全 文 ：作物学报 ACTA AGRONOMICA SINICA 2011, 37(7): 12591265 http://www.chinacrops.org/zwxb/
ISSN 0496-3490; CODEN TSHPA9 E-mail: xbzw@chinajournal.net.cn
This study was supported by SAGES funding from Agriculture and Agri-Food Canada and the Knowledge Innovation Program of the Chinese
Academy of Sciences (KZCX2-YW-443-3).
* Corresponding author: Nicolas Tremblay, E-mail: Nicolas.Tremblay@agr.gc.ca, Tel: +1-450-515-2102, Fax: +1-450-346-7740
Corresponding to the first author: E-mail: zjj0954@163.com
Received(收稿日期): 2010-12-20; Accepted(接受日期): 2011-03-27.
DOI: 10.3724/SP.J.1006.2011.01259
Responses of Corn (Zea mays L.) Nitrogen Status Indicators to Nitrogen Rates
and Soil Moisture
ZHU Juan-Juan1, LIANG Yin-Li1,2, and TREMBLAY Nicolas3,*
1 College of Life Science, Northwest A&F University, Yangling 712100, China; 2 Institute of Soil Water and Conservation, Chinese Academy of Sci-
ences and Ministry of Water Resources, Yangling 712100, China; 3 Horticulture Research and Development Centre, Agriculture and Agri-Food
Canada, 430 Bloul. Gouin, St-Jean-sur-Richelieu, Quebec, J3B 3E6, Canada
Abstract: Plants usually experience fluctuating water supply during their life cycle due to continuous changes in climatic factors.
Soil water content (SWC) is one of the most critical factors affecting nitrogen (N) availability, movement, and uptake by crops.
Consequently, SWC levels may confound the assessment of crop N status. The present study compared the sensitivity of tissue N
concentration, SPAD readings, Dualex readings, and SPAD/Dualex ratios for assessing corn (Zea mays L.) N status under different
water supply conditions. A greenhouse trial was conducted with four N fertilizer application rates (0, 50, 50+75, and 200 kg ha1)
and three watering levels (drought, drought followed by rewatering, and fully-watered). Tissue N concentration, SPAD, Dualex,
and SPAD/Dualex values were influenced significantly by N rates and by SWC. Tissue N concentration, SPAD, and SPAD/Dualex
increased with N rates, whereas Dualex decreased. In the first phase of reaction to drought, tissue N concentration, SPAD and
SPAD/Dualex decreased rapidly but Dualex increased; however, the opposite pattern of response was observed in the long term.
Under rewatering, tissue N concentration, Dualex and SPAD/Dualex gradually recovered, whereas SPAD values did not change
significantly as they did in the drought treatment. There were highly significant relationships between SPAD (r = 0.92), Dualex
(r = 0.86), or SPAD/Dualex (r = 0.63) and tissue N concentration. However, SPAD and Dualex were better predictors of tissue N
concentration under drought conditions (SPAD: r = 0.90; Dualex: r = 0.83) than under fully-watered conditions (SPAD: r = 0.39;
Dualex: r = 0.44) at the end of the trial. Among the indicators, Dualex is better able to discriminate N treatments, with consistent
results across SWC levels.
