Relationship between P and N concentrations in maize and wheat leaves

Journal Identification = FIELD Article Identification = 5466 Date: May 20, 2011 Time: 2:27 pm

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Field Crops Research 123 (2011) 28–37

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Field Crops Research

journa l homepage: www.e lsev ier .com/ locate / fc r

elationship between P and N concentrations in maize and wheat leaves

illes Bélanger ∗, Annie Claessens, Noura Ziadigriculture and Agri-Food Canada, Soils and Crops Research and Development Centre, 2560 Hochelaga Blvd., Québec, QC, Canada G1V 2J3

r t i c l e i n f o

rticle history:eceived 10 February 2011eceived in revised form 7 April 2011ccepted 8 April 2011

eywords:oncentrationhosphorusitrogeneaf

heatorn

a b s t r a c t

Simple plant-based diagnostic tools can be used to determine crop P status. Our objectives were to estab-lish the relationships between P and N concentrations of the uppermost collared leaf (PL and NL) ofspring wheat (Triticum aestivum L.) and maize (Zea mays L.) during the growing season and, in particular,to determine the critical leaf P concentrations required to diagnose P deficiencies. Various N applicationswere evaluated over six site-years for wheat and eight site-years for maize (2004–2006) with adequatesoil P for growth. Phosphorus and N concentrations of the uppermost collared leaf were determinedweekly and the relationships between leaf N and P concentrations were established using only the sam-pling dates from the stem elongation stage for wheat and from the V8 stage of development for maize.Leaf P concentration generally decreased with decreasing N fertilization. Relationships between PL andNL concentrations (mg g−1 DM) using all site-years and sampling dates were described by significantlinear–plateau functions in both maize (PL = 0.82 + 0.089 NL if NL ≤ 32.1 and PL = 3.7 if NL > 32.1; R2 = 0.41;P < 0.001) and wheat (PL = 0.02 + 0.106 NL if NL ≤ 33.2 and PL = 3.5 if NL > 33.2; R2 = 0.42; P < 0.001). Variationamong sampling dates in the relationships were noted. By restricting the sampling dates [413–496 grow-

◦
ing degree days (5 C basis) in wheat (i.e., stem elongation) and 1494–1579 crop heat units in maize (i.e.,silking), relationships for wheat (PL = 0.29 + 0.073 NL, R2 = 0.66; P < 0.001) and maize (PL = 1.04 + 0.084 NL,R2 = 0.66; P < 0.001) were improved. In maize, expressing P and N concentrations on a leaf area basis (PLA
and NLA) at silking further improved the relationship (PLA = 0.002 + 0.101 NLA, R2 = 0.80; P < 0.001). Predic-tive models of critical P concentration as a function of N concentration in the uppermost collared leaf ofwheat and maize were established which could be used for diagnostic purposes.

Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.

. Introduction

Adequate tissue P concentration is essential for maximizing croprowth and yield. The identification of a critical P concentrationver which there is no further yield increase is essential for diag-osing P deficiencies in crops. Models describing critical N and Poncentrations in whole plants based on the concept of critical dilu-ion curves were developed by Lemaire and Salette (1984) for Nnd extended to P by Salette and Huché (1991). Because of the rela-ively similar dilution of both P and N in increasing shoot biomass inerennial grasses, Salette and Huché (1991) and Duru et al. (1997)
roposed the concept of a critical P concentration in whole plantsxpressed as a function of N concentration. This approach was vali-ated for timothy (Bélanger and Richards, 1999; Bélanger and Ziadi,
Abbreviations: CHU, crop heat units; DM, dry matter; GDD, growing degree days;L, nitrogen concentration on a leaf dry matter basis; NLA, nitrogen concentrationn a leaf area basis; PL, phosphorus concentration on a leaf dry matter basis; PLA,hosphorus concentration on a leaf area basis.∗ Corresponding author. Tel.: +1 418 210 5034; fax: +1 418 648 2402.

E-mail address: [email protected] (G. Bélanger).

378-4290/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rioi:10.1016/j.fcr.2011.04.007

2008), wheat (Ziadi et al., 2008), and maize (Ziadi et al., 2007).Although this approach of determining critical P concentration hasbeen widely adopted in France for perennial grasses (Jouany et al.,2005), its feasibility is limited because of the need to sample, dry,and grind whole plants.

