1.Increasing sowing density increased PH and decreased SD,SLW,and SSW under the same Pbzapplication concentration.In both growing seasons,PH under D2P1,D3P1,and D4P1were significantly(P＜0.05)higher than that under D1P1,with an average increase of 14.28%,35.27%,and 45.49%,respectively.At the same sowing density,peanut plants treated with Pbzshowed a decrease in PH,FLBL,and HCG and an increase in SD,SLW,and SSW relative to those without Pbzapplication.Compared with D1P0,D2P0,D3P0,and D4P0,PH under D1P1,D2P1,D3P1,and D4P1significantly(P＜0.05)decreased by 15.75%,7.68%,9.92%,and 5.79% in the 2018 growing season,respectively.In addition,D3P1decreased PH and HCG and increased SD and SSW relative to D2P0in both growing seasons.

Table 1 Effects of sowing density and Pbz application on plants architecture parameters in the 2018 and 2019 growing seasons.

3.2.Lignin biosynthesis-related enzyme activity

Without the application of Pbz,PAL activity gradually decreased with the increase of sowing density(Fig.1A,E).At the pod-setting stage in the 2018 and 2019 growing seasons,D2P0,D3P0,and D4P0had an average decrease of 6.33%,13.20%,and 17.79%in PAL activity relative to D1P0,respectively.The application of Pbzincreased PAL activity at the same sowing density in the 2018 growing season.Compared with D3P0,D3P1significantly(P＜0.05)increased PAL activity by 8.66%,9.42%,and 5.71%at the flower-pegging stage,pod-setting stage,and pod-filling stage in the 2018 growing season,respectively.

Under the same Pbzapplication concentration,CCR activity was found to decrease with the increase of sowing density(Fig.1B,F).At the same sowing density,peanut plants treated with Pbzshowed a significant(P＜0.05)increase in CCR activity relative to those without the application of Pbz.For example,CCR activity under D3P1was significantly(P＜0.05)decreased by 8.75% relative to D1P1at the pod-setting stage in the 2018 growing season.In addition,D3P1significantly(P＜0.05)increased CCR activity by 5.07%compared with D3P0at the same growth stage in the 2018 growing season.

Similar to the trends observed in the activities of PAL and CCR,the activities of both CAD and POD decreased with increasing sowing density under the same Pbzapplication concentration(Fig.1C,G and D,H).At the same sowing density,Pbzapplication also significantly(P＜0.05)increased CAD and POD activities relative to without the application of Pbz.The activities of CAD and POD under D3P1increased by an average of 6.03%and 6.24%relative to those under D3P0,at the three growth stages in the 2018 growing season.

3.3.Stem lignin content

The results in Fig.2 showed that lignin content decreased with the increase of sowing density under the same Pbzapplication concentration.Lignin content under D2P0,D3P0,and D4P0was significantly(P＜0.05)lower than that under D1P0at the pod-setting stage in both growing seasons,with an average decrease of 7.49%,17.44%,and 28.68%,respectively.At the same sowing density,peanut plants treated with Pbzshowed significantly(P＜0.05)increased lignin content.Moreover,lignin content under D3P1was significantly(P＜0.05)higher than that under D2P0at the pod-filling stage in the 2018 and 2019 growing seasons.A similar outcome was observed for D3P0and D4P1,indicating that D4P1increased lignin content relative to D3P0.

Fig.1.Effects of sowing density and Pbz application on lignin biosynthesis-related enzyme activity in the 2018(A,B,C,D)and 2019(E,F,G,H)growing seasons.Vertical bars represent the standard error of the mean of three replicates(n=3).Different letters on bars at the same growth stage indicate significant differences among treatments,P＜0.05.D1P0,D2P0,D3P0,and D4P0 represent the respective combinations of the sowing densities of 1.95×105,2.40×105,2.85×105,and 3.30×105 plants ha-1 with the application of 0 mg L-1 paclobutrazol;D1P1,D2P1,D3P1,and D4P1 represent the combinations of the same sowing densities with the application of 100 mg L-1 paclobutrazol.PAL,phenylalanine ammonia-lyase;CCR,cinnamoyl-CoA reductase;CAD,cinnamyl-alcohol dehydrogenase;POD,peroxidase;FPS,flower-pegging stage;PSS,pod-setting stage;PFS,pod-filling stage.

