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JOURNAL OF PLANT NUTRITION, 23(9), 1251-1265 (2000)
Differences in Shoot Growth and Zinc
Concentration of 164 Bread Wheat
Genotypes in a Zinc-Deficient Calcareous
Soil
B. Torun,a G. Bozbay,a I. Gultekin,b H. J. Braun,c H. Ekiz,b
and I. Cakmaka,1
a
Cukurova University, Faculty of Agriculture, Dept. Soil Science and Plant
Nutrition, Adana, Turkey
b
International Winter Wheat Research Center, Konya, Turkey
c
CIMMYT, POB 39 Emek 06511 Ankara, Turkey
ABSTRACT
A greenhouse experiment was carried out to study severity of the zinc (Zn)
deficiency symptoms on leaves, shoot dry weight and shoot content and
concentration of Zn in 164 winter type bread wheat genotypes (Triticunt
aestivum L.) grown in a Zn-deficient calcareous soil with (+Zn=10 mg Zn
kg-1 soil) and without (-Zn) Zn supply for 45 days. Tolerance of the genotypes
to Zn deficiency was ranked based on the relative shoot growth (Zn efficiency
ratio), calculated as the ratio of the shoot dry weight produced under Zn
deficiency to that produced under adequate Zn supply. There was a substantial
difference in genotypic tolerance to Zn deficiency. Among the 164 genotypes,
108 genotypes had severe visible symptoms of Zn deficiency (whitish-brown
1
Corresponding author (fax #: 90-322-3386747; e-mail address: [email protected]).
1251
Copyright © 2000 by Marcel Dekker, Inc.
www.dekker.com
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1252
TORUNETAL.
necrotic patches) on leaves, while in 25 genotypes Zn deficiency symptoms
were slight or absent, and the remaining genotypes (e.g., 31 genotypes) showed
mild deficiency symptoms. Generally, the genotypes with higher tolerance
to Zn deficiency originated from Balkan countries and Turkey, while
genotypes originating from the breeding programs in the Great Plains of the
United States were mostly sensitive to Zn deficiency. Among the 164 wheat
genotypes, Zn efficiency ratio varied from 0.33 to 0.77. The differences in
tolerance to Zn deficiency were totally independent of shoot Zn concentrations,
but showed a close relationship to the total amount (content) of Zn per shoot.
The absolute shoot growth of the genotypes under Zn deficiency corresponded
very well with the differences in tolerance to Zn deficiency. Under adequate
Zn supply, the 10 most Zn- inefficient genotypes and the 10 most Zn-efficient
genotypes were very similar in their shoot dry weight. However, under Zn
deficiency, shoot dry weight of the Zn-efficient genotypes was, on average,
1.6-fold higher compared to the Zn-inefficient genotypes. The results of this
study show large, exploitable genotypic variation for tolerance to Zn deficiency
in bread wheat. Based on this data, total amount of Zn per shoot, absolute
shoot growth under Zn deficiency, and relative shoot growth can be used as
reliable plant parameters for assessing genotypic variation in tolerance to Zn
deficiency in bread wheat.
INTRODUCTION
Zinc deficiency has been shown as a most widespread micronutrient deficiency
in soils. The extent of Zn deficiency in soils is comparable to the extent of
macronutrient deficiencies (White and Zasoski, 1999). Zinc deficiency is a limiting
factor for wheat production in many areas of Australia (Graham et al., 1992),
Turkey (Cakmak et al., 1996a, 1999a) and India (Rathore et al., 1980).
When compared with rye, triticale and barley, wheat possesses a lower tolerance
to Zn deficiency (Cakmak et al., 1997a, 1998). Among wheats, durum wheats
have a greater sensitivity to Zn deficiency than bread wheats. However, large
variation for tolerance to Zn deficiency exists in bread wheat, indicating that
breeding genotypes with improved tolerance to Zn-deficient soils is warranted
(Rengel and Graham, 1995a; Cakmak et al., 1996b;,1999b; Kalayci et al., 1999).