In previous papers, we showed that the N concentration ofthe uppermost collared leaf could be used for diagnostic pur-poses in maize (Ziadi et al., 2009) and wheat (Ziadi et al., 2010)because of the close relationship between the N concentration ofthe uppermost collared leaf and the N nutrition index based onthe critical N concentration of whole plants. Based on the closerelationship between N and P concentration at the whole plantlevel described above, we hypothesized that P and N concen-trations of the uppermost collared leaf were also closely relatedand that a critical P concentration of the uppermost collaredleaf can be established using the relationship with N concentra-tion. These relationships would then make it possible to diagnoseboth N and P deficiencies using a single leaf rather than whole
plants.
The relationship between leaf P and N concentrations on a DMbasis (PL and NL, respectively) has been studied on the leaf oppo-site and below the maize ear at silking (Kamprath, 1987). Leaf N

ghts reserved.

dx.doi.org/10.1016/j.fcr.2011.04.007

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dx.doi.org/10.1016/j.fcr.2011.04.007


G. Bélanger et al. / Field Crops Research 123 (2011) 28–37 29

Table 1Site characteristics and cropping practices for the wheat experimental series.

2004 2005 2006

Lanoraie Ste. Victoire L’Acadie Lanoraie L’Acadie L’Acadie

Organic matter (%) 2.04 4.27 2.23 2.68 3.38 2.40P, kg ha−1a 1742 143 176 1318 125 212pH (water) 5.9 6.4 6.8 5.9 7.2 6.5Soil surface texture

Clay content (%)b 5 9 28 5 46 22Silt content (%)b 7 13 52 6 46 45Sand content (%)b 88 78 20 89 8 33

Soil classificationc Typic Haplorthods Typic Humaquepts Typic Humaquepts Typic Humaquepts Typic Humaquepts Typic HumaqueptsPrecipitation (mm)d 550 595 439 817 630 675Precipitation, 30-yr average (mm)e 607 759 569 607 569 569Temperature (◦C)d 16.0 17.4 18.6 17.2 19.1 19.2Previous crop Potato Soybean Soybean Potato Soybean WheatSeeding date 23 April 8 May 7 May 22 April 9 May 10 MayFertilizationf

Date 16 June 29 June 23 June 9 June 20 June 27 JuneDays after seeding 55 53 48 49 43 49

Harvesting date 10 August 18 August 17 August 3 August 10 August 16 August

a P extracted with Mehlich-3.b Evaluated in the A horizon.c Soil Survey Staff, 2006. Keys to Soil Taxonomy. USDA, Natural Resources Conservation Service, 10th edition.d Accumulated precipitation and average daily temperature from seeding to harvest.

clsGumcwo

blok

◦ ′ ◦ ′ ◦ ′ ◦ ′

TN

e 30-yr average from 1971 to 2000 (May to October).f Second N application at the start of stem elongation.

oncentration, however, decreases down the plant canopy due toeaf senescence, an increase in the proportion of structural leaf tis-ue, and progressive shading by newer leaves above (Lemaire andastal, 1997). Therefore Gastal et al. (2001) suggested using theppermost collared leaf which should remain unaffected by plantass accumulation and shading. The relationship between P and N

oncentrations in the uppermost collared leaf has never been testedith the objective of defining a critical P concentration, to the best

f our knowledge.The objective of this study was to establish the relationship

etween leaf P and N concentrations in the uppermost collaredeaf of wheat and maize during the growing season by using databtained under a wide range of levels of N nutrition and at sitesnown to have had adequate soil P for growth. More specifically, we

able 2itrogen fertilization treatments for the wheat and maize experimental series.

Wheat

N treatments N application at seedinga

N, kg ha−1Second Napplicationb

Lanoraie (2004, 2005) and Ste. Victoire (2004)

0 0 040 30 1080 30 50120 30 90160 30 130200 30 170120c 60 60L’Acadie (2004)0 0 030 30 070 30 40110 30 80L’Acadie (2005, 2006)30 30 060 30 3090 30 60120 30 90

a Nitrogen was surface-broadcast as ammonium nitrate.b Nitrogen was surface-broadcast as calcium ammonium nitrate at the start of stem eloc Nitrogen was surface-broadcast as calcium ammonium nitrate.d Nitrogen was surface-banded as calcium ammonium nitrate by hand 10 cm from the

wanted to determine the critical P concentration in the uppermostcollared leaf that could be used to quantify P deficiencies.

