Determination of the post-anthesis nitrogen status using ear critical nitrogen dilution curve and its implications for nitrogen management in maize and wheat

Determination of the post-anthesis nitrogen status using ear critical nitrogen dilution curve and its implications for nitrogen management in maize and wheat

European Journal of Agronomy 113 (2020) 125967 Contents lists available at ScienceDirect European Journal of Agronomy journal homepage: www.elsevier...

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European Journal of Agronomy 113 (2020) 125967

Contents lists available at ScienceDirect

European Journal of Agronomy journal homepage: www.elsevier.com/locate/eja

Determination of the post-anthesis nitrogen status using ear critical nitrogen dilution curve and its implications for nitrogen management in maize and wheat

T

Ben Zhaoa,*, Xiaoli Niub, Syed Tahir Ata-Ul-Karimc,**, Laigang Wangd, Aiwang Duana, Zhandong Liua, Gilles Lemairee a Key Laboratory of Crop Water Use and Regulation, Ministry of Agriculture, Farmland Irrigation Research Institute, Chinese Academy of Agricultural Sciences, 380 Hongli road, Xinxiang, Henan 453003, PR China b College of Agricultural Equipment Engineering, Henan University of Science and Technology, Luoyang, Henan 471000, PR China c Key Laboratory of Soil Environment and Pollution Remediation, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, Jiangsu, 210008, PR China d Institute of Agricultural Economics and Information, Henan Academy of Agricultural Sciences, Zhengzhou 450002, PR China e INRA Centre Poitou- Charentes Les Verrines CS80006, 86600 Lusignan, France

A R T I C LE I N FO

A B S T R A C T

Keywords: Ear Critical nitrogen concentration Nitrogen nutrition index Ear nitrogen accumulation Crop nitrogen management

Nitrogen (N) accumulation in plant reproductive organs during the post-anthesis growth phase of maize and wheat plays a crucial role in the formation of grain yield and quality. However, little is known about the effect of crop pre-anthesis and post-anthesis N status on ear N accumulation (NAE). This study endeavored to extend the crop N dilution theory already developed for vegetative growth period to determine ear critical N concentration (%NcE) during post-anthesis period of crop growth for analyzing the difference of NAE under various N levels. The data including the weight of dry mass (W) and N concentration of entire plant and ear, post-anthesis plant N uptake (PANU) from soil, grain number (GN), and grain weight (GW) were collected on wheat (two cultivars) and maize (three cultivars) from eight N rates (0–300 kg N ha−1) field experiments. The results revealed that the process of %N dilution exists in ear and it is plausible to extend the concept of %NcE curve till crop post-anthesis period. The %NcE curves as function of ear dry mass (WE) of wheat (%NcE = 2.85WE-0.17) and maize (% NcE = 2.22WE-0.26) were lower than those developed in maize and wheat on whole plant basis. This study revealed that the ear has the potential to diagnose ear N status under different N conditions and the increases in ear N nutrition index (NNIE) during the post-anthesis period with increasing N rate were well synchronized with plant NNI (NNIp) at anthesis. GN and GW of maize and wheat showed significantly positive correlation with NNIp at anthesis and NNIE at maturity under N-limiting treatments, and GN and GW could keep relatively stable under non-N limiting treatments. NNIp and NNIE showed the potential capacity to predict GN and GW of maize and wheat under N limiting condition. Ear critical N accumulation (NAcE) was calculated using ear Nc curve to investigate the differences mechanism of NAE under different N conditions. The difference of NAcE under different N treatments was deduced from the pre-anthesis N status of maize and wheat by determining GN. The ear N deficiency (NDE) between NAcE and NAE was co-regulated by plant pre-anthesis and post-anthesis N status, which in turn have potential to explain the variance of GW at maturity in both crops. The significantly attenuated effect of pre-anthesis N deficiency on ear potential N demand in maize and wheat indicated that the postanthesis N management must consider the pre-anthesis N status and the corresponding reduction of the postanthesis N input to prevent N loss under N limiting treatment in both crops. Maize was more dependent on postanthesis N status while wheat was more reliant on pre-anthesis N status for satisfying ear growth and producing optimum GN owing to the differential values of PANU/NAE in maize and wheat during post-anthesis period. This study provides a new viewpoint on post-anthesis N management of maize and wheat for enhancing N use efficiency and grain yield.



Corresponding author. Corresponding author. E-mail addresses: [email protected] (B. Zhao), [email protected] (S.T. Ata-Ul-Karim).

⁎⁎

https://doi.org/10.1016/j.eja.2019.125967 Received 11 May 2019; Received in revised form 28 October 2019; Accepted 29 October 2019 1161-0301/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

fertilization decision of for increasing yield and grain protein content (Ata-Ul-Karim et al., 2016, 2017c). The injudicious N diagnosis might results in declined yield and quality (plant N deficiency) or higher risk of crop lodging (plant N excess) and also excess of N flows to atmosphere and hydrosphere. Considering the centrality of ear growth during the post-anthesis period of crop, Plénet and Lemaire (1999) established the relationship between ear N concentration and ear W in maize. Yet, no attempt has been made to develop ear %Nc dilution curve to test the applicability of crop N dilution theory to diagnose the N status during post-anthesis period in maize across different N conditions. Besides, the independent data from different regions is required to validate the applicability of existing ear %Nc dilution curve of wheat developed in the eastern China by Zhao et al. (2016a). It is indispensable to establish the relationship between ear N status and plant post-anthesis N status for confirming the validity of crop N diagnosis during post-anthesis period of maize and wheat. The pre-anthesis N deficiency has been reported to significantly decrease ear N accumulation (NAE) by reducing the formation of grain number (GN) in wheat (Abbate et al., 1995; Demotes-Mainard et al., 1999; Duan et al., 2018), but little report was about the effect of pre-anthesis N deficiency on GN and NAE in maize. The attempt has not been made to investigate the impact of post-anthesis N status on NAE in maize and wheat at present, which might be related to grain weight (GW). Therefore, this study endeavored to develop ear %Nc dilution curve in maize, to validate the developed ear %Nc curve of maize and the existing ear %Nc curve of wheat (Zhao et al., 2016a), and to analyze the dynamic difference of NAE in relation with crop N status (pre-anthesis and post-anthesis) resulting from different level of N supply for maize and wheat using ear Nc dilution curves, and to analyze the influence of crop N status on GN and GW in maize and wheat. This approach aims at providing crop N diagnostic tools for guiding N management during the post-anthesis period of wheat and maize.

