Seedling defoliation of cereal crops increases peanut growth and yield in an intercropping system

Bo Huang, Xiaojin Zou, Huasen Xu, Jiayi Xu, Huiyu Liu, Wentao Sun, Liang Gong, Shiwei Niu,Liangshan Feng, Ning Yang, Lizhen Zhang, Zhanxiang Sun,*

a College of Land and Environment, Shenyang Agricultural University, Shenyang 110866, Liaoning, China

b Center of International Cooperation Service, Ministry of Agriculture and Rural Affairs, Beijing 100125, China

c Institute of Plant Nutrient and Environmental Resources, Liaoning Academy of Agricultural Sciences, Shenyang 110161, Liaoning, China

d Beijing Key Laboratory of Biodiversity and Organic Farming, College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China

Keywords:Plant height Strip intercropping Corn Growth Root

ABSTRACT Intercropping cereals and legumes is practiced widely in the world for improving yields and economic benefits. Shorter legume crops in intercropping are shaded by taller cereals, substantially reducing legume growth and yield. Reducing shade in intercropping by shortening the plant height of cereals by seedling defoliation has been proposed as a practical approach to increase crop yields and land productivity.A two-year field experiment was conducted to investigate the effect of defoliation of cereal crops at seedling stage on the growth and yield of peanut(Arachis hypogaea L.)intercropped with corn(Zea mays L.)or millet(Setaria italica L.).In comparison with non-defoliation controls,defoliation reduced final plant height by 29 cm on average for corn and 18 cm for millet.Photosynthetically active radiation on peanut in intercropping systems with corn or millet intercropping was respectively 27.0% and 22.8% higher than those in controls, significantly improving the light environment of intercropped peanut. Net photosynthetic rates of peanut were on average 25.5% higher in corn and peanut intercropping and 19.6% higher in millet and peanut intercropping than those in non-defoliation controls.Total biomass of intercropped peanut increased owing to increased root growth. Across two years, yield of peanut intercropped with corn was 27.7% and with millet 32.8% higher than those of controls. Defoliation of cereal crops did not affect corn yield but significantly decreased millet yield by 24.5%.Our results suggest that applying seedling defoliation in intercropped corn could increase peanut yield without compromising corn yield in an intercropping system.

1. Introduction

A major oilseed crop,peanut(Arachis hypogaea L.)is widely cultivated in China,on a total area of 4.7 million hectares[1],and has become one of the main cash crops in semi-arid regions.However,the yield of sole-cropped peanut suffers increased disease susceptibility [2], the so-called ‘‘consecutive monoculture problem”,which can lead to yield losses of up to 70% in long-term peanut monoculture. To solve the problem, intercropping is regarded as an effective and sustainable solution.Cereal and peanut intercropping shows diverse advantages in improving yields and environment by reducing wind erosion, and is widely practiced by small household farmers in semi-arid areas such as northeastern China[3,4].Corn(Zea mays L.)and millet(Setaria italica L.)are also widely grown in northeastern China.However,intercropping cereals with legumes has a drawback: the yield of shorter crops (legumes) is compromised by shading by taller ones (cereal).

Shading by cereal crops, such as corn, induces light stress by reducing by up to 31% the incident radiation on the neighboring legume[5].Shading by taller corps in millet and peanut intercropping reduced peanut yield from 30.8% to 50.2% in northeastern China from 2017 to 2020.In a two-year study,shading from neighboring corn reduced the yield of intercropped soybean by 44.3%in comparison with sole soybean [6]. Intercropping could reduce shaded-legume yields as high as 50% [6,7,8]. A three-year field experiment in semi-arid regions of China found that peanut yield decreased from 30.9% to 40.3% under intercropping with corn [9].Most legume crops were affected by shading from taller crops in intercropping, especially during germination and flowering stages[7,10]. Shading reduces the growth rate and numbers of flowers,pods,and nodules of intercropped legumes,and also impairs plant root growth and nutrient acquisition [11,12]. A study of corn and soybean intercropping in 2014–2015 showed that shaded soybeans had 31.0%–51.9% fewer pods per plant, reducing their yield by 50.6%–65.6% relative to a sole stand [13]. Shading accelerated flower abscission and delayed crop maturity, reducing final pod number and grain yield [7,14]. Peanut when intercropped with corn had 2.4%–33.0% less final aboveground dry matter and 16.0%–49.3% lower leaf area index than sole peanut. With an increase in duration and degree of shading, the negative effects on dry matter accumulation increased [15]. Shading is the main factor that limits the yield of intercropped legumes[5].Agronomic attempts have been made to alleviate yield losses of intercropped legumes by minimizing shading effects of taller crops on the legume[16].However,most of these attempts involved modifying the spatial and temporal niche differentiation of intercropped crops by optimizing row configuration and sowing window [17],with only a few studies focusing on modifying the height of the cereal canopy.