2. Method

2.1. Site description and treatments

2.1.1. WheatA field experiment was conducted at six site-years in Québec,

Canada: Lanoraie (45◦ 57′ N, 73◦ 19′ W) in 2004 and 2005, Ste. Vic-
toire (45 55 N, 73 06 W) in 2004, and L’Acadie (45 17 N, 73 20W) in 2004, 2005, and 2006. Site characteristics and cropping prac-tices are presented in Table 1; methods of soil analysis are reportedin Ziadi et al. (2008). The cultivar ‘AC Barrie’, a recommended spring
Maize

N treatments N application at seedingc

N, kg ha−1Second Napplicationd

St. Louis (2004, 2005), St. Basile (2004), and Ste. Catherine (2005, 2006)

20 20 050 20 30

100 20 80150 20 130200 20 180250 20 230

L’Acadie (2004)20 20 073 20 53

125 20 105178 20 158L’Acadie (2005, 2006)

30 30 083 30 53

135 30 105188 30 158

ngation, except for treatment 120c for which N was applied at the tillering stage.

maize row at the V6 to V10 stages of development.


3 ops Research 123 (2011) 28–37

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illing wheat cultivar in Québec, was used at all sites. Seeding andertilization dates were site-specific (Table 1). The seeding rate was50 kg ha−1.

Nitrogen fertilization treatments are presented in Table 2. Atach site, the treatments were arranged in a randomized completelock design with four replicates. The plot size was 10 m × 10 mt Lanoraie and Ste. Victoire, and 6 m × 10 m at L’Acadie with rowspaced at 0.15 m. At seeding, P and K fertilizers were applied accord-ng to soil analysis and local recommendations (Centre de référencen agriculture et agroalimentaire du Québec (CRAAQ), 2003).hus, 15 kg P ha−1 and 10 kg K ha−1 in 2004, and 22.5 kg P ha−1 and2.5 kg K ha−1 in 2005 and 2006, were surface-broadcast.

.1.2. MaizeA field experiment was conducted at eight site-years in Québec,

anada: St-Louis (45◦ 51′ N; 73◦ 00′ W), St-Basile-de-Portneuf,eferred to as St-Basile (46◦ 48′ N; 71◦ 46′ W), and L’Acadie45◦ 17′ N; 73◦ 20′ W) in 2004; St-Louis, Ste-Catherine-de-a-Jacques-Cartier, referred to as Ste-Catherine (46◦ 49′ N; 71◦

9′ W), and L’Acadie in 2005; and Ste-Catherine and L’Acadien 2006. Site characteristics and cropping practices are pre-ented in Table 3; methods of soil analysis are reported iniadi et al. (2007). Maize hybrids, seeding dates, and fertil-zation applications were site-specific (Table 3). Plot size wasm × 10 m and consisted of 12 maize rows except at L’Acadiehere plot size was 6 m × 10 m and consisted of eight maize

ows; a 0.75-m inter-row spacing provided a plant density of9,040 plants ha−1.

Nitrogen fertilization treatments are presented in Table 2. Atach site, the treatments were arranged in a randomized com-lete block design with four replicates. At seeding, P and Kertilizers were applied according to soil analysis and local recom-

endations (CRAAQ, 2003). Thus, 35 kg P ha−1 and 35 kg K ha−1 in004, 35 kg P ha−1 and 50 kg K ha−1 in 2005, and 35 kg P ha−1 and8 kg K ha−1 in 2006 were surface broadcast.

.2. Sample collection and analysis

Twenty plants within a 0.28-m × 0.72-m quadrate in each plotere selected to determine the wheat leaf N concentration. Sam-les were taken by removing weekly the uppermost collared leaff each plant for a period of 8 wk in 2004, 5 wk at Lanoraie andwk at L’Acadie in 2005, and 4 wk in 2006. The leaf number of theppermost collared leaf changed during the sampling period.