Nitrogen (N) is the most vital nutritional element for crop productivity and is highly mobile within the plant during crop growth process. During the vegetative period of crop, plant N uptake (PNU) is stored and distributed into different organs (leaves and stems), and the amount of PNU gradually increases with progress of crop growth (Greenwood et al., 1990). A stable allometric relationship between PNU and the weight of plant dry mass (W) under different N conditions has been previously confirmed by Salette and Lemaire (1981). Based on this relationship, plant N concentration showed a declining trend with the accumulation of crop dry mass, which can be seen as N dilution process in crop (Salette and Lemaire, 1981; Lemaire and Salette, 1984). The N dilution process during the pre-anthesis growth period of crop can be considered as the consequence of the plant allometry between its metabolic compartment with high N concentration associated to leaf area expansion and structural compartment with low N concentration associated to plant architecture and hydraulic conductivity as reviewed recently by Lemaire et al. (2019). According to the difference of crop N dilution process and the accumulation of plant W across different N treatments, plant critical N (%Nc) dilution curve has been developed in various crops including maize, wheat, barley and rice (Justes et al., 1994; Plénet and Lemaire, 2000; Ata-Ul-Karim et al., 2013, 2017a, Zhao, 2014), which have confirmed that the plant %Nc dilution curve can well diagnose plant N status during crop pre-anthesis period under different N treatments. Yet, there is no clear conclusion whether plant %Nc dilution curve can be used to diagnose plant N status during crop post-anthesis period at present. When maize and wheat enters into the reproductive period, the expansion of plant leaf area stops and the stored N of vegetative organ (metabolic and structural compartments) translocate to the reproductive organ (storage compartment) for grain filling, and post-anthesis plant N uptake (PANu) from soil gradually decreases due to the root senescence (Asseng et al., 2017; Bonelli et al., 2016), which results in the change of the N source-sink relationship of maize and wheat. Plant vegetative organs and soil both serve as the sources of N supply during the reproductive period of maize and wheat growth (Plénet and Lemaire, 1999), while plant reproductive organs served as the sink of N storage. The entire plant N dilution process might potentially be then modified due to the addition of a new sink corresponding to N storage that is used during grain filling and the addition of a new source by N remobilization from senescing leaves during post-anthesis period. The new N source-sink relationship during the post-anthesis period seems not to be very pronounced to modify N dilution process at whole-plant level as compared to that during pre-anthesis period (Reed et al., 1988; Coor et al., 1997; Herrmann and Taube, 2004). Yet, this change may be progressively more relevant with N dynamic process at the level of the reproductive organ during the post-anthesis period of crop. Consequently, it is imperative that the N dynamic process of the reproductive organ should be more explicitly analyzed. It is long-established by Plénet and Lemaire (1999) that ear N concentration gradually decreases with the accumulation of ear W during the post-anthesis period. This decline could be seen as a dilution process of ear N accumulation (NAE) based on W, which corresponds to the different proportion of grain protein (N) and starch (C) accumulation indispensable for ear weight accumulation process associated to cell division and cell expansion. Zhao et al. (2016a) also confirmed the conclusion of Plénet and Lemaire (1999) and developed an ear %Nc dilution curve of wheat. Maize and wheat are very important grain crops in China (Liu and Ju, 2003). The N status of maize and wheat poses significant effect on the formation of yield (Jeuffroy and Bouchard, 1999; Ciampitti and Vyn, 2012). Excessive rainfall events can lead to N losses via leaching or denitrification from maize and wheat fields and limit soil N availability, consequently, resulting in increased risk of post-anthesis N stress (Nasielski et al., 2019). Therefore, the accuracy of crop N diagnosis during post-anthesis period is of prime significance for fine-tuning of N

2. Materials and methods 2.1. Experimental design Eight multi-N fertilization rate field experiments were conducted during 2015–2017 growing seasons at Xinxiang (35°18′N, and 113°52′E) and Qinyang (35°08′N, and 112°92′E) situated in North China using two winter wheat and three summer maize cultivars. A detailed description of experiment designs and weather conditions are shown in Table 1 and Fig.1. The size of each plot was 60 m2 (6 m × 10 m) and 30m2 (6 m × 5 m) in maize and wheat experiments, respectively. N fertilizer in both crops was applied before sowing (50 %) and at the jointing stage (50 %). Adequate amounts of phosphorus and potassium fertilizers were applied in all experiments. The planting density was of 6.5–7.5 × 104 plant ha−1 and of 18–22 × 105 plant ha-1 for maize and wheat experiments, respectively. The other crop management practices were in accordance with local production practices. There was no obvious water, pest and disease stress observed during the entire crop growth period. N fertilizer application was the only limiting factor in all experiments. 2.2. Sampling and measurement Six plants per plot were harvested at anthesis stage and 10, 25, 40, and 55 days after anthesis in maize, while an area of 0.4 m2 per plot was harvested in wheat at anthesis and 7, 14, 21, and 28 days after anthesis and separated into the stems (leaf sheath), leaves, and ears (grain). The samples were then dried at 70℃ until constant weight to determine the corresponding dry mass. The samples after weighing were ground and passed through a 1-mm sieve, and stored in plastic bags for N concentration determination using Kjeldahl method (Bremner and Mulvancy, 1982). N accumulation of each plant organ was calculated as the product of W and %N from different organs. At maturity, wheat 2

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Table 1 Main characteristics of the eight field experiments. Experiment

Site

Year

Crop

N rate (kg N ha−1)

Cultivar

Soil total N* (g kg−1)

Sowing date

Soil type

Sampling stages

1

Xinxiang

2015

Maize

Silking+10day Silking+25day Silking+40day Silking+55day

2015

Maize

3

Xinxiang

2016

Maize

4

Qinyang

2016

Maize

0-90-180-270

DH605

1.14

5

Qinyang

2017

Maize

0-100-200

1.12

6

Xinxiang

2017

Maize

0-90-180-270

Dingyou919 (DY919) DH605

7

Qinyang

2016

Wheat

0-80-160-240

1.15

8

Xinxiang

2017

Wheat

0-100-200-300

Yumai58 (YM58) Aikang58 (AK58)

June 10 June 10 June 12 June 12 June 15 June 15 October 13 October 15

light loam

Xinxiang

Zhengdan958 (ZD958) Denghai605 (DH605) ZD958

1.03

2

0-75-150-225300 0-75-150-225300 0-90-180-270

0.94 0.98

1.05

1.04

light loam light loam light loam sandy light loam sandy

Anthesis+7day Anthesis+14day Anthesis+21day Anthesis+28day

light loam

* Soil total N indicate the sum of the organic and inorganic fraction of soil nitrogen, which is measure by the method of dry combustion (Lu, 1999).

plant were harvested in an area of 1 m2 and maize plant were harvested in 8 m length of two rows from each plots. Ear at maturity were separated and threshed to measure grain yield, GW and GN.