Researchers [18,19] have found that plant height of corn could be reduced by defoliation at seedling stage. Defoliation was first used on corn,wheat,and barley to assess yield loss by hail damage,and has been practiced for more than 100 years [16]. As early as the 19th century,crop tops were mown to feed livestock,followed by experiments with corn leaf removal [20]. Although the experiments on defoliation resulted in reductions in corn yield [20],Crookston and Hicks [21] found that crop yield after defoliation at early stage was increased. However, this knowledge obtained from sole cropping is rarely applied to intercropping and might benefit shaded crops.

Given that a previous study[18]found that plant heights of two corn hybrids were lowered by 34–37 cm after defoliation at seedling stage,we hypothesized that legume yield losses could be alleviated by less shading from cereals in intercropping systems. The objectives of this study were to (i) quantify the effect of corn and millet defoliation at seedling stage on growth and yields of intercropped crops;and to(ii)determine whether defoliation of the taller cereals could increase peanut yield by alleviating the shading caused by its neighbors.

2. Materials and methods

2.1. Experimental design and treatments

The field experiment followed a randomized complete block design with three replicates.Location of the experiment was Fuxin Long-term Observation and Experiment Station (42°06′N,121°75′E), Zhangwu, Liaoning, China. The climate is a temperate continental monsoon, with yearly mean air temperature of 7–8 °C. During the crop growth season from May to September, the total sunshine hours are 1200–1300 h,and the cumulated temperature above 10 °C is 2900–3400 °C. The frost-free period is 135–165 days.The yearly precipitation is 391 mm in the past 30 years.The soil type is sandy loam with a pH of 6.31,total N of 0.77 g kg-1,available P of 18.9 g kg-1,and available K of 63.1 g kg-1,within the 0–20 cm soil layer.The plot size was 48 m2(6 m×8 m).Plots were 1 m apart. Defoliation of corn and millet was performed manually at 3 cm plant height at 30 days after germination when corn and millet had 5 fully expanded leaves. The experimental treatments were: (1) no defoliation of corn (CK-CP), (2) defoliation of corn at 5-leaf stage by removal of the top three leaves(D-CP),(3)no defoliation of millet (CK-MP), and (4) defoliation of millet at 5-leaf stage by removal of the top three leaves (D-MP). All cropping was by strip intercropping following local farmer practice, with two rows of corn or millet and two rows of peanut in each strip.Row spacing for all crops was 50 cm in all treatments.Either cereal or peanut occupied 50%of land area in both intercropping systems.Corn(Jinboshi 825)was sown on May 9 and harvested on October 11 in 2019, and sown on May 8 and harvested on October 10 in 2020. Millet (Jingu 4) and peanut (Xiaobaisha 1016) were sown on May 21 and harvested on September 16 in 2019, and sown on May 22 and harvested on September 17 in 2020.The homogeneous plant densities of corn,millet and peanut were respectively 67,500,380,000, and 220,000 per ha. The experiment was rainfed without irrigation.Only basal fertilizer was applied,comprising 192 kg ha-1-N,120 kg ha-1P2O5,and 120 kg ha-1K2O,and other farming practices followed local farmer practice.

2.2. Measurements of photosynthesis, root characteristics, dry matter,and N, P, K concentrations

Photosynthetic parameters of peanut, namely net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2concentration (Ci), and transpiration rate (Tr), were measured with a portable photosynthesis system, LI-6400 (Li-COR, Lincoln, NE,USA)from 9:30 AM to 11:30 AM on a sunny day at flowering stage in both years. Five consecutive peanut plants were measured in each plot using the top fully expanded leaves of the canopy. Each measurement was repeated three times to minimize instrumental errors.