Ten plants within the 2-m section of a row in each plot wereelected to determine maize leaf N concentration. Samples wereaken by excising 0.71-cm2 disks midway between the stalk andhe tip, and midway between the midrib and leaf margin, on theppermost collared leaf of each plant, except at L’Acadie in 2004here 1.5-cm2 disks were excised. Ten disks per leaf in 2004 and 20isks per leaf in 2005 and 2006 were collected weekly for a periodf 8 wk in 2004, 7 wk in 2005, and 6 wk at Ste-Catherine and 5 wkt L’Acadie in 2006. The leaf number of the uppermost collared leafhanged during the sampling period.

Wheat leaves and maize leaf disks were dried at 55 ◦C in a forced-raft oven for 3 d, ground to pass through a 1-mm screen in ailey ED-5 mill, and stored at room temperature prior to labora-

ory analyses. Samples of 0.1 g of dried and ground leaves and disksere mineralized using a mixture of sulphuric and selenious acids,

s described by Isaac and Johnson (1976). The P and N leaf con-
entrations were quantified with an automated continuous-flownjection analyzer using the methods 13-115-01-2-A and 13-107-6-2-D, respectively (Model QuickChem 8000, Lachat Instruments,oveland, CO). Ta
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.3. Data analysis

Data normality was verified using the Shapiro-Wilk statisticnd the variance homogeneity was verified visually with graph-cs of the residuals (SAS Institute, 2004). Data for each samplingate and site-year were subjected to ANOVA using PROC MIXED ofAS (SAS Institute, 2004) and standard error of the means (SEM)ere calculated. Statistical significance was postulated at P ≤ 0.05.omparisons of least squares means were carried out using theredicted difference (PDIFF) option of SAS with the Tukey–Kramerdjustment. The P concentration was expressed as a function ofconcentration and the linear and linear–plateau functions were

omputed using the regression procedure of SAS (SAS Institute,004). Units of thermal time were used to define the samplingates. Growing degree days (GDDs) were calculated as the accu-ulation of daily mean temperatures as follows:

DD =∑

(Tm − 5 ◦C) (1)

here Tm is the mean daily temperature (Ontario Ministry ofgriculture, Food and Rural Affairs, 2008). Crop heat units (CHUs)ere calculated as the accumulation of daily average night and day

emperatures as follows:

HU =∑ Ymin + Ymax

2(2)

here

min = 1.8(Tmin − 4.4) (3)

max = 3.33(Tmax − 10.0) − 0.084(Tmax − 10.0)2 (4)

nd Tmax and Tmin are the daily maximum and minimum tem-eratures (Ontario Ministry of Agriculture, Food and Rural Affairs,008).

. Results and discussion

.1. P and N concentrations in the uppermost collared leaf

The PL concentration varied with time and N fertilization from aaximum of 7.2 mg g−1 leaf DM to a minimum of 0.48 mg g−1 leafM in wheat (Fig. 1) and from a maximum of 5.4 mg g−1 leaf DM

o a minimum of 2.0 mg g−1 leaf DM in maize (Fig. 2). In previousapers, we reported that NL concentrations varied from a maxi-um of 47.4 mg g−1 leaf DM to a minimum of 6.1 mg g−1 leaf DM

n wheat (Ziadi et al., 2010) and from a maximum of 42.8 mg g−1

M to a minimum of 17.3 mg g−1 DM in maize (Ziadi et al., 2009).aximal values of NL and PL were greater in wheat than in maize

ut minimal values were greater in maize. The range of values washerefore much less in maize than in wheat. These results were gen-rally consistent with the well-known lower N concentration of C4pecies compared with C3 species (Flénet et al., 2006; Ghannoumt al., 2008; Greenwood et al., 1990). Jacob and Lawlor (1991) alsobserved that wheat had a higher leaf P concentration than maize.

In maize, a slightly lower range of PL concentrations1.4–3.6 mg g−1 leaf DM) was observed in studies using the leafpposite and below the ear at silking (Follett et al., 1974; Kamprath,987; Mallarino, 1996). This result confirms the possibility of a gra-ient of leaf P concentration from younger to older leaves, similar tohat observed for leaf N concentration (Lemaire and Gastal, 1997).