In recent years, an increasing effort has been devoted to better understanding the
factors involved in genotypic variation for tolerance to Zn deficiency, which will
allow development of reliable screening procedures for assessing Zn-efficient (or
Zn-deficiency-tolerant) genotypes. The term "Zn efficiency" is defined here as
the ability of a genotype to grow and yield better under Zn-deficient conditions
compared to other given genotypes. This definition has often been used in previous
studies (Graham, 1984; Graham et al., 1992; Cakmak et al., 1997a).
Development of Zn-efficient wheat cultivars is important for several reasons.
Zinc-efficient genotypes can contribute not only to reducing the costs of fertilizer
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Zn CONCENTRATION OF 164 BREAD WHEAT GENOTYPES
1253
inputs, but also to overcoming the problems related to subsoil Zn deficiency. In
Zn-deficient areas, subsoil is particularly deficient in Zn which cannot be solved
by usual fertilizer application practices (Graham et al., 1992; Nable and Webb,
1993). In most cases, application of Zn fertilizers in the top soil cannot efficiently
meet crop requirement for Zn, as this part of soil is often dried and Zn cannot
move down the soil profile (Grewal et al., 1997; Grewal and Graham, 1999).
Rengel (1999) recently reported that expression of high Zn efficiency in a
genotype is caused by more than one mechanism. Several morphological and
physiological factors have been described to correlate with the growth performance
of genotypes under Zn-deficient conditions, such as relative shoot growth (Zn
efficiency ratio), shoot/root ratios, scoring visual Zn deficiency symptoms,
proportion of the fine roots in whole root system, exudation of Zn-mobilizing
phytosiderophores, or measurement of activities of Zn-containing enzyme (Graham
andRengel, 1993;Cakmaketal., 1998,1997b; Rengel andRömheld, 1999; Rengel,
1999). However, the previous and recent studies dealing with characterization of
genotypic differences in Zn efficiency were often limited in their conclusions
because in most experiments a) only few wheat genotypes were included, b) bread
wheats with high Zn efficiency were compared with durum wheats with very low
Zn efficiency, and c) plants were usually grown in nutrient solution.
Therefore, the aim of this study was to compare 164 bread wheat genotypes for
their tolerance to Zn deficiency in a calcareous soil with and without Zn fertilization.
All genotypes were evaluated for severity of Zn deficiency symptoms and relative
shoot growth (Zn efficiency ratio). In addition, all genotypes were tested for the
concentration and content of Zn in shoots.
MATERIALS AND METHODS
Seeds of 164 genotypes were obtained from the Turkey/CIMMYT/ICARDA
International Winter Wheat Improvement Program (IWWIP). The lines originated
from the IWWIP as well as breeding programs in Turkey, Eastern Europe, North
America, and China. Plants were grown under greenhouse conditions in plastic
pots filled with 2.2 kg soil. The soil used was a severely Zn-deficient soil obtained
from a Zn-deficient region in Central Anatolia, where growth and yield of wheat
is markedly reduced due to Zn deficiency in soil (Cakmak et al., 1996a). The soil
characteristics were: pH 8.04 (1:1 H2O:soil), CaCO3149 g kg 1 , organic matter 7
g kg 1 and soil texture was clay. Concentration of DTPA-extractable Zn was
extremely low, at 0.09 mg kg"1 soil as measured by the method of Lindsay and
Norvell (1978).
About 20 seeds were sown in each pot, and after emergence the seedlings were
thinned to 10 seedlings pot 1 . Plants were grown for 45 days with (+Zn=10 mg Zn
kg 1 soil) and without (-Zn) Zn fertilization in form of ZnSO4 7H2O, together with
a basal treatment of 200 mg N kg 1 soil as Ca(NO3)24H2O,100 mg P kg 1 soil and
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TORUNETAL.