This curve was developed at level of soil area for crop post-anthesis N diagnostic, therefore, %NcE is ear critical N concentration, WE is the weight of ear dry mass per hectare (t ha−1), parameter a is ear %Nc when WE = 1 t ha−1 and parameter b is a dimensionless coefficient, indicating the rate of decline in %NcE with the progression of WE accumulation. Based on ear %Nc dilution curve, N nutrition index of ear (NNIE) was calculated as follow:

2.3. Ear critical nitrogen dilution curve and nitrogen nutrition index If we consider the ear growth and development at post-anthesis period as an individual organ with its own dry mass (WE) and N concentration (%NE), then it is possible to determine the minimum ear N concentration for getting the maximum WE and to derive an ear %Nc dilution curve according to the methodology proposed by Justes et al. (1994). By using the same formalism as for the %Nc dilution curve for whole plant (Lemaire et al., 2008), this ear %Nc dilution curve can then be written as follow: %NcE = a(WE)−b

NNIE=%NE/%NcE

(2)

where %NE is the ear N concentration. NNIE < 1 would indicate that ear growth is limited by N supply, NNIE = 1 would indicate that ear N status is non-limiting for the ear development, and NNIE > 1 would indicate that luxury N is accumulated in ears. Alternatively, Eq (1) can be transformed by multiplying its two variables by WE, giving the critical N accumulation in ear (NAcE) as a

(1)

Fig. 1. Monthly mean air temperture (℃) and precipitation (mm) during 2015 to 2017 2015–2017 experimental years at Xinxiang and Qinyang (A: 2015 Xingxiang; B: 2016 Xinxiang; C: 2017 Xinxiang; D: 2015 Qinyang; E: 2016 Qinyang; F: 2017 Qinyang). 3

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function of ear WE: NAcE = 10a(WE)1−b

(3)

According to Eq (3), it is possible to estimate the quantity of NAcE during post-anthesis period of maize and wheat. The optimum NAcE in ear is imperative for its maximum growth. 2.4. The related nitrogen index of plant and ear at post-anthesis stage of maize and wheat Parameters referring to whole plant and ear N status after anthesis were determined as follows: Post-anthesis plant N uptake (PANu, g N m2) = Plant N accumulation at maturity (g N m2) -Plant N accumulation at anthesis (g N m2) Ear N accumulation (NAE, g N m2) = Ear N concentration at maturity (%) × Ear dry mass at maturity (g m2) Ear N deficiency (NDE, g N m2) = Ear critical N accumulation at maturity (g N m2) - Ear actual N accumulation at maturity (g N m2) Difference of ear critical N accumulation between different N treatments (Difference of NAcE, g N m2) = Ear critical N accumulation under high N treatment at maturity (g N m2) - Ear critical N accumulation under low N treatment at maturity (g N m2)

Fig. 2. The critical %N values used to define the ear %Nc curve of maize. The solid line represents ear %Nc dilution curve of maize while the dotted lines represent the distribution of ear dry mass and %Nc values among different N treatments at different sampling dates which allow calculating the ear %Nc points. The horizontal error line represented the standard error of the weight of ear dry mass, and the vertical error line represented the standard error of ear % N and %Nc value.

2.5. Statistical analysis Experiment data groups acquired from eight field experiments were analyzed using one-way analysis of variance function (F test) of SPSS software package (SPSS Inc., Chicago, IL, USA) to compare the statistical difference of WE, plant and ear N related indices as well as yield components across different N treatments. The significance level for all hypotheses test was designed as P < 0.05. The univariate linear regression model (y=Ax+B) of SPSS software was used to analyze the relationships between crop N status, plant and ear N related indices, and yield components. The negative power model was used to develop the relationship between WE and %NcE.

(Table 3). Due to the effect of plant pre-anthesis N status, the NNIE values during post-anthesis period were higher under higher N application treatments in both crops and were significantly influenced across different growth stages, growing seasons, and cultivars (Fig. 3). The independent experimental data (DY919 and DH605, Exp. 5 and 6 of maize; YM58 and AK58, Exp. 7 and 8 of wheat) were used to calculate the NNIE values of maize and wheat to validate the suitability of the developed %NcE curve of maize (Fig.2) and the existing %NcE curve of wheat (Zhao et al., 2016a). The NNIE values were ranged from 0.67 to 1.24 (Fig. 3, A and B) and 0.6–1.18 (Fig. 3, C and D) in maize and wheat, respectively. According to the calculation method of NNIE (Eq.2), three different types of ear N status could be discriminated using the %NcE curve across different cultivars of maize and wheat. For instance, during 2017 seasons of wheat (AK58, Fig.3D), NNIE values of N0 and N100 treatments were ranged from 0.75 to 0.94 (lower than 1) indicating that N was limiting ear growth., while for N200 treatments, NNIE values were ranged from 0.97 to 1.07 (approximately around 1) indicating that N was not limiting ear growth. Moreover, for N300 treatment, the NNIE values were ranged from 1.1 to 1.18 (all higher than 1) indicating that excessive N exist in the ear. Additionally, the grain yields of both crops were used to validate the diagnosis result of the %NcE curve across different N treatments and cultivars. The grain yield of maize and wheat increased with the increasing of NNIE, however, this increase was not significant when NNIE value of wheat was higher than one (Fig. 3, C and D). This also indicated that the %NcE curve can well diagnose ear N status of maize and wheat under different N and cultivars conditions (Supplementary Fig. A,). Besides, the strong synchrony between ear and plant N status in maize and wheat was shown using NNIE at post-anthesis +10 days and + 7 days with NNIp at anthesis on maize and wheat, respectively (Fig. 4, exp. 1–8). When NNIp was equal to one, NNIE of maize was 0.83, and NNIE of wheat was 0.95. Ear N status of wheat was higher than that of maize for the same NNIp values. The linear relationship between NNIE and NNIp indicated that the linear model accounted for 97 % and 92 % of the variance across different cultivars of maize and wheat, respectively. The two linear models betweeen NNIE and NNIP were compared according to the methodology proposed by Mead and Curnow (1983). The results of comparison showed that there was significant difference between maize and wheat (Fcalculated = 21.19 > Ftabulated (1–31) = 4.16, α = 0.05). Therefore, a