The PL concentration of wheat generally decreased with thermalime expressed in growing degree days at all site-years (Fig. 1). In

aize, however, the changes in PL concentration with thermal time
xpressed in crop heat units varied with site-years with a decreaset some of the site-years, a decrease followed by an increase at otherite-years, or a limited change (Fig. 2). There are few reports on thehanges of PL concentration during the growing season. Roberts and
search 123 (2011) 28–37 31

Rhee (1993) and Bullock and Anderson (1998) report decreasingPL concentrations with time in maize. A decrease in NL concentra-tion with time or developmental stage has been reported for wheat(Giunta et al., 2002; Hocking, 1994; Ziadi et al., 2010) and maize(Bullock and Anderson, 1998; Smeal and Zhang, 1994; Ziadi et al.,2009) and was attributed to the increasing proportion of structuralleaf tissue with the increasing size of successive leaves (Hoppo et al.,1999; Lemaire and Gastal, 1997) and to the redistribution of thiselement from vegetative to reproductive parts (Hocking, 1994). Ingeneral, the pattern of change with thermal time were similar forboth P and N concentrations even though N concentrations seemedto decrease more rapidly after an N application as illustrated at twoof the site-years (Fig. 3). Salette (1990) attributes this positive rela-tionship to the association that exists between P and N in proteinbuilding, particularly in young tissues.

In maize, N fertilization applied during the growth cycle sig-nificantly increased PL concentration (Fig. 2). For wheat, however,the effect of N fertilization on PL concentration was less than formaize (Fig. 1). The positive effect of increased N fertilization on rootgrowth and the physiological capacity of the root to absorb P wouldprobably be less for P uptake in wheat than for maize. In previouspapers, we reported that N fertilization increased wheat and maizeNL concentration (Ziadi et al., 2009, 2010). The positive effect of Nfertilization on NL concentration has also been reported for wheat(Follett et al., 1992) and maize (Smeal and Zhang, 1994). The pos-itive effect of N fertilization on leaf P concentration, however, hasonly been reported for maize on the leaf opposite and below theprimary ear (Follett et al., 1974; Kamprath, 1987).

3.2. Relationship between leaf P and N concentrations during thegrowing season

The main objective of this study was to determine species-specific relationships between PL and NL concentrations that couldbe used to determine the critical P concentration from leaves ofwheat and maize. The relationship between PL and NL concen-trations, using all site-years and sampling dates, is expressed bysignificant linear–plateau functions for wheat:

PL = 0.02 + 0.106 NL if NL ≤ 33.2 and

PL = 3.5 if NL > 33.2 (R2 = 0.42; P < 0.001) (5)

and maize:

PL = 0.82 + 0.089 NL if NL ≤ 32.1 and

PL = 3.7 if NL > 32.1 (R2 = 0.41; P < 0.001) (6)

which account for 42% of the variation in wheat and 41% of thevariation in maize (Fig. 4).

The linear–plateau relationship indicates that the PL concen-tration of wheat was constant at 3.5 mg g−1 leaf DM for a NLconcentration > 33.2 mg g−1 leaf DM but decreased with decreas-ing NL concentrations below 33.2 mg g−1 leaf DM (Fig. 4). A similarrelationship was observed for maize (Fig. 4) in which a constant PLconcentration of 3.7 mg g−1 leaf DM was obtained for a NL concen-tration > 32.1 mg g−1 leaf DM.

The NL concentrations at which the PL concentration ceased toincrease were close to the critical NL concentrations previouslydetermined for wheat (35.1 mg g−1 leaf DM; Ziadi et al., 2010) andmaize (31.0 mg g−1 leaf DM; Ziadi et al., 2009). Therefore, we canassume that below these critical NL concentrations, plants wereunder limiting N conditions, whereas above these NL concentra-
tions, there was luxury N consumption. The linear portion of theserelationships illustrates the adjustment of PL concentrations to NLconcentrations under limiting N conditions. This adjustment hasalso been reported in many studies investigating the relationship


32 G. Bélanger et al. / Field Crops Research 123 (2011) 28–37

Fig. 1. Changes in leaf P concentration on a leaf DM basis (PL) versus thermal time (growing degree days) for wheat fertilized with various N applications in an experimentalseries conducted over six site-years. Arrows represent the date of the second N application; 120c treatment represents the conventional application of 60 kg N ha−1 at seedingand 60 kg N ha−1 at tillering. The vertical bars represent SEM values at each sampling date after the second N application and ns indicates non-significance (P > 0.05).