125 mg K kg 1 soil as KH2PO4. All nutrients were mixed thoroughly with soil
before potting. All pots were randomized every 3-4 days in greenhouse and watered
daily with deionized water.
Before harvesting, plants grown in Zn-deficient soil were scored for the severity
of leaf symptoms due to Zn deficiency, i.e., whitish-brown necrotic patches on
the leaves. The severity of symptoms was assessed by giving a score from 1 (very
severe symptoms) to 5 (symptoms slight or absent). At harvest, plants were at
the beginning of stem elongation. Only shoots were harvested, dried at 70°C and
ashed at 550°C for determination of Zn concentration in whole shoot. Zinc
concentration was measured by an atomic absorption spectrometry after dissolving
the ash in 3.3% HC1. The Zn efficiency ratio (relative shoot growth) was calculated
as the ratio of shoot dry matter produced under Zn deficiency (-Zn) to that under
adequate Zn supply (+Zn). As the amount of Zn in seeds can influence the Zn
efficiency of genotypes (Rengel and Graham, 1995c), the Zn concentration of
some genotypes used in this study was determined as described above. Due to
limited number of seeds, two pots with 20 homogeneous seedlings were used for
each treatment. After termination of the experiment and determination of the 10
most Zn-efficient and the 10 most Zn-inefficient genotypes, the experiment was
replicated with these 20 genotypes (5 seedlings pot"1), and the results showed that
the differences in genotypic tolerance to Zn deficiency were very similar to the
experiment with 164 genotypes. Therefore, the results of the first experiment are
presented here. All measurements were taken in 3 replications (3 samples from 2
pots). For statistical treatments see legends of the tables.
RESULTS
The first visual symptom of Zn deficiency was the reduction in shoot elongation.
Thereafter, youngest leaves became light yellowish and middle-aged leaves
developed whitish-brown necrotic patches, mostly on the middle parts. Among
the 164 wheat genotypes, large differences were observed in the severity of visual
symptoms of Zn deficiency on leaves. Mostly, genotypes from programs located
in the Great plains of the United States showed higher sensitivity to Zn deficiency
while in genotypes from East European countries, in particular Romania and
Bulgaria, Zn deficiency symptoms were mild or slight. Genotypes from Mexico
were found among either the most tolerant or the most sensitive ones. From the
164 wheat genotypes, leaf symptoms of Zn deficiency were very severe in 23,
severe in 85, mild in 31, slight in 10 and very slight or absent in 15 genotypes
(Table 1). The genotypes showing very severe Zn deficiency symptoms had the
lowest shoot dry weight under Zn deficiency, but these genotypes showed the
highest shoot dry weight under adequate Zn supply (Table 1). Accordingly, the
lowest Zn efficiency ratio was found in genotypes with very severe Zn deficiency
symptoms.
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Zn CONCENTRATION OF 164 BREAD WHEAT GENOTYPES
1255
TABLE 1. Effect of Zn supply (+ZN=10 mg Zn kg 1 soil) on the shoot dry weight
and relative shoot growth (Zn efficiency ratio) of the genotypes grouped according
to the score of Zn deficiency symptoms. Plants were grown in a Zn-deficient
calcareous soil for 45 days.
Leaf symptoms of
Zn deficiency*
Shoot dry weight
Zn Efficiency
+Zn
-Zn
ratio**
1
(g plant' )
(%)
1 (n=23)
0.33±0.44
0.77±0.08
43
2 (n=85)
0.38±0.06
0.73±0.08
52
3 (n=31)
0.38±0.06
0.72±0.10
53
4 (n=10)
0.41±0.06
0.71±0.08
58
5(n=15)
0.44±0.05
0.69±0.07
64
Mean
0.39
0.72
54
*l-very severe, 2=severe, 3=mild, 4=slight, and 5=very slight or absent.