Photosynthetically active radiation (PAR) above the peanut canopy at seedling stage was measured on a sunny day between 9:00 AM and 12:00 AM, at week 2 and week 5 after defoliation in both years, with a linear quantum meter (AccuPAR LP-80 Ceptometer; Decagon Devices, Pullman, WA, USA). A grid sampling method was used,with the canopy divided into five vertical layers and horizontal grids 25 cm wide each from the middle of the cereal rows to the peanut rows.Five PAR readings were recorded at each position. Each measurement was repeated three times.

The monolith root sampling method of Wang et al. [22] was shown to be more suitable than the auger core method in cereal and peanut intercropping. For each plot, three peanut and three cereal root monoliths were collected (monolith size was 0.6 ×0.6 × 0.6 m for peanut and 1.0 × 1.0 × 1.0 m for corn or millet).Root samples were rinsed in tap water and adhering soil particles were removed with brushes. The root samples were then scanned with a root-scanning instrument (LAD2400, Sintek, Shanghai,China). Images were processed with WinRhizo (Sintek) to yield total root length, diameter, and surface area. Means of three root samples per plot were calculated.

In both years,aboveground parts of three peanut and three cereal plants per plot were sampled at harvest to determine aboveground dry matter. Samples were separated into stems and leaves. Root samples for measuring dry weight were monoliths as described above. All above- and belowground samples were oven-dried for 1 h at 105°C and then for 72 h at 75 °C to constant weight. Samples of peanut were then ground to fine powder for measurement of total N, P, and K concentrations for crop nutrient acquisition analysis [23].

2.3. Statistical analysis

The ANOVA analysis was performed using SAS 8.2 (SAS, Cary,NY, USA). The defoliation treatment and intercropping pattern were entered as fixed factors,while year and block(replicate)were entered as random factors. A one-way ANOVA was used to estimate the effects of defoliation on the yields of crops, number and weight of peanut root nodules, available PAR, photosynthesis (Pn,Gs, Ci, and Tr) and root morphological traits of peanut. Tukey post-hoc tests(P ＜0.05)were performed to determine the statistical significance of difference between means.

3. Results

3.1. Plant heights and yields

The plant heights of both corn and millet decreased significantly after defoliation in comparison with non-defoliation treatments (Fig. 1). The height difference of corn at harvest time was 29 cm across two years. The height difference of millet was 21.5 cm before harvest and 18.5 cm after harvest in 2019–2020.

Fig. 1. Plant height of corn and millet in intercropping with or without defoliation. CKCP refers to intercropping of peanut and corn without defoliation; DCP refers to intercropping of peanut and defoliated corn; CKMP refers to intercropping of peanut and millet without defoliation; DMP refers to intercropping of peanut and defoliated millet.

There was no significant difference in corn yield between defoliation and non-defoliation treatments in 2019,but defoliated corn showed 7.94%higher yield than the non-defoliated control in 2020.The yield of defoliated millet was 24.5%lower than that of the nondefoliation control(Fig.2).In comparison with the non-defoliation treatment, the aboveground dry matter of both intercropped corn and millet was significantly decreased after defoliation at seedling stage.

3.2. PAR and photosynthesis of peanut plant

The PAR transmittance of the peanut canopy was significantly improved after defoliation of the taller crops in both intercropping systems. In comparison with non-defoliation treatments, PAR at the top of the peanut canopy under defoliation of intercropped corn increased by 27.9% in 2019 and 25.2% in 2020 (Fig. 3). The PAR at the top of the peanut canopy under defoliation of intercropped millet increased by 22.8% across both years.

Net photosynthetic rates of peanut in intercropping were 25.5%(with corn)and 19.6%(with millet)higher under defoliation of taller crops than in non-defoliation controls across two years(Table 1). Stomatal conductance and Trshowed similar trends to Pn. There was no significant difference in Cibetween defoliation and non-defoliation treatments. All photosynthesis parameters of intercropped peanut showed no differences between two the intercropping systems.

Table 1 Net photosynthetic rate(Pn),stomatal conductance(Gs),intercellular CO2 concentration(Ci),and transpiration rate(Tr)of peanut intercropped with defoliated corn or millet,2019 and 2020.