Fig.2.Effects of sowing density and Pbz application on stem lignin content in the 2018(A,B,C)and 2019(D,E,F)growing seasons.Vertical bars represent the standard error of the mean of three replicates(n=3).Different letters indicate significant differences among treatments,P＜0.05.D1P0,D2P0,D3P0,and D4P0 represent the respective combinations of the sowing densities of 1.95×105,2.40×105,2.85×105,and 3.30×105 plants ha-1 with the application of 0 mg L-1 paclobutrazol;D1P1,D2P1,D3P1,and D4P1 represent the combinations of the same sowing densities with the application of 100 mg L-1 paclobutrazol.

3.4.Stem puncture strength,stem breaking strength,and lodging percentage

Under the same Pbzapplication concentration,increasing sowing density reduced SPS and SBS(Fig.3).SPS and SBS under D2P0,D3P0,and D4P0were significantly(P＜0.05)lower than those under D1P0at the pod-setting stage in the 2018 growing season,which were decreased by 7.54%,13.36%,and 24.88% and 7.17%,28.09%,and 44.44%,respectively.The application of Pbzsignificantly(P＜0.05)increased SPS and SBS compared with without Pbzapplication at the same sowing density.Compared with D3P0,D3P1significantly(P＜0.05)increased SPS and SBS by 6.73% and 29.59%at the pod-setting stage in the 2018 growing season.In addition,SPS and SBS under D3P1were higher than those under D2P0at the podfilling stage in both growing seasons.

The results showed that lodging percentage was affected by the sowing density and Pbzapplication concentration(Fig.S2).Lodging percentage gradually increased with the increase of sowing density under the same Pbzapplication concentration.In addition,the application of Pbzdecreased lodging percentage relative to without Pbzapplication at the same sowing density.Moreover,D3P1decreased lodging percentage relative to D2P0in the 2019 growing season,and the lodging percentage under D4P1was also lower than that under D3P0.

3.5.Canopy light transmission ratio

The LTR of different canopy layers in the 2018 and 2019 growing seasons are shown in Fig.S3.Increasing sowing density decreased the LTR at the middle leaf strata and bottom leaf strata under the same Pbzapplication concentration.Compared with D1P0,the LTR at the middle leaf strata and bottom leaf strata under D2P0,D3P0,and D4P0were significantly(P＜0.05)decreased by 27.78%,39.08%,and 60.36% and 25.24%,54.26%,and 67.01% in the 2018 growing season,respectively.At the same sowing density,peanut plants treated with Pbzshowed significantly(P＜0.05)increased LTR at the middle leaf strata and bottom leaf strata relative to those without the application of Pbz.D3P1significantly(P＜0.05)increased the LTR at the middle leaf strata and bottom leaf strata compared with D3P0in both growing seasons,with an average increase of 37.07% and 60.41%,respectively.

3.6.Relative chlorophyll content and net photosynthetic rate

The results showed that SPAD values and Pndecreased with the increase of sowing density under the same Pbzapplication concentration in the 2018 growing season(Fig.4).SPAD values and Pnunder D3P0were significantly(P＜0.05)lower than those under D1P0at the pod-setting stage in the 2018 growing season,with a decrease of 4.50%and 6.97%,respectively.In addition,the application of Pbzincreased SPAD values and Pnrelative to without Pbzapplication at the same sowing density.Compared with D1P0,D2P0,D3P0,and D4P0,SPAD values and Pnunder D1P1,D2P1,D3P1,and D4P1significantly(P＜0.05)increased by 4.77%,6.91%,7.20%,and 4.31% and 4.47%,4.59%,4.08%,and 5.49% at the pod-setting stage in the 2018 growing season,respectively.Moreover,D3P1increased SPAD values and Pnrelative to D2P0at the pod-setting stage in both growing seasons,and SPAD values and Pnunder D4P1were also higher than those under D3P0at the pod-setting stage in the 2018 growing season.