**Zn efficiency=(dry weight at -Zn/dry weight at +Zn) x 100.
TABLE 2. Effect of Zn supply (+Zn= 10 mg kg"1 soil) on concentration and content (total
amount of Zn in shoots of the genotypes grouped according to the severity of Zn deficiency
symptoms.
Leaf symptoms of
Zn deficiency*
Zn concentration
-Zn
+Zn
Zn content
-Zn
1
+Zn
1
(mgkg' dry wt.)
(ug plant' )
1 (n=23)
7.3±0.7
44±5
2.4±0.4
34±4
2 (n=85)
6.7±0.8
45±7
2.6±0.5
33±6
3(n=31)
6.9±0.8
44±8
2.6±0.6
31±6
4 (n=10)
6.8±0.9
40±8
2.8±0.4
28±6
5 (n=15)
7.5±1.1
42±7
3.3±0.5
29±5
Mean
7.0
43
2.7
31
*l-very severe, 2=severe, 3=mild, 4=slight, and 5=very slight or absent.
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1256
TORUNETAL.
Irrespective of the severity of Zn deficiency symptoms, all genotypes had more
or less similar Zn concentrations under Zn deficiency (6.8 to 7.5 mg kg-1 DW,
Table 2). Also under adequate Zn supply, shoot Zn concentrations did not vary
much between the genotypes, and ranged between 40 to 45 mg kg"1 DW. In the
case of Zn content, there was an increase in Zn content with a decrease in the
severity of Zn deficiency symptoms in plants without Zn supply (Table 2).
Similar effects were also found regarding the Zn efficiency ratio in the second
experiment with the 10 best and the 10 worst genotypes from 164 wheat genotypes
tested in the first experiment. The average score of the Zn deficiency symptoms
of the 10 most Zn-efficient genotypes was 3.8, while the 10 most Zn-inefficient
genotypes had an average score of 1.6 (Table 3). Under Zn deficiency, all Znefficient genotypes produced more shoot dry weight than the Zn-inefficient
genotypes. In contrast, in the Zn supply treatment, Zn-inefficient genotypes had,
on average, more shoot dry matter production (Table 3), indicating that lower
shoot dry weight of Zn-inefficient genotypes under Zn deficiency is really caused
by their Zn-inefficiency and not by a genotypically slower shoot growth rate.
Therefore, there was a highly significant correlation between the shoot dry weight
of the genotypes under Zn deficiency and Zn efficiency ratio (Figure 1).
The differences in tolerance to Zn deficiency among the genotypes seem to be
inherent and not related to the seed content of Zn. Due to limited number of seed,
we analyzed seeds of only some Zn-efficient and Zn-inefficient genotypes for the
Zn content. As the seed weights of the genotypes were similar, seed Zn
concentrations did not differ between the genotypes. For example, seed Zn contents
of Zn-efficient genotype 602-156-22 and Zn-inefficient genotype SB-396-6 were
similar (around 0.58 ng Zn seed1). Also, seed Zn contents of Zn-efficient genotype
4206/3/911138.10 and the most Zn-inefficient genotype PYN//TAM101/AMIGO
were similar (0.46 ug Zn seed1).
On average, the most Zn-efficient and the most Zn-inefficient genotypes did
not differ in shoot Zn concentration under deficient and adequate Zn supply (Table
4). Under Zn deficiency, the lowest and the highest Zn concentrations in shoot
were found within Zn-efficient genotypes, namely the genotypes MV9 with 5.5
mg Zn kg"1 dry wt and 602-156-22 with 8.9 mg Zn kg 4 dry wt. In contrast to Zn
concentration, Zn content per shoot showed larger difference between Zn-efficient
and Zn-inefficient genotypes (Table 4). Under Zn deficiency, the Zn-efficient
genotypes contained much more Zn than the Zn-inefficient genotypes. The lowest
Zn content per shoot was found in the most Zn-inefficient genotype PYN//
TAM101/AMIGO with 1.6 ug Zn per shoot. The second most Zn-efficient
genotype 602-156-22 showed the highest Zn content per shoot, i.e., 4.3 ug Zn
(Table 4).