3. Result 3.1. Determination of ear critical nitrogen curve in maize and wheat The %NcE curves in maize (Eq. 4) and wheat (%NcE = 2.85WE−0.17; Zhao et al. (2016a) were developed following the computation method of Justes et al. (1994). In this study, 12 out of 16 data groups fulfilled the statistical criteria and were used to evaluate %NcE values during post-anthesis period of maize (ZD958 and DH605, Exp. 1–4). The %NcE values were determined by the intercept between oblique and vertical lines through the selected data groups during post-anthesis period of maize. The %NcE value decreased with the increasing WE, which can be fitted by a power function (Fig.2). It was represented by the following equation: %NcE = 2.22WE−0.26 R2 = 0.89**

(4)

where %NcE is the critical N concentration in the ear and WE is the weight of ear dry mass in t ha−1 of maize. 3.2. Changes of plant and ear nitrogen nutrition index across varied nitrogen treatments Plant N status was determined by plant NNI (NNIp; Lemaire et al., 2008). The NNIp at anthesis represented plant N status during pre-anthesis period of maize and wheat and was calculated using plant W based %Nc curves of maize (%Nc = 3.4W−0.37; Plénet and Lemaire, 1999) and wheat (%Nc = 5.35W-0.44; Justes et al., 1994). The NNIp values were ranged from 0.65 to 1.13 for maize (Exp. 5–6) and from 0.68 to 1.25 for wheat (Exp. 7–8) across different N treatments 4

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Fig. 3. Changes of ear nitrogen nutrition index and harvested yield for maize (YD919 and DH605) and wheat (YM58 and AK58) at post-anthesis stage under varied nitrogen rates (A and B: Exp.5 and 6 of maize; C and D: Exp.7 and 8 of wheat). Vertical bars represent the values of least significance difference at each N treatment.

square meter in maize and wheat, respectively. The GN of maize was much lower than that of wheat. Further, NAcE across different N treatments were ranged from 9.22 to 14.08 g N m2 and 9.21–16.65 g N m2 for maize and wheat, respectively. The values of NAcE increased with the increasing GN of maize and wheat-based on pre-anthesis N status (Table 2). The differences of GN were more obvious than that of NAcE values between maize and wheat. Plant NNI values of maize and wheat at anthesis under low N (N0) treatments were ranged from 0.65 to 0.76 and the value of NAcE/GN were varied from 3.1–3.33 and from 0.38 to 0.42 for maize and wheat, respectively. In contrast, NNIp values at anthesis for maize and wheat were from 1.13 to 1.27 under the higher N (more than 225 kg N ha−1) treatments having the NAcE/GN values ranging from 4.12 to 4.17 and 0.43 to 0.47 for maize and wheat, respectively. At the same N input condition (300 kg N ha-1) of maize (Exp. 2) and wheat (Exp. 8), the NNIP value (1.27) at anthesis of maize was very close with the NNIp value (1.25) at anthesis of wheat. However, the NAcE/GN value (4.17) of maize was far greater than that (0.47) of wheat (Table 2). The differences of NAcE/GN values exists between maize and wheat due to the higher GN of wheat than maize.

Fig. 4. Relationship between ear nitrogen nutrition index (NNI) at post-anthesis period and plant NNI at anthesis of maize (Exp. 1–6, ZD958, DH605, DY919) and wheat (Exp. 7–8, YM58, AK58). Plant NNI of maize was calculated based on the curve (%Nc = 3.4W−0.37) of Plénet and Lemaire (1999), and plant NNI of wheat was calculated based on the curve (%Nc = 5.35W-0.44) of Justes et al. (1994).

3.4. Use of ear nitrogen nutrition index for analyzing crop N dynamics during post-anthesis period

unified model was not used to represent the relationship between NNIE and NNIP on maize and wheat (Fig. 4).

Post-anthesis plant N uptake, NAE and GW were related to NNIE and were significantly affected by different N treatments and increased with the increasing N application rates (Table 3). NNIE at maturity of maize and wheat under different N treatments were ranged from 0.54 to 1.21 of maize (Exp. 1–6) and from 0.59 to 1.12 of wheat (Exp. 7–8) and were used to estimate post-anthesis N status of ear. NAE were ranged from 4.43 to 14.12 g N m2 and 6.12–19.04 g N m2 for maize and wheat, respectively. The NNIE at maturity (1.12) was the same at the Exp. (1) of maize and Exp. (8) of wheat, but the NAE value (19.04) of wheat was

3.3. Changes of ear critical nitrogen accumulation and grain number in maize and wheat at maturity based on the pre-anthesis N status Ear NAc and GN of maize and wheat were affected by plant preanthesis N status. NNIp at anthesis, GN, and NAcE all increased with the increasing N application rate on maize and wheat (Table 2). The GN was ranged from 2.7 to 3.38 × 103 per m2 and 22.46–36.52 × 103 per 5

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Table 2 The one-way analysis of variance function (F test) of plant nitrogen nutrition index at anthesis, ear critical nitrogen accumulation and grain number under different nitrogen treatments at maturity of maize and wheat within each experiment (Exp.1–8) Crop

Experiment

N treatment (kg N ha−1)

Plant NNI at anthesis

Grain number (×103 m2)

NAcE at maturity (g N m2)

NAcE /GN

Maize

Exp 1

N0 N75 N150 N225 N300 N0 N75 N150 N225 N300 N0 N90 N180 N270 N0 N90 N180 N270 N0 N100 N200 N0 N90 N180 N270 N0 N80 N160 N240 N0 N100 N200 N300

0.68e 0.82d 0.92c 1.05b 1.19a 0.72e 0.88d 0.98c 1.14b 1.27a 0.76d 0.88c 1.03b 1.18a 0.69d 0.87c 1.07b 1.23a 0.74c 0.89b 1.08a 0.65d 0.81c 0.94b 1.13a 0.74d 0.85c 0.96b 1.15a 0.68d 0.86c 1.08d 1.25a

2.82b 3.08ab 3.23a 3.3a 3.2a 2.74b 3.08ab 3.23a 3.26a 3.38a 2.9b 3.12a 3.3a 3.26a 2.7b 3a 3.18a 3.23a 2.97b 3.17ab 3.4a 2.86b 3.1ab 3.21a 3.34a 24.23c 32.36b 34.58a 36.52a 22.46c 28.85b 32.55a 35.25a