Fig.3.Effects of sowing density and Pbz application on stem puncture strength and stem breaking strength in the 2018(A,B)and 2019(C,D)growing seasons.Vertical bars represent the standard error of the mean of three replicates(n=3).Different letters on bars at the same growth stage indicate significant differences among treatments,P＜0.05.D1P0,D2P0,D3P0,and D4P0 represent the respective combinations of the sowing densities of 1.95×105,2.40×105,2.85×105,and 3.30×105 plants ha-1 with the application of 0 mg L-1 paclobutrazol;D1P1,D2P1,D3P1,and D4P1 represent the combinations of the same sowing densities with the application of 100 mg L-1 paclobutrazol.FPS,flower-pegging stage;PSS,pod-setting stage;PFS,pod-filling stage.

3.7.Peanut yield and yield components

Whether with or without the application of Pbz,peanut pod yield showed a parabolic change with increasing sowing density(Fig.5A,B).The highest peanut yield under the application of Pbzoccurred when the sowing density was D3(2.85×105plants ha-1),whereas without the application of Pbzthe peanut yield was highest at the sowing density of D2(2.40×105plants).Compared with D2P0,D3P1increased peanut pod yield by 6.78% and 5.85% in the 2018 and 2019 growing seasons,respectively.These results indicated that the optimum sowing density was raised by the application of Pbz,because the yield components were significantly affected by the sowing density and Pbzapplication concentration(Table S2).The present study showed that the number of pods per plant decreased with the increase of sowing density,but the application of Pbzincreased the number of pods per plant at the same sowing density.Moreover,increasing sowing density increased the number of pods per kilogram,and the number of plants per hectare increased significantly(P＜0.05)with the increase of sowing density.However,the application of Pbzhad no significant effect on the number of plants per hectare.

3.8.Correlation analysis among plant architecture parameters,lodging percentage,and photosynthetic characteristics

The relationships among plant architecture parameters,lodging resistance,and lodging percentage are shown in Fig.S4.These results showed that lodging percentage was significantly(P＜0.05)positively correlated with PH and significantly(P＜0.01)negatively correlated with SD,SPS,SBS,and lignin content.The activities of PAL,CCR,CAD,and POD showed significantly positive correlations with lignin content(P＜0.01).Lignin content was significantly and positively correlated with SPS and SBS(P＜0.01).In addition,the linear regression models of lodging percentage on photosynthetic characteristics were obtained by regression analysis(Fig.S5).The present study indicated that lodging percentage was significantly(P＜0.01)negatively correlated both with SPAD values and Pn.In other words,a decrease in lodging percentage could enhance the photosynthetic capacity.

4.Discussion

4.1.Effects of sowing density and Pbz application on plant architecture and lodging resistance

Stem lodging,which limits the improvement of crop yield,is a common problem in many crops[24–26].The present studies[27,28]have indicated that lodging was correlated with plant architecture and stem mechanical strength.Plant architecture and stem mechanical strength can influence plant growth and development[29]and are regulated by sowing density and exogenous plant growth regulators[30,31].Researchers have proposed[32]that Pbzis an effective exogenous plant growth regulator that can regulate plant architecture and reduce lodging risk,especially at higher planting density.

Fig.4.Effects of sowing density and Pbz application on relative chlorophyll content and net photosynthetic rate in the 2018(A,B)and 2019(C,D)growing seasons.Vertical bars represent the standard error of the mean of three replicates(n=3).Different letters on bars at the same growth stage indicate significant differences among treatments,P＜0.05.D1P0,D2P0,D3P0,and D4P0 represent the respective combinations of the sowing densities of 1.95×105,2.40×105,2.85×105,and 3.30×105 plants ha-1 with the application of 0 mg L-1 paclobutrazol;D1P1,D2P1,D3P1,and D4P1 represent the combinations of the same sowing densities with the application of 100 mg L-1 paclobutrazol.FPS,flower-pegging stage;PSS,pod-setting stage;PFS,pod-filling stage.

Fig.5.Effects of sowing density and Pbz application on peanut pod yield in the 2018(A)and 2019(B)growing seasons.Vertical bars represent the standard error of the mean of three replicates(n=3).Different letters indicate significant differences among treatments,P＜0.05.D1P0,D2P0,D3P0,and D4P0 represent the respective combinations of the sowing densities of 1.95×105,2.40×105,2.85×105,and 3.30×105 plants ha-1 with the application of 0 mg L-1 paclobutrazol;D1P1,D2P1,D3P1,and D4P1 represent the combinations of the same sowing densities with the application of 100 mg L-1 paclobutrazol.