The relationship between Zn efficiency, Zn concentration and Zn content is
shown in Figure 2 for all 164 wheat genotypes. Zinc efficiency ratios did not
show any relation with Zn concentration (rMXOOOS), but significantly correlated
with the Zn content of genotypes (^=0.282***).
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TABLE 3. The effect of Zn supply (+Zn=10 mg kg 1 soil) on leaf symptoms of Zn deficiency, shoot dry weight, and Zn efficiency
ratio of the 10 most Zn-efficient genotypes selected among 164 bread wheat genotypes grown in a Zn-deficient calcareous soil for
45 days. Selection of 10 Zn-efficient and 10 Zn-inefficient genotypes was based on the Zn efficiency ratio of the genotypes.
Zn-efficient genotype
Origin of genotype
21031/CO652142//MARA/SUT/4/CERCO
602-156-22
ZOMBOR
420673/911B8.10
AGRI//BJY7/VEE
ID800994.W/VEE
YAN7578.128
MV9(ARPATHAN-9)
DAGDAS
JUWEL/LV24//LV32
Mean
Turkey
Zn-inefficient genotype
ARAPAHOE
HARUS
TEMU39.76/CHAT//CUPE/3/M1223-3D-1D/ALD
63-122-77-2/NO66//LOV2/3/KVZ/HYS/4/RJJ/5/BLL
CNN/KHARKOV//YKT/3/NO64/4/SR11/5/SX
SB-396-6
KS82142
SN64//SKE/2*ANE/3/SX/4/BEZ/5/SERI
RSK/NAC
PYN//TAM101/AMIGO
Mean
Leaf symptoms of
Zn deficiency»
Hungary
Bulgaria
Mexico-USA-Turkey
Mexico-Turkey
China
Hungary
Turkey
Romania
USA
Canada
USA-Turkey
Turkey
Turkey
USA
USA
Mexico-USA-Turkey
Mexico
USA
*l-very severe, 2=severe, 3=mild, 4=slight, and 5=very slight or absent.
**Zn efficiency=(dry weight at -Zn/dry weight at +Zn) x 100.
Shoot dry matter
-Zn
+Zn
1
4
5
5
5
2
2
5
4
3
3
3.8
(g plant" )
0.51±0.06
0.66±0.02
O.48±O.O8
0.63±0.03
0.44±0.01
0.62±0.02
0.45±0.03
0.64±0.02
0.59±0.07
0.84±0.11
0.45±0.09
0.65±0.06
0.5O±0.03
0.74±0.06
0.46±0.01
0.69±0.06
0.43±0.06
0.65±0.01
0.41±0.01
0.62±0.04
0.47
0.67
1
2
3
1
1
1
3
1
2
1
1.6
0.29±0.01
0.33±0.04
0.22±0.01
O.30±0.05
0.30±0.02
O.32±O.O3
0.26±0.03
0.25±0.01
0.30±0.01
0.23±0.03
0.29
0.74±0.06
0.85±0.05
0.71±0.01
0.79±0.14
0.80±0.01
0.86±0.16
0.71±0.07
0.69±0.04
0.87iO.01
0.70±0.06
0.77
Zn efficiency
ratio**
(%)
77
76
71
70
70
69
68
67
66
66
70
39
39
38
38
38
37
37
36
34
33
37
§
o
I
M
O
O
V3
1258
TORUNETAL.