Keywords: Indicators; N status; drought; water recovery
不同水氮处理对玉米氮素诊断指标的影响
朱娟娟 1 梁银丽 1,2 TREMBLAY Nicolas3,*
1 西北农林科技大学生命科学学院, 陕西杨陵 712100; 2 中国科学院水利部水土保持研究所, 陕西杨凌 712100; 3 加拿大农业与农业食品部
园艺研究与发展中心, St-Jean-sur-Richelieu, J3B 3E6, Canada
摘 要: SPAD-502叶绿素仪与 Dualex-3多酚仪均可诊断作物的氮素营养状况, 为了探讨不同水分条件下该指标对作物施
氮量的响应, 以玉米品种 Pioneer 38B84为研究对象, 在温室盆栽条件下, 研究 3个土壤水分水平(干旱、干旱后复水和水
分良好)和 4个施氮水平(0、50、50+75和 200 kg hm2), 即 12个处理对植物含氮量、SPAD值、Dualex值以及 SPAD/Dualex
比值的影响。结果表明, 植物含氮量、SPAD值和 SPAD/Dualex比值均随施氮量的增加而增加, Dualex值随施氮量的增加
而减少。干旱胁迫初期, 植物含氮量、SPAD值和 SPAD/Dualex比值迅速下降, Dualex值迅速增加; 随着干旱胁迫时间的
推进, 各指标逐渐呈相反趋势变化。干旱复水后, 植物含氮量、Dualex值和 SPAD/Dualex比值逐渐恢复, 然而 SPAD值恢
复较小。SPAD值(r = 0.92)、Dualex值(r = 0.86)及 SPAD/Dualex值(r = 0.63)与植物含氮量呈极显著的相关性, 但试验后
期, 由于生长阶段的不同, SPAD值和 Dualex值与植物含氮量在干旱条件下(SPAD: r = 0.90; Dualex: r = 0.83)的相关性高
于在水分良好条件下(SPAD: r=0.39; Dualex: r = 0.44)。通过比较不同水分条件下, SPAD值、Dualex值和 SPAD/Dualex值
对不同施氮量的响应, 发现 Dualex值在不同水分条件下均能较好地反应施氮水平。
关键词: 诊断指标; 氮素; 干旱; 复水
1260 作 物 学 报 第 37卷
The SPAD-502 Chlorophyll Meter (Soil Plant Analy-
sis Development, Minolta Camera Co., Ltd., Japan) and
the Dualex (contraction of Dual + excitation, Force-A,
Paris, France) are leaf-clip instruments that can be used
to assess crop nitrogen (N) status quickly and
non-destructively. The SPAD instrument is based on the
measurement of leaf chlorophyll (Chl) content, whereas
Dualex measures polyphenolics (Phen) concentrations.
SPAD values have been demonstrated to be positively
correlated with leaf N concentration in a wide range of
investigations involving several crops. Many studies
have established critical SPAD readings below which
crop yield responds to N fertilization. SPAD readings
above 45.4, 52.1, 55.3, and 58.0 at corn (Zea mays L.)
stages of three to four, six to seven, and ten to eleven
fully expanded leaves and at silking, respectively, have
been recommended in order to obtain high grain yields[1].
The range of 52 to 56 SPAD units is defined as critical
for separating N sufficiency from N deficiency at the 1/4
milk line stage of corn [2]. N stress may be present at
SPAD readings lower than 38–40, and a value above 44
at GS-45 has been linked to excess N uptake in spring
wheat (Triticum aestivum L.) [3]. However, problems have
been reported with the use of SPAD for N status assess-
ment in plants. One problem relates to the saturation of
the Chl level [4] which results in an indication of excess N
because Chl is only partly affected by N uptake [5]. SPAD
measurements are also affected by crop variety, water
and cold stress, location, season, and insect damage [6-7].
The Dualex can be used to measure polyphenolics
(Phen) concentrations in plants based on a Chl fluores-
cence assessment. A highly significant relationship with
leaf extracted Phen concentration was demonstrated (R2 =
0.94 for Goulas et al. [8] and R2 = 0.81 for Cartelat et al. [9]).
Phen are a diverse class of plant secondary metabolites
which have many functions [9-10]. Polyphenol synthesis
and accumulation in plants is generally stimulated in re-
sponse to biotic and abiotic stresses [11]. Phen are prima-
rily composed of carbon (C), which is assumed to be the
limiting resource for their production. N availability de-
termines C fixation rates and C availability. According to
the Protein Competition Model (PCM) [12], the competi-
tive relationship that exists between protein and Phen is
due to the common precursor (the amino acid phenyla-
lanine [PHE]). When N is a limiting factor for crops, leaf
growth and C fixation increase with N fertilizer applica-
tion rates. As a result, the amount of PHE allocated for
protein synthesis is enhanced with increasing N supply,
whereas the amount of PHE allocated for Phen synthesis
declines. Therefore, the Dualex has been used success-
fully to evaluate N status in many crops, based on a strong
negative correlation with leaf N concentration [4,9-10].