PH,FLBL,and HCG are the main factors that determine the architecture of peanut plants.In the present study,PH increased with increasing sowing density but decreased with the application of Pbz(Table 1).However,SD,SLD,and SSD decreased with the increase of sowing density but increased with Pbzapplication(Table 1).Thus,the lodging percentage was affected by the changes in plant architecture.Our results showed that lodging percentage was significantly and positively correlated with PH,but negatively correlated with SD,SPS,and SBS(Fig.S4).As a result,increasing sowing density improved the risk of lodging,but Pbzapplication reduced lodging percentage(Fig.S2),which is consistent with the findings of previous studies[33–35].

Additionally,SPS and SBS are the main stem mechanical strength traits of peanut plants and play an important role in enhancing lodging resistance,which depends on the content of chemical constituents such as lignin[36].In the present study,lignin content was significantly and positively correlated with SPS and SBS(Fig.S4).Our results also showed that lignin biosynthesis-related enzymes(PAL,CCR,CAD,and POD)activities,lignin content,SPS,and SBS all decreased with the increase of sowing density(Figs.1,2,and 3).Our results were similar to those of the previous study[23],in which an increase in sowing density resulted in a decrease in stem lignin accumulation and stem mechanical strength.The application of Pbzreduced the risk of lodging by increasing lignin accumulation and mechanical strength in maize stalks[37].Similarly in our study,peanut plants treated with Pbzshowed an increase in lignin content,SPS,and SBS relative to those without Pbzapplication(Figs.2 and 3).This increase was due mainly to the increased activities of lignin biosynthesis-related enzymes(Fig.1).More importantly,D3P1decreased PH,increased lignin content,and improved SPS and SBS relative to D2P0(Table 1;Figs.2 and 3).As a result,the present study showed that appropriately increasing sowing density in combination with the application of Pbzalso could decrease PH,promote lignin accumulation,and improve stem mechanical strength(Fig.6).

4.2.Effects of sowing density and Pbz application on photosynthetic characteristics

Sowing density and Pbzapplication play an important role in promoting the coordinated development of individuals and populations by optimizing plant architecture and regulating the transmission and effective utilization of light energy in canopy structures[38].Our results showed that LTR at both the middle and bottom leaf strata decreased with increasing sowing density(Fig.S3),possibly because the fraction of light intercepted by the plant increased with increasing planting density[39].The application of Pbzincreased LTR at the middle and bottom leaf strata at the same sowing density(Fig.S3).Pbzapplication reduced the elongation of the main stem and lateral branches,resulting in a shorter and more compact plant architecture.As a result,more light was transmitted to the middle and bottom leaf layers,increasing the photosynthetic capacity of leaves.Furthermore,the photosynthetic capacity of leaves can be affected by sowing density and Pbzapplication concentration.In the present study,SPAD values and Pndecreased with increasing sowing density(Fig.4),mainly because increasing sowing density reduced the allocation of leaf nitrogen to the photosynthetic apparatus[40].In a previous study[41],maize leaves treated with Pbzregulated photosynthetic capacity and antioxidant system and displayed higher chlorophyll contents and photosynthetic rate.The present study also reached the same conclusions,which showed that Pbzapplication increased SPAD values and Pn(Fig.4).LTR at the middle and bottom leaf strata,SPAD values,and Pnunder D3P1were higher than those under D2P0(Figs.S3 and S4).These findings suggested that appropriately increasing sowing density in combination with Pbzapplication could improve light distribution and photosynthetic capacity(Fig.6).

4.3.Effects of sowing density and Pbz application on peanut yield and yield components

Fig.6.Diagram illustrating the effects of sowing density combined with the application of Pbz on peanut plant architecture,lodging resistance,and photosynthetic characteristics.

To cultivate high-yielding peanut,it is important to construct a reasonable population structure,promote the coordinated development of individuals and populations,and improve the number and plumpness of pods[38,42].Peanut is an infinitely growing type of crop,which has the following biological characteristics:a long flowering period,a large number of flowers,and a high production potential of the individual plant[43].Sowing density and Pbzapplication can achieve a higher yield by cultivating strong individuals,constructing reasonable populations,and giving full play to the production potential of the individual plant,which may explore a new way for high-yielding cultivation of peanut.The application of Pbzcan control the excessive vegetative growth,enhance the photosynthetic capacity of leaves,and promote the distribution and transportation of photosynthetic products to the pods[21].Peng et al.[18]reported that wheat plants treated with 150 mg L-1Pbzshowed an increase in early tillers,and increased panicles number and grain yield.The application of 300 mg L-1Pbzincreased maize yield by delaying leaf senescence and regulating the antioxidant system during the grain-filling stage[41].In addition,the foliar spraying of Pbzwith a concentration of 120 mg L-1improved rice yield by increasing the length and weight of rice roots and improving the number of effective panicles and grain weight[44].Thus,the optimum application concentration and the effects of spraying Pbzvaried with plant type.