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DISCUSSION
The results of the present study showed existence of substantial differences in
tolerance to Zn deficiency. Zinc-efficient genotypes originate mainly from
programs in Eastern Europe and Turkey. Since Zn-deficiency is widespread on
the Central Anatolian Plateau of Turkey, it is understandable that Turkish breeders
inadvertently selected for Zn efficiency. The reason for the high level of Zn
efficiency among cultivars from Bulgaria and Romania is not understood at present,
since in both countries Zn-deficient soils are not a constraint. Most of the Zninefficient cultivars originated from breeding programs in the Great Plains of the
United States. No cultivar from the Great Plains was among the 30 most efficient
cultivars. Lack of Zn efficiency in genotypes from the Great Plains might explain
why only one line from the Great Plains, Bolal from Nebraska, was ever grown
on large scale rainfed areas in Turkey, although climate and rainfall patterns are
similar. During the last 30 years, several thousand lines from the United States
were tested unsuccessfully in Turkey as potential varieties for rainfed areas, but
were heavily used in the crossing program. With the identification of Zn deficiency
as adaptation limiting factor, introduced germplasm can be screened more
effectively and utilized more efficiently in Turkish wheat breeding programs.
Equally important, the causes of the recognized limited opportunity for the direct
release of introduced wheat cultivar is now better understood.
Despite existence of considerable variation in severity of Zn deficiency
symptoms and Zn efficiency ratios, no variation was, however, found in Zn
concentration between genotypes under Zn deficiency (Table 2 and Figure 2).
By contrast, the amount of Zn per shoot of the Zn-deficient genotypes showed a
better relation with the Zn efficiency ratio. This finding indicate that Zn content
per shoot, but not Zn concentration per unit of shoot dry weight, is a reliable
criterion for assessing genotypes for their tolerance to Zn deficiency, confirming
the previous results with genotypes of different cereal species (Graham and Rengel,
1993; Rengel and Graham, 1995b; Cakmak et al., 1997b, 1998). In most cases,
tissue Zn concentration was not considered as a suitable measure for determination
of the Zn nutritional status of plants (Ghoneim and Bussler, 1980; Cakmak and
Marschner, 1987). Higher Zn content of the Zn-efficient genotypes (Tables 2
and 4) might be a reflection of greater capacity of these genotypes to absorb Zn
from soil. However, higher Zn content of the Zn-efficient genotypes under Zn
deficiency must not be necessarily related to enhanced Zn uptake capacity of the
genotypes. This could be a consequence of a higher dry matter production of the
Zn-efficient genotypes under Zn deficiency. Possibly, Zn-efficient genotypes
inherently have a greater utilization efficiency for Zn at the cellular level than the
Zn-inefficient genotypes, which thereby results in a better growth with a
corresponding enhancement in total Zn uptake. In studies with bread wheat
genotypes differing in Zn efficiency it has been found that Zn uptake rate did not
show a relation to differential Zn efficiency in bread wheat genotypes (Cakmak et
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Zn CONCENTRATION OF 164 BREAD WHEAT GENOTYPES
40
50
60
70
1259
80
Zn Efficiency (%)
FIGURE 1. Relationship between Zn efficiency and shoot dry weight of 164 bread wheat
genotypes grown in a Zn-deficient calcareous soil for 45 days.
al., 1998; Erenoglu et al., 1999). Therefore, in future experiments, a special
attention should be given to the measurement of the pool of physiological active
Zn within the cells.
For some genotypes, there was a poor relation between the severity of leaf
symptoms of Zn deficiency and the ratio of Zn efficiency (Table 3). KS82142
had a visual score of 3 (mild deficiency), but showed a large decrease in shoot
growth by Zn deficiency and thus had a very low Zn efficiency ratio (e.g., 37%).
By contrast, among the Zn-efficient genotypes AGRI//BJY//VEE and
ID800994.W/VEE showed severe Zn deficiency symptoms, but had a high Zn
efficiency ratio (e.g., 70%).