Table 2 Root morphological traits of intercropped crops under different defoliation treatments in 2019–2020.

Fig.2. Aboveground dry matter and yields of intercropped corn and millet in 2019 and 2020. CKCP refers to non-defoliation in corn and peanut intercropping, DCP refers to defoliating intercropped corn,CKMP refers to non-defoliation in millet and peanut intercropping, and DMP refers to defoliation of intercropped millet.Different lowercase letters indicate significant difference between defoliation treatments within the same intercropping system and year at P ＜0.05.

Fig.3. Effect of defoliation of cereal crops on photosynthetically active radiation(PAR)at the top of peanut canopy when peanut was intercropped with corn(A)or millet(B).CKCP refers to non-defoliation in corn and peanut intercropping, DCP refers to defoliating intercropped corn, CKMP refers to non-defoliation in millet and peanut intercropping and DMP refers to defoliation of intercropped millet.Different lowercase letters indicate significant difference between defoliation treatments within the same intercropping system and year at P ＜0.05.

3.3. Peanut root and nodule growth

Total root length and surface area per plant of corn and millet decreased significantly under defoliation treatments in comparison with non-defoliation controls in both years. Plant root length and surface area of peanut in defoliation treatments in the mixing with corn were significantly increased by respectively 22.1% and 22.4%in comparison with non-defoliation controls and by 33.9% and 23.6% in the mixture with millet (Table 2). In comparison with non-defoliation controls, peanut intercropped with defoliation showed significantly increased nodule numbers by 93.1% (with corn)and 36.3%(with millet)(Fig.4),whereas no significant difference in nodule weight was found between the defoliation and nondefoliation treatments.

3.4. Peanut nutrient acquisition and grain and biomass yields

In comparison with non-defoliation controls,peanut in the mixture with corn under defoliation of corn and millet showed significantly increased N, P, and K acquisition (Fig. 5). Intercropping peanut with defoliated corn captured 28.1% more N, 19.8%more P and 24.1% more K in comparison with the non-defoliation control, while peanut in the mixture with defoliated millet captured 34.0% more N, 45.9% more P and 35.2% more K.

Fig.4. Nodule number and weight of peanut roots in intercropping with corn(A)and millet(B).CKCP refers to non-defoliation in corn and peanut intercropping,DCP refers to defoliating intercropped corn, CKMP refers to non-defoliation in millet and peanut intercropping, and DMP refers to defoliation of intercropped millet. Different lowercase letters indicate significant difference between defoliation treatments within the same intercropping system and year at P ＜0.05.

Fig. 5. Nutrient acquisition of peanut in intercropping under different defoliation treatments in 2019–2020. Different lowercase letters indicate significant difference between defoliation treatments within the same intercropping system and year at P ＜0.05.

In comparison with non-defoliation controls, the aboveground dry matter of peanut intercropped with defoliated corn and millet increased by 22.1%and 29.5%for both years,respectively(Table 3).No significant difference in shelling percentage was observed.However, peanut intercropped with defoliated corn and millet showed 21.1% and 23.3% more seed pods than those in nondefoliation controls(Table 3).Peanut intercropped with defoliated corn showed 7.49%higher 100-seed weight and 9.45%higher seedhull ratio than those from the control, while peanut intercropped with defoliated millet showed 8.46% higher 100-seed weight and 4.39% higher seed-hull ratio than those from the control. Peanut intercropped with defoliated corn and millet showed on average 27.7%and 32.8%higher yields than those from the controls in both years.

Table 3 Effect of defoliation of cereal crops on the yield of peanut in intercropping in 2019–2020.