The gradients of both Chl and Phen along wheat
leaves increased from the base to the tip [9]. In order to
alleviate this variability, Cartelat et al. [9] suggested that
the simple Chl/Phen ratio would be a good indicator of N
status. The discrimination between unfertilized control
and rich-fertilized treatment in corn obtained with
SPAD/Dualex (Chl/Phen) ratio was much higher than for
Dualex or SPAD readings alone [10]. Many reports have
confirmed that the SPAD/Dualex ratio is a good indicator
of N status in other species [5,13].
Plants usually experience fluctuating water supply
during their life cycle due to continuously changing cli-
matic factors. Water is one of the most important envi-
ronmental factors regulating plant growth and secondary
metabolites [14]. It is the most critical factor influencing N
availability, movement, and uptake in crops [15]. N-uptake
ability was reduced to about 20% of the well-watered
control when soil water content (SWC) was decreased to
5%; however, N-uptake ability recovered after rewater-
ing [16]. Many studies have been done on the response of
SPAD readings to water stress. Schröder et al. [5] showed
that SPAD readings decreased in corn under water stress,
but Martìnez et al. [17] indicated that SPAD readings in-
creased by almost three units when leaf relative water
content (RWC) decreased by 6.5% in wheat. Contradic-
tions related to water status have also been reported for
Phen concentration in leaves. Cheruiyot et al. [14] found
that Phen concentration was reduced under drought stress
in tea (Camellia sinensis L.), which is in agreement with
Scalabrelli et al. [18] in grapevine (Vitis vinifera L.).
However, Estiarte et al. [19] reported a 10% increase of
Phen in wheat leaves under drought stress.
Users of N diagnosis instruments are interested in ob-
taining measurements that are as specific as possible for
plant N status and robust against confounding factors
such as water supply. Many studies have investigated the
influence of N and water status on individual N indica-
tors. Very few have actually compared the performance
of several N status indicators in a context of contrasted
and transient water status. The aim of this study was
therefore to understand the influence of N rates and soil
water status (as well as their interaction) on N concentra-
tion in leaves, SPAD readings, Dualex readings, and
SPAD/Dualex ratios in order to identify the best water
and N stress indicator.
1 Materials and methods
1.1 Experimental design
The greenhouse experiment was carried out at Agri-
culture and Agri-Food Canada’s Horticulture Research
and Development Centre in St-Jean-sur-Richelieu
(45º18′N, 73°15′W, elevation 47 m), Quebec, Canada,
using the corn cultivar Pioneer 38B84. Soil used in the
greenhouse trial was obtained from the L’Acadie experi-
mental farm (0–30 cm layer), Quebec, Canada in 2009; it
was kept at room temperature in sealed barrels and sieved
to pass a 1 cm mesh. The soil was a clay loam (Aquolls,
Humaquepts). A pre-sowing soil test gave the following
mean values: soil pH (water) 6.6; organic matter 4.0%;
第 7期 朱娟娟等: 不同水氮处理对玉米氮素诊断指标的影响 1261
NO3-N 35.5 mg kg1; available P (Mehlich 3) 86.8 mg
kg1; and available K (Mehlich 3) 138.9 mg kg1. Corn
was sown on 11 January, 2010. Seeds were sown in plas-
tic pots (outside diameter: 20.3 cm, height: 15.9 cm)
containing 2.85 kg of soil. A previous experiment showed
that this soil type tends to dry fast and get very hard in
pots. A rockwool block (length: 10 cm, width: 10 cm,
height: 5 cm) was put in each container to provide a wa-
ter reserve buffer within the soil volume. Before starting
the measurements, the smallest and largest plants were
removed; seven plants were kept in each pot to ensure
uniform plant. Phosphate and potash fertilization was
applied using 2.6 g of triple super-phosphate (0–46–0
N-P-K) and 5.8 g of potassium-magnesium sulphate
(0-0-22-11 N-P-K-S) per pot. P, K and N (except the top-
dressing N) fertilizers were applied in the surface (0–10
cm layer) layer of soil and mixed thoroughly with the soil
before sowing.