9.52d 10.82c 11.52b 12.86a 14.25a 9.22e 10.04d 10.38c 12.16b 14.08a 9.65c 10.15b 10.96a 12.48a 9.36d 10.42c 11.84b 13.46a 9.32c 10.42b 11.76a 8.86d 9.32c 11.45b 13.75a 9.21c 13.74b 15.07a 15.68a 9.45c 13.24b 15.78a 16.65a

3.38d 3.51c 3.57c 3.9b 4.45a 3.36c 3.26c 3.21c 3.73b 4.17a 3.33b 3.25b 3.32b 3.83a 3.47c 3.47c 3.72b 4.16a 3.13c 3.28b 3.46a 3.1c 3c 3.57b 4.12a 0.38b 0.42a 0.44a 0.43a 0.42b 0.46a 0.48a 0.47a

Exp 2

Exp 3

Exp 4

Exp 5

Exp 6

Wheat

Exp 7

Exp 8

Different letters behind the numbers indicate that significance at P ≤ 0.05.

average NAcE value corresponding to the maximum value of NAcE. The diagonal red line (1:1) in Fig. 5 represented NAE values under different N treatments. It was possible to visualize the difference of NAcE for each limiting N treatment as compared to the non-limiting N conditions as represented by the arrow (1) in Fig. 5. The difference of NAcE gradually decreased with the increasing NAE. The maximum difference of NAcE was observed between the N0 treatments and non-N limiting treatments, which was 4.88 g N m2 and 7.33 g N m2 of maize and wheat, respectively. The difference of NAcE value was lower for maize than that of wheat. Moreover, the arrow (2) represented the NDE between ear NAcE and NAE across different N treatments and allowed the estimation of the NDE as compared to its potential due to the deficiency in ear N status at post-anthesis stage of maize and wheat. The maximum NDE between ear NAcE and NAE (4 g N m2) of maize was higher than that (2.49 g N m2) of wheat. The relationships established between PANu and NAE showed a significantly positive correlation of maize and wheat (Fig. 6A), and the regression model was stable across different cultivars of maize and wheat, respectively. The X intercept (2.59 g N m2 for maize and 3.44 g N m2 for wheat) exemplified the minimum N accumulation in ear. As the X intercept of the model between PANu to NAE was different with 0, the value of PANu/NAE increased with increasing NAE (Fig. 6B). The contribution of PANu became imperative to NAE under non-N limiting condition. The linear-plateau regression model could well represent the relationships of GN with NNIp and NNIE across different cultivars and N treatments of maize and wheat (Fig. 7). GN of maize and wheat under N limiting treatments were significantly and positively correlated with NNIP at anthesis (Fig. 7, A and B). The linear part of this regression

higher than that (13.45) of maize at the same NNIE condition. The GW of maize (315.2–389.5 g) were obviously higher than that of wheat (30.5–38.4 g) under different N treatments (Table 3). Post-anthesis plant N uptake was a component of NAE in maize and wheat in complement with the quantity of N translocated into ear from leaves and stems. The trends of PANu were similar to that of NAE across different N treatments and increased with the increasing N application rates (Fig. B, supplement material online). The value of PANu/NAE, representing that the contribution of de novo N absorption to NAE, is higher for high N treatments than that under lower N treatments (Table 3). NNIE at maturity was lower than 0.7 for both species under low N treatments and corresponded to PANu within the range of 1.53–1.91 g N m2 for maize and 1.42–1.68 g N m2 for wheat (Fig. C, supplement material online). These quantities represented 34%–36% of NAE for maize and 23%–26% of NAE on wheat. In contrast, under the higher N treatments, PANu was ranged from 5.42 to 7.06 g N m2 and from 5.86 to 6.22 g N m2 for maize and wheat, respectively, which represented 40%–50% for maize and 31%–36% of NAE for wheat, respectively. The value of PANu/ NAE of wheat (0.31, Exp. 8) was lower as compared with maize (0.48, Exp. 1) at the same NNIE (1.12) condition. The relationships established between NAE and NAcE for maize and wheat depicted the significant positive correlation. NAcE increased with the increasing NAE at maturity (Fig. 5), and the regression models were stable across different cultivars of maize and wheat. The linear model of maize and wheat accounted for 87 % and 96 % of the variance, respectively. The slope of the linear model for maize was lower than that for wheat. In contrast, the intercept of the linear model for maize was higher than that of wheat. The horizontal red line in Fig. 5 indicated the 6

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Table 3 The one-way analysis of variance function (F test) of ear nitrogen nutrition index, post-anthesis plant nitrogen uptake, ear actual nitrogen accumulation and thousand-grain weight under different nitrogen treatments at maturity of maize and wheat within each experiment (Exp.1–8) Crop

Experiment

N treatment

Ear NNI at maturity

PANU (g N m2)

NAE (g N m2)

PANU/ NAE

Thousand-grain weight (g)

Maize

Exp 1

N0 N75 N150 N225 N300 N0 N75 N150 N225 N300 N0 N90 N180 N270 N0 N90 N180 N270 N0 N100 N200 N0 N90 N180 N270 N0 N80 N160 N240 N0 N100 N200 N300

0.57e 0.76d 0.85c 0.94b 1.12a 0.65e 0.81d 0.91c 1.04b 1.21a 0.71d 0.84c 0.92b 1.07a 0.54d 0.75c 0.95b 1.12a 0.65c 0.81b 0.98a 0.54d 0.75c 0.86b 1.02a 0.62d 0.76c 0.91b 1.04a 0.59d 0.78c 0.95b 1.12a

1.53d 2.83c 4.42b 5.43a 6.22a 1.62e 3.05d 4.65c 5.86b 7.06a 1.86d 3.03c 4.95b 6.01a 1.91d 2.76c 4.83b 5.96a 1.75c 3.51b 5.02a 1.86c 3.32b 4.82a 5.42a 1.42c 2.93b 5.46a 6.22a 1.68c 2.75b 5.13a 5.86a

4.43d 6.85c 9.85b 12.45a 13.45a 4.83e 7.03d 10.56c 12.25b 14.12a 5.13d 7.23c 11.35b 13.24a 5.26d 7.72c 11.05b 13.12a 5.07c 7.12b 11.85a 5.23d 7.65c 11.43b 13.56a 6.12c 10.48b 15.83a 17.65a 6.58c 11.53b 17.15a 19.04a