4. Discussion

4.1. Does defoliation of corn and millet at seedling stage affect its yields in intercropping?

Defoliation effects on crop yield vary widely in mono-cropping and depend on species,cultivar,defoliation time,and environment[24]. A two-year experiment conducted in Hebei province, China,showed that defoliation could lead to a yield loss of up to 4.6%–21.7% [29]. Crookston and Hicks [21] reported that corn yield varied from -14% to +53% across a 2-year period. Corn yields can range from -8% to +13% under defoliation before 5th leaf stages(V5) [25]. Corn yields could drop nearly 22% in some situations but increase by 48% in others across different hybrids under defoliation at V3–V4 stages [21,26–28]. However, a recent study [25]showed no evidence for yield loss under defoliation at the 9-leaf stage in Kentucky, US. In a study conducted in Yunnan, China, all of 12 barley cultivars showed significantly reduced plant height,grain yield, and biomass under defoliation [29], whereas a twoyear study conducted in the Loess Plateau of China showed that defoliation did not affect wheat yield in dry seasons, but reduced biomass and grain yields in wet seasons [30]. Another two-year experiment performed in Sichuan province of China reported that paddy rice yield decreased 34% after defoliation in comparison with controls[31].In our study,defoliating millet at seedling stage reduced crop yield but defoliating corn increased yield in the intercropping system. The positive effects on intercropping corn under seedling defoliation suggested that this practice would be useful in intercropping systems, not only for improving corn yield but also for alleviating the negative effect of shade on understory species.

4.2. Could early defoliation of tall cereals increase the yield of understory crops in intercropping?

Peanut yield was increased by early defoliation of the tall cereals (corn and millet) in the intercropping experiment, a result attributed to positive interspecific interaction both above and below ground[33],which mainly due to the reduction of the shading and the increase of the light interception of peanut[34,35].Our results supported previous studies that PAR at the top of the peanut canopy under defoliation treatments increased by 26.6% in intercropping with corn and 22.8%with millet.Because of the alleviation of light competition, peanut net photosynthetic rate increased,thus resulting in more biomass[2,36,37].Similar results were found by Raza et al. [38] in corn and soybean intercropping.

In comparison with no-defoliation controls,the root length and surface area of peanut increased when it was intercropped with defoliated corn and millet.This finding was as expected,given that reduction by defoliation of crop root growth, root length, and surface area of cereals decreased after defoliation in both years, in agreement with a previous study[32].However,some researchers[19,39,40]reported that defoliation at seedling stage could improve corn root growth and development after defoliation.Belowground interspecific interactions can increase crop yields by complementarity and facilitation effects,resulting in more efficient use of soil resources including N and P [33,41–44]. In our study, negative effects on root growth of corn and millet defoliation at the early stage could have promoted the root growth and nutrient (N, P,and K)acquisition of shaded peanut,resulting in a yield increase.

4.3. Economic benefits of early defoliation of tall cereals in intercropping

In corn and peanut intercropping, when the cereal plants underwent early defoliation, the yield of both corn and peanut increased in present study, especially for peanut because alleviating the negative effect of shading. Because in local markets,the price of peanut is much higher than that of corn, an increase of peanut yield by 548 kg ha-1(based on Table 3 and the price of current market) increases farmer profit. After accounting for the cost of renting a defoliation machine and other overhead costs, the net profit of corn and peanut intercropping can reach 6172 RMB Yuan per ha. However, the net income of millet and peanut intercropping under early defoliation is negative.

5. Conclusions

In comparison with non-defoliation, early defoliation of cereal crops increased the yield of intercropped peanut without compromising corn yield but reducing millet yield. The increased yield of intercropped peanut resulted partially from increased root development that allowed peanut to acquire more nutrients and increased its photosynthesis, and partially from increased light interception achieved by shortening the plant height of neighboring corn or millet. Our results provide strong evidence that early defoliation of corn in the intercropping can increase crop yield.Especially in corn and peanut intercropping, defoliation of corn at the seedling stage could be an effective agronomic technique for increasing productivity and farmer income.

CRediT authorship contribution statement

Bo Huang:Writing-original draft.Xiaojin Zou:Funding acquisition,Writing-original draft.Huasen Xu:Data curation.Jiayi Xu:Software.Huiyu Liu:Investigation.Wentao Sun:Methodology.Liang Gong:Writing - review & editing.Shiwei Niu: . Liangshan Feng:Funding acquisition.Ning Yang:Investigation.Lizhen Zhang:Writing - review & editing.Zhanxiang Sun:Supervision.

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 work was funded by National Key Research and Development Program of China (2016YFD0300202), China Agriculture Research System(CARS-08-G09),National Natural Science Foundation of China(32071551),and Liao Ning Revitalization Talents Program (XLYC1907089).