A completely randomized block design was used with
seven replications. Nitrogen was provided in the form of
calcium ammonium nitrate (CAN). Four N treatments
were provided at sowing (0, 0.64, 0.64, and 2.55 g of N
pot1), and one N treatment at 21 DAS referred to as
“50+75” (V2-V3; topdressing stage) (0, 0, 0.96, and 0 g
of N pot1, respectively) (Table 1). The N rates tested for
the greenhouse experiment were therefore estimated as a
“field equivalent” of 0, 50, 50+75, and 200 kg N ha1.
Three watering treatments were established: 1) drought
(D, 40–45% of field moisture capacity); 2) drought fol-
lowed by rewatering (D-R, 40–45% of field moisture
capacity before 47 DAS and 80–90% of field moisture
capacity thereafter); 3) fully-watered (W, 80–90% of
field moisture capacity). The soil water regime in all pots
was maintained at a fully-watered level for all treatments
until N topdressing (21 DAS). Soil water content (SWC)
in the pots was tested thereafter using a type HH2 soil
moisture meter (Delta-T Devices Ltd. Cambridge, UK).
SWC (0–10 cm layer) was measured every two days
prior to rewatering and every day thereafter, based on
five sampling points in each pot. When measurements
reached the lower limit of the field capacity established
for each treatment, SWC was increased to the upper limit
of the established range.
1.2 Sampling
Plant samples were taken seven times (at 20 [V2], 32
[V3-V4], 39 [V4-V5], 46, 53, 60, and 67 DAS, respec-
tively). Due to treatment effects on plant growth, differ-
ent growth stages occurred at different watering levels
from 46 DAS onwards. They were V4-V5, V5-V6,
V6-V7, and V7-V8 for drought condition; V6-V7, V7-V8,
V11-V12, and VT-R1 for fully-watered condition; V4-V5,
V6, V7-V8, and V11-V12 for drought followed by rewa-
tering condition. At each sampling time, one plant was
randomly selected from each pot for the SPAD, Dualex,
aboveground biomass, and tissue N concentration meas-
urements.
1.3 Plant measurements
Readings were taken with a Minolta SPAD-502 (Soil
Plant Analysis Development, Minolta Camera Co., Ltd.,
Japan) and a Dualex-3 (Force-A, Orsay, France) on the
uppermost fully expanded leaves of the plants. The Du-
alex-3 is a portable instrument for the evaluation of leaf
flavonoids concentration from the measurement of UV
(375 nm) absorbance of the leaf epidermis by double
excitation of Chl fluorescence [8]. The instrument makes
use of a feedback loop that equalizes the fluorescence
level induced by a reference red light to the UV-light-
induced fluorescence level [7]. All measurements were
made on the lamina, avoiding midribs, except measure-
ments at V2 and V3-V4 because the leaves were not wide
enough. Since either leaf side can be used to assess Phen
status [7], only adaxial readings were made for SPAD and
Table 1 Description of soil water content (SWC) and nitrogen (N) treatments (g pot1)
SWC treatment N treatment N at sowing N at topdressing Total N
N0 0 0 0
N50 0.64 0 0.64
N50+75 0.64 0.96 1.60
Drought
(D, 40–45% of field capacity)
N200 2.55 0 2.55
N0 0 0 0
N50 0.64 0 0.64
N50+75 0.64 0.96 1.60
Drought followed by rewatering from 46 DAS
(D-R, 40–45% of field capacity first; then
80–90% of field capacity)
N200 2.55 0 2.55
N0 0 0 0
N50 0.64 0 0.64
N50+75 0.64 0.96 1.60
Fully watered
(W, 80–90% of field capacity)
N200 2.55 0 2.55
1262 作 物 学 报 第 37卷
Dualex in order to reduce sampling time. No specific
time of day was set for taking the measurements. A
minimum of 20 readings were taken at different places on
each leaf sampled, and the averaged data were used for
statistical calculations.