0.35 b 0.41a 0.45 a 0.44 a 0.48 a 0.34 b 0.43a 0.44 a 0.48a 0.50a 0.36 b 0.42 a 0.44a 0.45a 0.36 b 0.36 b 0.44a 0.45 a 0.35 b 0.49 a 0.42 ab 0.36 b 0.43 a 0.42 a 0.40 ab 0.23 b 0.28 b 0.34 a 0.36 a 0.26 b 0.24 b 0.30 ab 0.31 a

325.4c 345.2b 367.3a 375.2a 389.2a 332.4c 348.5b 374.2a 389.5a 387.1a 315.2c 347.5b 372.1a 376.5a 325.6c 347.2b 375.8a 386.5a 308.3c 342.5b 368.5a 317.8c 348.5b 367.4a 374.2a 30.5c 32.4b 35.4a 36.2a 31.3c 34.5b 37.5a 38.4a

Exp 2

Exp 3

Exp 4

Exp 5

Exp 6

Wheat

Exp 7

Exp 8

Different letters behind the numbers indicate that significance at P ≤ 0.05.

model accounted 78 % and 84 % variance of GN using NNIP at anthesis when it was lower than 0.94 and 0.97 for maize and wheat, respectively. The plateau part of this regression model confirmed that GN does not increase under non-N limiting treatments. The performance of the NNIE and GW relationship was slightly better than NNIp and GN relationship under N limiting treatments. 83 % and 86 % variance of GW were explained using NNIE at maturity based on the linear stage of the

linear-plateau regression model across different cultivars of maize and wheat. There was also a plateau phenomenon on GW under non-N limiting treatments, GW would not increase when NNIE at maturity was higher than 1.02 and 1.04 on maize and wheat, respectively (Fig.7, C and D).

Fig. 5. Relationship between ear critical nitrogen accumulation (NAcE) and ear actual nitrogen accumulation (NAE) across different N treatments on maize (Exp. 1–6,ZD958, DH605, DY919) and wheat (Exp. 7–8, YM58, AK58). The horizontal red line indicated the average NAcE value corresponding to the maximum value of NAcE. The diagonal red line (1:1) represented NAE values under different N treatments. The arrow (1) visualize the difference value of NAcE for each limiting N treatment as compared to the non-limiting N conditions. The arrow (2) represents the N deficiency value between NAcE and NAE across different N treatments and allowed the NDE estimation as compared to its potential (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 7

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Fig. 6. Relationship between post-anthesis plant nitrogen uptake (PANu) and ear actual nitrogen accumulation (NAE, Fig. 6A) and PANU/NAE and NAE (Fig. 6B) across different N treatments at post-anthesis stage of maize (Exp. 1–6, ZD958, DH605, DY919) and wheat (Exp. 7–8, YM58, AK58).

4. Discussion

wheat under the same NAE condition. The differences of coefficient a in this study were in agreement with the previous report by Greenwood et al. (1990) that C4 crops need less N to produce the same W as compared to C3 crops at the plant level owing to the differences of photosynthetic N use efficiency between C3 and C4 crops. Conversely, the higher value (0.26) of coefficient b in maize than that (0.17) of wheat indicated that the process of N dilution in maize ear was more important than that of wheat. This difference between species should be linked to the intrinsic proportion of C and N accumulated in grain during grain filling process according to starch vs protein ratio (Plénet and Lemaire, 1999). The longer post-anthesis period of maize (55 days after anthesis) than that (28 days after anthesis) of wheat resulted in prolonged duration for WE accumulation in maize. This prolonged

4.1. Comparison with the existing critical nitrogen dilution curves of maize and wheat This study validated the existed %NcE curve of wheat (Zhao et al., 2016a) and the developed %NcE curve of maize in this study (Eq. 4). The coefficients a (2.22) and b (0.26) of the ear %NcE curve developed for maize (Fig. 2) were very close to those obtained (a = 2.3 and b = 0.25) under non-N limiting condition by Plénet and Lemaire (1999). The coefficient a represented the %NcE value when WE 1 t ha−1. The coefficient a (2.22) value of maize was lower than that (2.85) of wheat (Fig.8). The WE accumulation of maize was higher than that of

Fig. 7. Relationship between grain number and plant nitrogen nutrition index (NNI) at anthesis (A and B) and between grain weight and ear NNI at maturity (C and D) across different N treatments on maize (Exp. 1–6, ZD958, DH605, DY919) and wheat (Exp. 7–8, YM58, AK58). 8

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grain filling period. The higher value of PANu/NAE of maize as compared with wheat during post-anthesis period (Table 3) encouraged the longer retaining ability of canopy to stay-green and optimize plant N status during post-anthesis period of maize (Gaju et al., 2014). 4.2. Implication of pre-anthesis and post-anthesis N status to crop N management The whole plant and ear %Nc dilution curves could be used to diagnose crop pre-anthesis and post-anthesis N status (Fig. 3 and 4). Crop N status was characterized using NNI based on %Nc dilution curve in this study. Crop N status during pre-anthesis period mainly affect GN at the level of soil area. Jeuffroy and Bouchard (1999) showed the effect of plant N deficiency on GN for wheat using the integrated NNI (NNIint; duration and intensity of NNI) and revealed that NNIint can explain 96 % of the variation in GN of wheat. Plénet and Cruz (1997) reported that GN was highly correlated to NNIint estimated during the period from seedling to 20 days after silking of maize. Our result also validated this conclusion using NNIp at anthesis in wheat and maize grown in China (Fig. 7, A and B). Masle (1981a, b) confirmed that early N deficiency stops tillering appearance and growth of developing tillers during crop pre-anthesis period, thereby reducing GN per square meter. Late N deficiency does not affect tillers appearance and growth per square meter during crop pre-anthesis period. However, it results in decreased GN per ear due to the decreased survival of differentiated flowers (Abbate et al., 1995). Demotes-Mainard et al. (1999) indicated that the GN was determined by W and %N in the ears at anthesis of wheat. Further, Duan et al. (2018) showed that the %N of non-ear organs had a higher R2 value with GN compared with W of non-ear organs from booting to anthesis of wheat. Plant N deficiency always existed during the pre-anthesis period of maize and wheat under N limiting treatments in this study (Table 2). The pre-anthesis N deficiency in wheat not only resulted in lower tillering and their growth but also reduced the survival of differentiated flowers per square meter under the N limiting condition (Table 2). Therefore, the decline in GN of wheat was very obvious due to the N deficiency during its pre-anthesis growth period. However, pre-anthesis N deficiency does not have the similar negative influence on GN of maize due to its growth characteristic. There is no tiller formation during the growth process of maize. Consequently, the early N deficiency has little impact to GN formation in maize. GN is particularly sensitive to late N deficiency from silking to 20 days after silking on maize (Nasielski et al., 2019). Our study also confirmed that the potential ear N demand of maize and wheat was determined by GN (Abbate et al., 1995; Demotes-Mainard et al., 1999; Duan et al., 2018). The NAcE was the indicator of the potential N demand essential for ear growth and development under different N conditions. The value of NAcE/GN represented the potential N demand of pre ear under different N conditions at the level of soil area. The potential N demand was lower under N limiting treatments as compared with non-N limiting treatments (Table 3). Therefore, the determined GN of the pre-anthesis N status seemed to be an N demand sink which control the NAcE during post-anthesis period of maize and wheat. When maize and wheat entered into the post-anthesis phase, the NAE resulted in the difference of ear N status (Fig. 5). NNIE could be used to represent ear N status during post-anthesis period of maize and wheat (Fig. 3). The NAE increased with the increasing NNIE at maturity, which gradually reduced the difference between NAE and ear potential N demand with the increasing rate of N fertilizer application for satisfying ear potential growth. Due to the ear potential N demand determined by GN, the NAE was considered to be co-regulated by GN and ear potential growth (Plénet and Lemaire, 1999). Additionally, GW showed a strong correlation with NNIE at maturity on maize and wheat when NNIE was lower than one (Fig.7, C and D). GW was dependent on ear N status during post-anthesis period of maize and wheat under N limiting conditions and the higher NAE could produce more GW of the ear (Zhu et al., 2009). When NNIE was higher than one, GW would not