Fig. 2. Changes in leaf P concentration on a leaf DM basis (PL) versus thermal time (crop heat units) for maize fertilized with various N applications in an experimental seriesconducted over eight site-years. Arrows represent the date of the second N application. The vertical bars represent SEM values at each sampling date after the second Napplication, ns indicates non-significance (P > 0.05), and nd indicates means not determined because of insufficient plant material in each replicate.



F s therw 120ca econd

bi22ttrlc

aicaNfawtaL

ig. 3. Changes in leaf N and P concentrations on a leaf DM basis (NL and PL) versuith various N applications. Arrows represent the date of the second N application;

t tillering. The vertical bars represent SEM values at each sampling date after the s

etween P and N concentrations in whole plants fertilized with var-ous N sources (Bélanger and Richards, 1999; Bélanger and Ziadi,008; Duru and Ducrocq, 1997; Salette, 1990; Ziadi et al., 2007,008). Under non-limiting N conditions, however, the PL concen-ration was no longer adjusting to the NL concentration, whereas inhe wheat and maize whole plants, this adjustment was still occur-ing (Ziadi et al., 2007, 2008). Mallarino (1996) also observed theimited capacity of leaf tissue to accumulate PL beyond the optimaloncentrations.

Although the relationships were significant when all site-yearsnd sampling dates were used, significant variability was observedn the PL values for a given NL concentration. In wheat, for a NL con-entration of 33.2 mg g−1 leaf DM, PL concentrations varied frompproximately 2.6 to 4.1 mg g−1 leaf DM (Fig. 4). In maize, for aL concentration of 32.1 mg g−1 leaf DM, PL concentration varied

rom approximately 3.2 to 4.2 mg g−1 leaf DM (Fig. 4). This vari-bility was mainly due to the fact that these relationships varied
ith site-years in both species. For instance, at a given NL concen-
ration, PL concentrations of wheat were greater at L’Acadie thant Ste. Victoire and PL concentrations of maize were greater at St.ouis in 2005 than in St. Basile in 2004 (Fig. 5). Furthermore, these

mal time for wheat at Ste. Victoire and for maize at Ste. Catherine in experimentsrepresents the conventional application of 60 kg N ha−1 at seeding and 60 kg N ha−1

N application and ns indicates non-significance (P > 0.05).

relationships also varied during the growing season (Fig. 6). Forinstance, in maize at a given NL concentration of 32.1 mg g−1 leafDM, the PL concentration varied from 3.2 mg g−1 leaf DM on July 11(1084 CHU) to 4.3 mg g−1 leaf DM on July 25 (1433 CHU; Fig. 6).

This variability in the relationship between PL and NL concentra-tions with site-years and sampling dates was not expected. In ourprevious papers that considered the relationship for whole plants,we did not report variation in the relationship between samplingdates and sites (Ziadi et al., 2007, 2008). A closer look at the data inthose two papers, however, indicates that there were possible dif-ferences among sites-years. In a timothy experiment with differentP applications, P fertilization increased whole plant P concentra-tion while DM yield was not affected (Bélanger and Ziadi, 2008).This result suggests the possibility of luxury P consumption. In thisstudy, we assumed that P was not limiting shoot growth. It is likelythat luxury P consumption occurred at some of the sites and thiswould affect the relationship between P and N concentrations for
both whole plants and a single leaf.
The variability in the relationship between PL and NL concentra-tions with sampling dates was possibly related to the differentialchanges in leaf P and N concentrations over time in the growing sea-



F N cona de onb

scom

3s

uiwcdi

Fab

ig. 4. Relationship between leaf P concentration on a leaf DM basis (PL) and leafpplications in two experiments conducted at various site-years. Data points incluefore. Solid lines represent the linear–plateau relationships.

on. Our results suggest that the P concentration of the uppermostollared leaf decreases less with time than the N concentration. Inther words, the P concentration of the uppermost collared leaf isore stable over time than the N concentration.