1.4 Laboratory analyses
Shoots were cut at ground level and oven-dried at
70°C for 7 d, after which the dried biomass was weighed.
Samples were ground through a 1-mm screen in a Wiley
mill, and stored at room temperature before laboratory
analyses. Samples of 0.5 g of dried biomass were miner-
alized using a mixture of sulphuric and selenious acids,
as described by Isaac and Johnson [20]. The tissue N con-
centration was measured on a QuikChem 8000 Lachat
autoanalyzer (Lachat Instruments, Milwaukee, WI) using
Lachat method 15-501-3 [21].
1.5 Statistical analysis
The database was subjected to correlation analysis by
the SAS [22] PROC CORR procedure and analysis of
variance (ANOVA) by the SAS PROC MIXED proce-
dure followed by orthogonal contrast analyses of linear,
quadratic, and other similar effects for quantitative treat-
ments [23]. When both linear and quadratic analyses were
significant, the trend was referred to as curvilinear [24].
Time-repeated measures analysis was used to determine
the influence of N rates, SWC, and growth stages on
SPAD, Dualex, and SPAD/Dualex values by the SAS
PROC MIXED procedure [25].
2 Results
2.1 Biomass
Shoot biomass data are presented here to show the ef-
fect of treatments on crop growth, and as a reference for
the response of N status indicators in the same context.
Overall, the effects of treatments on shoot biomass fol-
lowed the expected pattern. Shoot biomass increased
with DAS but the degree of increase interacted with both
SWC and N treatments (Fig. 1-A). N rates and improve-
ments of SWC increased shoot biomass at each sampling
date. Nitrogen was the most important limiting factor for
growth, as evidenced by the N0 treatment, which resulted
in minimal growth, irrespective of the SWC treatments.
2.2 Response of N status indicators to different
levels of N and SWC
2.2.1 Tissue N concentration Tissue N concentra-
tion decreased gradually with DAS and the degree of
reduction was influenced mainly by the SWC treatment
(Fig. 1-B). Tissue N concentration was lower in the
drought (D) treatment after 32 DAS. N treatment effects
were significant. The N0 treatment stood apart from the
others with very low tissue N levels.
2.2.2 SPAD, Dualex, and SPAD/Dualex SPAD
readings (Fig. 1-C) and SPAD/Dualex ratio (Fig. 1-E)
decreased gradually with DAS, while Dualex readings
increased (Fig. 1-D). Already from the first sampling date
(20 DAS), the N0 treatment stood out for all indicators,
irrespective of SWC treatment. SPAD levels for the other
treatments decreased with DAS without a clear separa-
tion among treatments. Dualex measurements of the N0
treatment increased steeply at 32 DAS. They levelled off
after that point for the fully-watered (W) treatment, but
decreased for the drought (D) treatment. Dualex readings
were clearly segregated according to N and water treat-
ments from the rewatering stage onwards (47 DAS). Im-
provement of water supply conditions clearly increased
Dualex levels. The SPAD/Dualex ratio was characterized
by a wide range of levels, attributable mainly to SWC
treatments before rewatering (47 DAS). The levels
tended to decrease with DAS. The N0 treatment always
had the lowest SPAD/Dualex values. The fully watered
treatment produced higher SPAD/Dualex values than the
drought treatment before the rewatering stage (47 DAS)
but lower values thereafter. The rewatering treatment (D-
R) tended to produce slightly lower SPAD/Dualex values
than the drought treatment. At each date, increases in N
supply were positively related to SPAD/Dualex levels.