Plants number per unit area,pods number per plant,and pods number per kilogram are the main yield components of peanut.In the present study,peanut pod yield first increased and then decreased with the increase of sowing density(Fig.5A,B).A previous study also found that pod yield increased with increasing sowing density,but that excessive sowing density was not conducive to achieving higher pod yield[30].This is mainly because the higher number of plants cannot compensate for the decrease in pods number per plant and the increase in pods number per kilogram(the decrease in weight per pod).Interesting,the optimum sowing density was D2(2.40×105plants ha-1)when without Pbzapplication,but the optimum sowing density was raised to D3(2.85×105plants ha-1)under the application of Pbz(Table S2).Thus,the optimum sowing density was raised due to the application of Pbz.This is because the application of Pbzcan create the potential to increase peanut yield by optimizing plant architecture,enhancing lodging resistance,and improving photosynthetic capacity(Fig.6).Moreover,Pbzapplication can alter endogenous hormone levels in inflorescences,promoting flower development and grain setting[45,46].However,applying Pbzdecreased pod yield at the lower sowing density(D1,1.95×105plants ha-1).This is because Pbzapplication severely inhibited the growth of the main stem and lateral branch at the sowing density of D1(Table 1),which was not conducive to the transport of nutrients to pods and the accumulation of dry matter in pods.Therefore,the present study showed that D1P1increased the number of pods per kilogram(decreased weight per pod)relative to D1P0(Table S2).In addition,a full stand of seedlings,uniform emergence of seedlings,and strong seedlings are the basis for achieving a higher peanut yield.Therefore,precision seed selection and seed dressing with bactericidal and insecticidal coating agents are required for achieving higher yield under the SSPS mode[6].

5.Conclusions

A two-year field optimization experiment showed that sowing density and Pbzapplication influenced plant architecture,lodging resistance,photosynthetic characteristics,and peanut yield.In short,the application of Pbzdecreased PH and HCG,enhanced lodging resistance,and improved the photosynthetic capacity of leaves at the same sowing density.When the sowing densities were D2,D3,and D4,Pbzapplication increased pod yield compared with without the application of Pbz.However,increasing sowing density improved PH,decreased SPS and SBS,increased lodging percentage,and decreased the photosynthetic capacity of leaves under the same Pbzapplication concentration.In terms of yield,peanut pod yield showed a parabolic change trend with increasing sowing density.Pod yield was highest at the sowing density of D2(2.40×105plants ha-1)when without Pbzapplication,while the highest pod yield was obtained at the sowing density of D3(2.85×105plants ha-1)under the application of Pbz.Therefore,the optimum sowing density was raised due to the application of Pbz.In summary,our results suggested that a production strategy using a combination of the sowing density of 2.85×105plants ha-1and the application of 100 mg L-1Pbzunder the SSPS mode may thus be the optimal agricultural production practice for creating greater peanut production potential.

CRediT authorship contribution statement

Jihao Zhao:Writing–review & editing.Huajiang Lai:Investigation,Data curation,Resources.Chen Bi:Investigation,Resources.Mengjie Zhao:Investigation,Data curation.Yanling Liu:Investigation.Xiangdong Li:Conceptualization,Funding acquisition,Supervision,Writing–review & editing.Dongqing Yang:Conceptualization,Funding acquisition,Supervision,Writing–review &editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the National Key Research and Development Program of China(2020YFD1000902),the Shandong Key Research and Development Program(2018YFJH0601-3),the Major Agricultural Applied Technological Innovation Projects in Shandong Province(SD2019ZZ11),and the Shandong Modern Agricultural Technology and Industry System(SDAIT-04-01).

Appendix A.Supplementary data

Supplementary data for this article can be found online at https://doi.org/10.1016/j.cj.2022.05.012.