Fig. 8. Comparison of different tissue critical nitrogen (%Nc) curves on maize and wheat. The symbol ○ represents the ear %Nc curve of maize (% NcE = 2.22WE−0.26) based on ear dry mass (W) in China. The symbol ◇ represents the ear %Nc curve of wheat (%NcE = 2.85WE-0.17) based on ear W in China. The symbol × represents the %Nc curve of winter wheat (%Nc = 5.35W0.44 ) based on plant W in France (Justes et al., 1994). The symbol △ represents the %Nc curve of maize (%Nc = 3.4W-0.37) based on plant W in France (Plénet and Lemaire, 1999).

period of WE accumulation resulted in obvious N dilution phenomenon in maize (Yue et al., 2012, 2014), which resulted in higher PANu in maize as compared to wheat at maturity (Table 2 and 3). The %Nc at ear level gradually decreased with the progression of crop growth and followed the similar trends as that in case of %Nc at plant level (Fig. 8). The decline in ear %N during post-anthesis period was attributed to crop N dilution process (Lemaire et al., 2007; Zhao, 2014). PANu from soil ceased gradually and N remobilization was the major source of N supply to ear for grain filling. The %N decline of glume and stalk during post-anthesis period was attributed to the translocation of N nutrition of glume and stalk to grain for yield and protein formation (Gaju et al., 2014). Consequently, values of the coefficient a (2.22 and 2.85) of ear based curves were lower than those of (3.4 and 5.35) plant-based curves in maize and wheat, respectively. The value of plant %Nc was higher when tissue W was lower than 10 t ha−1 (Fig. 8, Plénet and Lemaire, 1999; Zhao et al., 2017) and was attributed to the allometric relationship between PNu and plant W accumulation, which existed during the pre-anthesis period of maize and wheat (Lemaire and Gastal (1997);Lemaire et al., 2008). Conversely, when plants attained W of approximately 10 t ha−1 in maize and wheat, they entered in post-anthesis stage with gradual decline in PNU due to the stop of leaf area expansion, the differences between plant and ear % Nc gradually diminish with the growth process of maize and wheat (Fig. 8). On the other hand, the level of ear %N dilution (0.17 and 0.26) in maize and wheat was not as marked as that on whole plant basis (0.37 and 0.44). Compared with plant %N dilution, the slower decline of ear %N was attributed to the higher translocation of N from metabolic (leaf) and structural (stem) components to storage component of plants (ear) to ensure yield and protein formation in maize and wheat. The positive relationships between NNIE and NNIp during post-anthesis period of maize and wheat (Fig. 4) confirmed the possibility of extending the concept of %Nc dilution curve to diagnose plant N status during the post-anthesis period of maize and wheat and was in agreement with the previous report on maize (Herrmann and Taube. 2004). Ear N status well-represented plant N status under varied N conditions during post-anthesis period of maize and wheat. There was a synchronic change of N status between plant and ear during post-anthesis phase, which indicated that ear growth results in N storage and serve as a sink (Barraclough et al., 2014; Yuan et al., 2017). The lower NNIp (0.83) of maize than that (0.95) of wheat for NNIp equal to one might be related with the difference of PANu between two crop species. Maize needed to absorb more N from soil than wheat for satisfying ear N demand during 9

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wheat. Owing to the higher contribution of plant NU within NAE, maize required the relatively high level of soil N supply during its post-anthesis stage. This study was conducted to understand ear N demand and optimize crop post-anthesis N management for increasing N use efficiency.

increase with N application rate, but the ear %N continue to increase under non-N limiting treatments, which is more important for grain quality (Zhao et al., 2016a). The slope of the regression between GW and NNIE at maturity was 123.98 and 14.34 on maize and wheat, which imply that a reduction of NNIE (0.1) at maturity lead to a reduction of the thousand GW of 123.98 and 14.34 g in maize and wheat. Owing to the higher PANu for grain filling in maize, the post-anthesis NDE had more obvious effect on GW of maize than wheat. Mi et al. (2000) compared that the effect of different ear sizes on PANu of wheat and reported that the cultivar with large ears has high potential for PANu. Analysis of ear N status during the post-anthesis period of maize and wheat made it possible to determine various characteristics of ear N nutrition. The decrease in GW at maturity could be deduced from the decline of ear N status during the entire post-anthesis period, which is controlled by the post-anthesis source/sink ratio (Hisse et al., 2019). Consequently, NNIE seemed to be a simple index which is very useful for the GW prediction of maize and wheat under various N conditions. Besides, NNIp was already shown to be a good indicator for the prediction of GN, radiation interception, radiation use efficiency, grain yield and quality, and nitrogen requirement of different crops (Bélanger et al., 1992; Jeuffroy and Bouchard, 1999; Salvagiotti and Miralles, 2008; Yuan et al., 2016; Zhao et al., 2016b; Ata-Ul-Karim et al., 2016, 2017b, c). The integrated analysis of crop pre-anthesis and post-anthesis N status showed that the determination of crop post-anthesis N status is closely linked with crop pre-anthesis N status owing to the effect of crop pre-anthesis N status on crop GN that control the ear potential N demand. The decline of ear potential N sink under N limiting condition indicated that ear require less N to satisfy the demand for its potential growth and grain filling. Therefore, when plant N deficiency affected GN formation of crop at pre-anthesis stage in the agricultural production, the N management of crop post-anthesis needed to reduce the unnecessary N input for preventing N loss into soil and atmosphere (Raun and Gordon, 1999). Although crop post-anthesis N demand had the same regularity under N limiting treatment, the post-anthesis N management between different crops had some differences for satisfying grain filling. The contribution of PANu within NAE was higher for maize than for wheat (Fig. 6). Maize need to absorb more soil N for satisfying ear growth during post-anthesis period. It indicated that the top-dressing N value for maize is higher than that for wheat under the similar N condition for N management during post-anthesis period.