.3. Relationship between leaf P and N concentrations at apecific stage of development

Because the relationship between P and N concentrations in theppermost collared leaf varied with developmental stage, narrow-

ng this range could reduce variability among site-years. Therefore,
e limited our analysis of the relationship between P and N con-
entrations to sampling dates corresponding to a given stage ofevelopment. Because stages of development were not determined

n our study, we used thermal units. In maize, we restricted our

ig. 5. Relationship between leaf P concentration on a leaf DM basis (PL) and leaf N conpplications in an experimental series conducted at some site-years. Data points includeefore. Solid lines represent the linear regressions.

centration on a leaf DM basis (NL) for wheat and maize fertilized with various Nly the sampling dates for which the second N application was applied at least 5 d

analysis to sampling dates with CHU ranging from 1494 to 1579units after seeding; this should correspond to the silking stageof development (Ontario Ministry of Agriculture, Food and RuralAffairs, 2008). In wheat, only sampling dates with GDD from seed-ing that ranged from 413 to 496 were used. In wheat, phenologicaldevelopment is also associated with GDD (Bauer et al., 1985;Klepper et al., 1982; West et al., 1991). This accumulation of GDDafter seeding should correspond to stem elongation for the cultivarAC Barrie (Saiyed et al., 2009).

The relationship between PL and NL concentrations is expressedby significant linear functions for wheat at approximately the stem
elongation stage:
PL = 0.29 + 0.073 NL (R2 = 0.66; P < 0.001) (7)

centration on a leaf DM basis (NL) for wheat and maize fertilized with various Nonly the sampling dates for which the second N application was applied at least 5 d



F conceC ns. Da

a

P

wvelarsAgss

Fawd

ig. 6. Relationship between leaf P concentration on a leaf DM basis (PL) and leaf Natherine in 2005 at three sampling times in experiments with various N applicatiopplied at least 5 d before. Solid lines represent the linear regressions.

nd maize at approximately the silking stage:

L = 1.04 + 0.084 NL (R2 = 0.66; P < 0.001) (8)

hich accounts for 66% of the variation in wheat and 66% of theariation in maize (Fig. 7). Kamprath (1987) reports a positive lin-ar relationship between PL and NL concentrations in the maizeeaf opposite and below the ear at silking but PL concentrations forgiven NL concentration were lower than in our study (Fig. 7). This

esult could be due to the fact that they used the maize leaf oppo-ite and below the ear while we used the uppermost collared leaf.
s mentioned earlier, there is a gradient in leaf N concentrationsoing down the plant canopy, which is due, in part, to the progres-ive shading by younger leaves (Lemaire et al., 1997). Our resultshow significant relationships between PL and NL in both wheat and
ig. 7. Relationship between leaf P concentration on a leaf DM basis (PL) and leaf N conpplications in an experimental series conducted over various site-years. Data points inclheat (i.e., stem elongation) and crop heat units ranging from 1494 to 1579 for maize (i.eotted line represents the linear relationship between P and N in the leaf opposite and be

ntration on a leaf DM basis (NL) for wheat at Ste. Victoire in 2004 and maize at Ste.ata points include only the sampling dates for which the second N application was

maize; by limiting our analysis to a specific developmental stage,these relationships were improved.

Although the functions for wheat and maize were not statisti-cally compared, we note that for a given NL concentration, the PLconcentration was greater in maize than in wheat. In other words,the critical leaf PL concentration is greater for maize than for wheat.For the whole plant, the P concentration for a given N concentrationwas also greater in maize than in wheat (Ziadi et al., 2007, 2008).For instance, the predicted P concentration of whole plants was3.61 mg g−1 DM in wheat and 3.85 mg g−1 DM in maize for a whole
plant N concentration of 25.0 mg g−1 DM. This result indicates thatthe adjustment between N and P concentrations for the leaf and forthe whole plant levels are not the same for wheat and maize. Thereason for this result is not clear. It might be related to the different
centration on a leaf DM basis (NL) for wheat and maize fertilized with various Nude only the sampling dates with growing degree days ranging from 413 to 496 for., silking). Solid lines represent the linear relationships between PL and NL, and thelow the ear at silking (P = 0.856 + 0.0699N; Kamprath, 1987).



F N coni the saa he sol

mi

3b

LaawtmPh

ul

P

wtvTP(

P

iwTtp

3

ietwp

ig. 8. Relationship between leaf P concentration on a leaf area basis (PLA) and leafn an experimental series conducted over eight site-years. Data points include onlynd only the sampling dates corresponding to silking (∼1500 crop heat units) (b). T

etabolic pathways for the two species (C3 and C4). More researchs needed to confirm this result and to seek an explanation.