2.3 Relationships between N status indicators and
N tissue concentration
There were strong linear correlations between SPAD
readings, Dualex readings, and SPAD/Dualex ratio and
tissue N concentration at each sampling date and all
SWC levels (Table 2). Positive correlations were ob-
served for SPAD and SPAD/Dualex ratio, whereas nega-
tive correlations were found for the Dualex readings, as
expected. Correlations with tissue N concentration were
on average strongest for SPAD, followed by Dualex and
then SPAD/Dualex. The strength of the relationships
tended to decline for the fully-watered treatment as of 46
DAS.
3 Discussion
In this study, as expected, significant correlations be-
tween SPAD, Dualex, or SPAD/Dualex and tissue N
concentration were found (Table 2). SPAD values and
SPAD/Dualex ratios increased with N rates, whereas Du-
alex values decreased. The selected treatments produced
the expected effects on shoot biomass accumulation (Fig.
1-A). The degree of variation of SPAD, Dualex, and
SPAD/Dualex ratio in relation to the N treatments was
consistent with previous studies [4,10].
The Dualex quantifies the UV light that crosses the
leaf epidermis and results in excitation of Chl fluores-
cence [8]. The UV filtering capacity of the leaf epidermis
is activated primarily by UV rays from the sun, but also
by stress factors such as N deficiency and drought. The
effect of drought stress, in conjunction with N deficiency,
has not been quantified up to now. In the greenhouse
set-up, the greenhouse glass likely reduced the UV levels
reaching corn leaves. Nonetheless, Dualex readings were
on average 65% higher in the N0 as compared to the fer-
第 7期 朱娟娟等: 不同水氮处理对玉米氮素诊断指标的影响 1263
Fig. 1 Responses of (A) shoot biomass (g d m plant1), (B) tissue N concentration, (C) SPAD, (D) Dualex, and (E) SPAD/Dualex to N rates
and SWC at different days after sowing
N topdressing was applied at 21 DAS (V2-V3; the first vertical broken line) and rewatering was done at 47 DAS (the second vertical broken line).
Time-repeated analysis was applied before and after rewatering. The *, **, and *** indicate the difference at P ≤ 0.05, 0.01, and 0.001, respectively.
L and Q indicate linear and quadratic components with DAS, respectively, based on repeated analysis.
tilized treatments, which indicates that the Dualex is able
to show the effects of N treatment on corn in a green-
house environment. Growth stages significantly affected
SPAD [1,10] and Dualex measurements. This is likely due
to growth stage-associated changes in the estimated pa-
rameters themselves, such as polyphenolics [19] concen-
1264 作 物 学 报 第 37卷
Table 2 Pearson’s correlation coefficients (r) for the relationship between N status indicators and tissue N concentration
Indicator SWC 1) level 20 DAS
2)
(–26)
32 DAS
(–14)
39 DAS
(–7) 46 DAS (+0
3)) 53 DAS (+7) 60 DAS (+14) 67 DAS (+21)
D / 0.81***4) 0.90*** 0.94*** 0.92*** 0.90*** 0.90***
D-R / / / / 0.90*** 0.90*** 0.79***
SPAD
W 0.92*** 0.80*** 0.87*** 0.86*** 0.91*** 0.77*** 0.39*
D / –0.86*** –0.80*** –0.80*** –0.78*** –0.82*** –0.83***
D-R / / / / –0.72*** –0.73*** –0.63***
Dualex
W –0.86*** –0.89*** –0.68*** –0.81*** –0.73*** –0.49 ** –0.44*
D / 0.47*** 0.56*** 0.78*** 0.84*** 0.67 *** 0.61***
D-R / / / / 0.48** 0.63 *** 0.68***
SPAD/Dualex
W 0.63*** 0.47 *** 0.50*** 0.82*** 0.79*** 0.70 *** 0.56**
1) D: 40–45% of field moisture capacity; D-R: 40–45% of field moisture capacity before 47 DAS and 80–90% of field moisture capacity thereafter;
W: 80–90% of field moisture capacity. 2) DAS: days after sowing. 3) The number in parentheses is the days before () or after (+) rewatering. 4) *, **, and
*** indicate significant difference at P≤ 0.05, 0.01, and 0.001 levels, respectively.
trations for the Dualex [9], as well as to the fact that the
leaf selected for sampling differed among the growth
stages. The SPAD/Dualex ratio has been described as a
better indicator of crop N status than either SPAD or Du-
alex alone [4,9-10]. The current study supports this finding,
as the range of values was greater and the difference be-
tween the unfertilized control and rich-fertilized N treat-
ments was always larger in SPAD/Dualex ratios (89%)
than that in SPAD (53%) or Dualex (70%) alone.