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. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Acknowledgments This study was supported by the National Natural Science Foundation of China (51609247, 51709263), the China Agriculture Research System (CARS-02, CARS-3-1-30), the Central Public-interest Scientific Institution Basal Research Fund (Farmland Irrigation Research Institute, CAAS, FIRI2017-04), the National Key R&D Program of China (No. 2016YFD0300609), the China Postdoctoral Science Foundation (2018M630617), the Chinese Academy of Science (2018PC0067). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.eja.2019.125967. References Abbate, P.E., Andrade, F.H., Culot, J.P., 1995. The effects of radiation and nitrogen on number of grains in wheat. J. Agric. Sci. (Camb.) 124, 352–360. Asseng, S., Kassie, B.T., Labra, M.H., Amador, C., Calderini, D.F., 2017. Simulating the impact of source-sink manipulations in wheat. Field Crops Res. 202, 47–56. Ata-Ul-Karim, S.T., Yao, X., Liu, X., Cao, W., Zhu, Y., 2013. Development of critical nitrogen dilution curve of Japonica rice in Yangtze River reaches. Field Crops Res. 149, 149–158. Ata-Ul-Karim, S.T., Liu, X., Lu, Z., Yuan, Z., Zhu, Y., Cao, W., 2016. In-season estimation of rice grain yield using critical nitrogen dilution curve. Field Crops Res. 195, 1–8. Ata-Ul-Karim, S.T., Zhu, Y., Lu, X.J., Cao, Q., Tian, Y.C., Cao, W., 2017a. Estimation of nitrogen fertilizer requirement for rice crop using critical nitrogen dilution curve. Field Crops Res. 2017, 32–40. Ata-Ul-Karim, S.T., Liu, X., Lu, Z., Zheng, H., Cao, W., Zhu, Y., 2017b. Comparison of different critical nitrogen dilution curves for nitrogen diagnosis in rice. Sci. Rep. 7, 42679. Ata-Ul-Karim, S.T., Zhu, Y., Cao, Q., Rehmani, M.I.A., Cao, W., Tang, L., 2017c. In-season assessment of grain protein and amylose content in rice using critical nitrogen dilution curve. Eur. J. Agron. 90, 139–151. Barraclough, P.B., Lopez-Bellido, R., Hawkesford, M.J., 2014. Genotypic variation in the uptake: partitioning and remobilization of nitrogen during grain-filling in wheat. Field Crops Res. 156, 242–248. Bélanger, G., Gastal, F., Lemaire, G., 1992. Growth analysis of a tall fescue sward fertilized with different rates of nitrogen. Crop Sci. 32, 1371–1376. Bremner, J.M., Mulvancy, C.S., 1982. Nitrogen-total. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soil Analysis, Part 2-Chemical and Microbiological Properties. American Society of Agronomy, Madison, pp. 595–624. Bonelli, L.E., Monzon, J.P., Cerrudo, A., Rizzalli, R.H., Andrade, F.H., 2016. Maize grain yield components and source-sink relationship as affected by the delay in sowing date. Field Crops Res. 198, 215–225. Ciampitti, I.A., Vyn, T.J., 2012. Physiological perspectives of changes over time in maize yield dependency on nitrogen uptake and associated nitrogen effificiencies: A review. Rev. Interpretation 133, 48–67. Demotes-Mainard, S., Jeuffroy, M.-H., Robin, S., 1999. Spike dry matter and nitrogen accumulation before anthesis in wheat as affected by nitrogen fertilizer: relationship to kernels per spike. Field Crop Res. 64, 249–259. Duan, J., Wu, Y., Zhou, Y., Ren, X., Shao, Y., Feng, W., Zhu, Y., Wang, Y., Guo, T., 2018. Grain number responses to pre-anthesis dry matter and nitrogen in improving wheat yield in the Huang-Huai Plain. Sci. Rep. 8, 7126–7135. Gaju, O., Allard, V., Martre, P., Le Gouis, J., Moreau, D., Bogard, M., Hubbart, S., Foulkes, M.J., 2014. Nitrogen partitioning and remobilization in relation to leaf senescence, grain yield and grain nitrogen concentration in wheat cultivars. Field Crop Res. 155, 213–223. Greenwood, D.J., Lemaire, G., Gosse, G., Cruz, P., Draycott, A., Neeteson, J.J., 1990. Decline in percentage N of C3 and C4 crops with increasing plant mass. Ann. Bot. 66,

5. Conclusion The present study developed an ear %Nc dilution curve (% NcE = 2.22WE−0.26) of maize. The ear %Nc dilution curves of maize and wheat were validated using the independent experimental data, and their performance was acceptable. NNIE increased with the increasing N application. The significantly positive relationships between NNIE and NNIp in maize and wheat exhibited the strong synchrony between plant and ear N status. Plant %Nc dilution curve was still applicable during the post-anthesis period for diagnosing the N status of maize and wheat. The significantly positive relationships between plant (ear) NNI and yield components were observed for maize and wheat under N limiting treatments. The proposed relationships allowed plant (ear) NNI to predict yield components of maize and wheat subjected to various N fertilizer strategies. The significantly positive relationships between NAE and NAcE were developed based on ear %Nc dilution curve on maize and wheat. Moreover, the differences between NAE and NAcE were calculated between supra-optimal and sub-optimal N treatments were determined by GN, which in turn was related with the pre-anthesis N status of maize and wheat. NDE was co-regulated by the preanthesis and post-anthesis N status of maize and wheat. The post-anthesis N management requires to further consider the effect of GN on NAE under N limiting condition. The pre-anthesis N deficiency could reduce ear N demand during the post-anthesis period of maize and 10

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