.4. Relationship between P and N concentrations on a leaf areaasis in maize

Because light interception is an area-based phenomenon,emaire et al. (1997) suggested expressing leaf N concentration onn area basis. They assumed that leaf N concentration expressed onn area basis would be more stable during the growing season thanhen it is expressed on a DM basis. We hypothesized, therefore,

hat the relationship between P and N concentrations in the upper-ost collared leaf could be improved by expressing leaf N (NLA) and(PLA) concentrations on an area basis. We could only verify thisypothesis in maize because discs of a known area were sampled.

The relationship between PLA and NLA concentrations in maize,sing all site-years and sampling dates, is expressed by a significant

inear function:

LA = 0.003 + 0.082 NLA (R2 = 0.52; P < 0.001) (9)

hich accounts for 52% of the variation (Fig. 8a). This improveshe relationship between N and P concentrations but there is stillariation among site-years and sampling dates (data not shown).herefore, we restricted our analysis of the relationship betweenLA and NLA concentrations to the silking development stageFig. 8b). The linear relationship

LA = 0.002 + 0.1011 NLA (R2 = 0.80; P < 0.001) (10)

s significant and explains a greater proportion of the variation thanhen leaf N and P concentrations are expressed on a DM basis.

hese results suggest that expressing both leaf N and P concentra-ions on a leaf area basis has more potential for diagnostic purposes,ossibly because they are more stable during the growing season.

.5. Implications for P diagnostic

We previously reported the possibility of determining the crit-cal P concentration for wheat (Ziadi et al., 2008) and maize (Ziadi
t al., 2007) by using the relationship between P and N concentra-ions of whole plants. However, the need to sample, dry, and grindhole plants limits the feasibility of using this approach. In thisaper, we propose a simpler approach based on the sampling of a
centration on a leaf area basis (NLA) for maize fertilized with various N applicationsmpling dates for which the second N application was applied at least 5 d before (a)id lines represent the linear relationships.

single leaf. Our results confirm our initial hypothesis that P and Nconcentrations of the uppermost collared leaf are closely relatedand that a critical P concentration of the uppermost collared leafcan be established using the relationship with N concentration.

An index of P nutrition could be calculated by dividing the leafP concentration in given situations by the critical P concentrationpredicted by Eq. (7) for wheat or by Eq. (8) for maize. Combined withan index of N nutrition based on the critical leaf N concentration(Ziadi et al., 2009, 2010), this approach could diagnose both P andN deficiencies in wheat and maize during the growing season.

Contrary to N deficiencies, P deficiencies cannot be easily alle-viated with later applications during the same season. This Pdiagnostic tool is therefore more adapted to adjust P fertilizationin the following growing season. In this study, the relationshipsbetween leaf P and N concentrations were established at sitesknown to have adequate soil P for growth. Further research isneeded to determine how effective these models of critical leaf Pconcentrations would be to differentiate situations of P deficiencyand sufficiency in situations with different P supply.

For maize, expressing P and N concentrations per unit of leaf arearesulted in a stronger relationship and, consequently, the diagnos-tic of P deficiency is likely to be more reliable. Sampling severalleaf discs of a known area in maize is an easy procedure because ofthe large size of the leaves. However, it would be more difficult toachieve in wheat and other grass species with smaller leaves.

4. Conclusions

Leaf P was significantly related to leaf N during the growingseason in both species. These relationships, however, varied withsite-years and sampling dates. By restricting the sampling dates toapproximately 450 GDD for wheat (i.e., stem elongation) and 1500CHU for maize (i.e., silking), the relationships were improved. Fur-thermore, by expressing P and N concentrations on a leaf area basisand restricting the sampling dates to approximately 1500 CHU, therelationship between P and N concentrations in maize was furtherimproved.

Acknowledgements

This study was funded by Synagri Inc. and Agriculture andAgri-Food Canada (AAFC) through a matching investment initia-


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ive program and the GAPS program of AAFC. The assistance of Alainarouche, Sylvie Michaud, and Danielle Mongrain is greatly appre-iated. The editorial assistance of Christina McRae, EditWorks, islso gratefully acknowledged.

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Relationship between P and N concentrations in maize and wheat leaves

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