These results suggest that visual leaf symptoms of Zn deficiency (whitish-brown
patches on leaves) cannot be always reliable for screening genotypes for Zn
efficiency. Supporting this suggestion, Carrol and Loneragan (1968) showed
very severe reductions in shoot growth of different plant species by Zn deficiency
before the visible symptoms of Zn deficiency became apparent. As the decreases
in shoot elongation and leaf size precede the appearance of necrotic patches on
leaves (Cakmak et al., 1998; Cakmak and Braun, 1999), measurement of the rates
of leaf or shoot elongation might be a more reliable parameter for assessing
genotypes for their Zn efficiency. This point warrants further studies with
genotypes differing in Zn efficiency.
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TABLE 4. Effect of Zn supply (+Zn=10 mg kg1' soil) on the concentration and content (total amount) of Zn in shoots of the 10
most Zn-efficient and 10 most Zn-inefficient genotypes of among 164 bread wheat genotypes grown for 45 days in a Zn-deficient
calcareous soil. Selection of the 10 Zn-efficient and Zn-inefficient genotypes was based on the Zn efficiency ratio of the genotypes
for the formula see Table 3).
Zn-efficient genotypes
Origin of genotypes
21031/CO652142
602-156-22
ZOMBOR
4206/3/911B8.10
YAN7578.128
AGRI//BJY//VEE
ID800994.W/VEE
MV9 (ARPATHAN-9)
DAGDAS
JUWEL/LV24//LV32
Mean
Turkey
Bulgaria
Hungary
Bulgaria
China
Mexico-USA-Turkey
Mexico-Turkey
Hungary
Turkey
Romania
Zn-inefficient génotypes
ARAPAHOE
HARUS
TEMU39.76/CHAT//CTJPE/3/M1223-3D-1D/ALD
63-122-77CNN/KHARKOV//YKT/3/NO64/4/SR11/5/SX
SB-396-6
KS82142
SN64//SKE/2*ANE/3/SX/4/BEZ/5/SERl
RSK/NAC
PYN//TAM101/AMIGO
Mean
USA
Canada
USA-Turkey
Turkey
Turkey
USA
USA
Mexico-USA-Turkey
Mexico
USA
Zn concentration
-Zn
+Zn
(mgkg"1drywt.)
6.1±1.3
49±1
8.9±1.1
52±3
45±1
7.8±0.2
47±.l
8.2±0.4
6.8±1.3
34±3
41±3
6.7±0.4
7.3±0.3
48±0
38±0
5.5±0.8
6.6±1.3
41±1
6.7±0.3
37±3
7.1
43
7.2±0.9
6.8±0.6
7.4±1.8
5.9±0.8
6.6±0.9
8.2i0.2
6.4±0.9
7.8±0.3
7.U0.1
7.H1.6
7.1
46±4
45±1
49±2
44±1
51±3
41±5
52±6
40±2
42±1
40±l
45
Zn content
-Zn
+Zn
(ug planf')
3.1±0.3
32±1
4.3±1.2
33±3
28±0
3.4±0.0
30±l
3.7±0.4
3.4±0.1
25±1
3.9±0.2
34±2
3.3±0.6
31±3
26±2
2.5±0.4
27±2
2.9±0.9
23±0
2.8±0.2
3.3
29
2.1±0.3
2.2±0.4
2.0±0.5
1.7±0.0
1.9±0.4
2.6±0.3
1.7±0.4
2.0±0.0
2.1±0.1
1.6±0.2
2
34±0
38±1
34±2
34±6
41±2
34±2
37±8
27±3
36±2
27±2
34
o
Zn CONCENTRATION OF 164 BREAD WHEAT GENOTYPES
1261
u —
•
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9 -
•
•«
8 -
•
4
-
•
•
7 6 o
(J
5 4 30
.*• .V ::•••
•.«• ••• • •
i
i
40
50
«.
•
.* «
»
• • • •
§
O
•
2 -
N
r : = 0.0008
r = 0.282***
l
60
•»*
3 -
70
Zn Efficiency (%)
80
30
1
I
I
I
40
50
60
70
80
Zn Efficiency (%)
FIGURE 2. Relationships between Zn efficiency, Zn concentration and Zn content in
shoot of 164 bread wheat genotypes grown in a Zn deficient calcareous soil for 45 days.