It has been suggested that the SPAD method is more
sensitive to water than to N stress [26-27], since reduced
crop cell turgor could influence the transmittance of NIR
(the near infrared region) energy through the leaf [7] and
this transmission of energy is the principle behind the
SPAD. Under drought stress, chlorophyll content was
strongly reduced in corn [28] and Phen content increased
by 10% in wheat [19]. In the current trial as well, the re-
duction in SPAD values and the increase in Dualex val-
ues under N0 were already apparent on the first meas-
urement date, but the effect of different SWC levels was
not as apparent. There was a trend change in tissue N
(Fig. 1-B) and SPAD levels (Fig. 1-C) between the D and
the W treatments before and after rewatering. These in-
dicators were on average lower under drought stress [28]
than in the fully-watered treatment before rewatering but
higher after. This may be explained by: 1) plant N uptake
reduced by drought stress; 2) reduction of leaf growth by
drought and the induction of shrinking [7] and; 3) main-
tenance of NH4+ and NO3− concentrations in the soil in
the drought treatment, which then became available for
uptake when water supply was restored [29]. As of 46
DAS, Dualex was lower in the drought stress treatment
than in the fully-watered treatment (Fig. 1-D), which is
inconsistent with the positive relationship reported earlier
between Phen accumulation and water stress [30]. How-
ever, a similar observation was made in a previous study [9]
and explained by leaf rolling, which results in reduction
of the Phen content of the measured epidermis due to less
sunlight accumulation. Another possibility is that
fully-watered plants gradually experienced a relative N
deficiency as they grew due to the limited N supply in the
pots, a situation that would result in higher Phen levels.
Crops have developed strategies for adapting to declines
in SWC. Drought modifies the balance between N ab-
sorption, remobilization, and incorporation and the N
cycling that occurs through the roots [31]. Rewatering after
severe drought leads to resumption of root growth [16] and
root vigour is stimulated with increasing N levels [32].
Buljovcic and Engels [16] reported that nitrate uptake ability
was restored 2 d after rewatering. In the current experi-
ment, the recovery of N-uptake ability after rewatering
was demonstrated by the increase in shoot biomass (Fig.
1) following rewatering, and evidenced by the changes in
SPAD, Dualex, and SPAD/Dualex ratio. Chl content has
been found to recover as part of the transition to a normal
watering regime [28]; however, the present study showed
that SPAD levels did not change significantly as they did
in the drought treatment (Fig. 1). Maybe the small size of
the pots resulted in a stress-induced lag in Chl recovery [28].
By contrast, Dualex readings were distinctly higher in the
rewatered treatment as compared to the drought treatment.
This may be an indirect consequence of the quick re-
sumption of growth as water supply was re-established,
leading to strong demand for the limited bulk N available
in the pots at this stage. After rewatering, SPAD/Dualex
ratios (Fig. 1) remained significantly higher in the
drought treatment as compared to the D-R treatment and
the W treatment.
4 Conclusion
SPAD and Dualex were better predictors of tissue N
concentration under drought conditions than under fully-
watered conditions as the experimental period progressed.
However, Dualex measurements showed a greater sensi-
tivity to N treatments and SWC changes resulting in bet-
ter discriminative power among treatments throughout
the experiment than SPAD or SPAD/Dualex ratio.
Therefore, Dualex is better to evaluate N fertilizer status
第 7期 朱娟娟等: 不同水氮处理对玉米氮素诊断指标的影响 1265
than SPAD across SWC levels.
Acknowledgements: The authors thank Edith Fallon and
Marcel Tétreault.
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