Appearance of severe symptoms of Zn deficiency in the highly Zn-efficient
genotypes AGRI//BJY//VEE and ID800994.W/VEE (Table 3) may indicate that
Zn in these genotypes is re-translocated at greater amounts from older leaves into
meristematic tissues, allowing better growth under deficient supply of Zn, but
causing faster occurrence of visible symptoms of Zn deficiency on older leaves.
Zinc is a relatively highly phloem mobile micronutrient (Marschner, 1995) and
shows a high remobilization from older leaves into sink organs, especially under
deficient supply of Zn (Pearson and Rengel, 1994). No information is available
about genotypic differences in Zn remobilization from source to sink organs.
This interesting topic needs to be clarified in future experiments. The role of a
better nutrient remobilization from older to youngest/growing tissues in expression
of high nutrient efficiency has been documented for Ca-efficient cowpea (Horst
et al., 1992) and P-efficient sorghum genotypes (Furlani et al., 1984).
Shoot dry weight of genotypes under Zn deficiency appeared to be a suitable
parameter in differentiating genotypes for their tolerance to Zn deficiency. Under
Zn deficiency, all Zn-efficient genotypes had more shoot dry weight than Zninefficient genotypes, whereas under adequate Zn supply, Zn-efficient and Zninefficient genotypes were similar in shoot dry matter production (Table 1), even
most Zn-efficient genotypes accumulated, on average, less dry matter than Zninefficient genotypes (Table 3). This suggests that shoot growth under Zn-deficient
conditions can be a useful trait for distinguishing genotypes for their tolerance to
Zn deficiency. Recently, Rengel and Römheld (1999) showed in an experiment
with 10 wheat cultivars that shoot dry weight under Zn deficiency is a suitable
criterion to separate genotypes for their tolerance to Zn deficiency. Shoot dry
weight was also considered as a reliable plant parameter for screening genotypes
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for P efficiency in tomato (Coltman et al, 1985), rice (Fageria et al., 1988) and
wheat (Fageria and Baligar, 1999). However, this parameter should carefully be
used for screening genotypes for nutrient efficiency, when genotypes inherently
differ in habitus, growth rate or harvest index (i.e., tall and dwarf cultivars). Under
low supply of Zn, genotypes having higher growth rate (i.e., higher Zn demand)
can cause Zn deficiency stress more rapidly and decreases in growth than the
genotypes with slower growth rate (lower Zn demand) (Marschner, 1995).
Recently, we showed that shoot dry weight within and among Aegilops species
(Cakmak et al., 1999b) and diploid, tetraploid and hexaploid wheats (Cakmak et
al., 1999c) under Zn deficiency was not well related to the severity of Zn deficiency
symptoms or Zn efficiency ratios, because of their differential shoot growth rate.
However, in the present study, the genotypes tested showed more or less similar
growth rate under sufficient Zn supply. Therefore, there was a very close
relationship between Zn efficiency ratio and the absolute shoot dry weight under
Zn deficiency (Figure 1).
In conclusion, the present study showed existence of a substantial genotypic
variation for tolerance to Zn deficiency among 164 wheat genotypes, but no
variation was observed for shoot Zn concentration. For the genotypes tested in
the present study, the absolute and relative shoot growth (efficiency ratio) were
the most reliable parameters that can easily be used in breeding/selecting genotypes
with high tolerance to Zn deficiency. As the shoot growth may, however, vary
between the genotypes, irrespective of the level of Zn deficiency tolerance, the
shoot growth parameter can be used together with the scores of the Zn deficiency
symptoms.
ACKNOWLEDGMENTS
This work was supported by the DANIDA Project coordinated by the International
Food Policy Research Institute, Washington, DC.
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