Annals of Botany 88: 941±945, 2001 doi:10.1006/anbo.2001.1522, available online at http://www.idealibrary.com on
S H O R T CO M M U N I CAT I O N
Zinc (Zn)-phosphorus (P) Interactions in Two Cultivars of Spring Wheat (Triticum aestivum L.) Diering in P Uptake Eciency Y.- G . ZH U * , S . E . S M I T H and F. A . S M I T H Cooperative Research Centre for Molecular Plant Breeding, Centre for Plant Root Symbioses, Department of Soil and Water, The University of Adelaide, SA 5005, Australia Received: 8 May 2001 Returned for revision: 19 June 2001 Accepted: 11 July 2001 Zinc-phosphorus (Zn-P) interactions were investigated in two wheat cultivars (Brookton and Krichau) diering in P uptake eciency. The experiment was carried out in a growth chamber. Rock phosphate or CaHPO4 were used as P sources, and ammonium nitrate or nitrate only as nitrogen sources. Two Zn levels were used: 0.22 and 2.2 mg ZnSO4.5H2O kgÿ1. The results con®rmed that Brookton had a higher P uptake eciency than Krichau under low P conditions, irrespective of nitrogen and Zn supply. Zn supply had little eect on tissue P concentration and P uptake per unit of root weight in either cultivar, irrespective of nitrogen supply. An increase in P availability caused a signi®cant reduction in Zn uptake per unit of root weight, and tissue concentration of Zn in both cultivars. The reduction in tissue Zn concentration cannot be explained entirely by a dilution eect. Zn uptake by, and Zn concentrations in, Brookton (with high P uptake eciency) were signi®cantly lower than those of Krichau. Zn concentrations in Brookton were more sensitive to P uptake than those in Krichau. It is suggested that high P uptake eciency may depress plant uptake of Zn, and therefore cause a reduction in the concentration (density) of Zn in grains # 2001 Annals of Botany Company of wheats grown in low P (and possibly low Zn) soils. Key words: Phosphorus eciency, translocation, uptake, zinc-phosphorus interaction, wheat.
I N T RO D U C T I O N Phosphorus (P) and zinc (Zn) de®ciencies are widespread nutritional constraints on crop production in many parts of the world, and phosphorus-zinc interactions have been widely investigated (Marschner, 1995). Increasing the availability of P in the growth medium can induce Zn de®ciency in plants by altering soil and plant factors (Robson and Pitman, 1983), but little is known about speci®c mechanisms. Many studies have shown that a low Zn supply but a high P supply markedly enhance P concentration in plant tissues, which may cause P toxicity and contribute to symptoms resembling Zn de®ciency (Loneragan et al., 1979, 1982; Cakmak and Marschner, 1986; Webb and Loneragan, 1988, 1990). However, little information is available on the eect of P availability on plant uptake of Zn, and Zn concentration in plant tissues. In sand-based pot culture, Lu et al. (1998) demonstrated that an increase in P availability did not cause a signi®cant decrease in Zn concentration in oilseed rape (Brassica napus). However, their results indicated that P application did not markedly aect tissue P concentration either. Singh et al. (1988) and Gianquinto et al. (2000) observed that an increase in P supply depressed Zn concentration in Phaseolus vulgaris, but they ascribed the dierence to a dilution eect of plant growth. Therefore it remains unclear * For correspondence. Fax 00 61 8 830 3 6511, e-mail yongguan.
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whether an increase in P availability in the growth medium can reduce Zn uptake by plant roots. Crop plants show genotypic variation in P uptake eciency (i.e. total P uptake; mg P per plant), and breeding for P-ecient crop cultivars has been recognized as one approach to the management of P-de®cient soils (Graham, 1984; Caradus, 1994). Since P-ecient crop cultivars are designed to be grown in soils with limited P supply, and with limited Zn supply in many cases, interactions between P uptake eciency and Zn uptake are of practical importance. Firstly, Zn supply may aect the expression of P uptake eciency in pot assays or under ®eld conditions. For example, Huang et al. (2000) recently observed that Zn de®ciency (low tissue Zn concentrations) causes an increase in the expression of P transporter genes in barley roots. Secondly, enhancing P uptake eciency may cause a decrease in plant uptake of Zn, leading, potentially, to low Zn concentrations (densities) in food. There are increasing concerns over the eects of low levels of micronutrients (including Zn) in food on human health (Welch and Graham, 1999). Nevertheless, interactions between P uptake eciency and Zn uptake remain largely unknown. The present experiment was undertaken to investigate the interactive eects of P and Zn supply on plant growth, root uptake and tissue concentration of P and Zn in two wheat cultivars diering in P uptake eciency. The experiment was also used to evaluate the eects of nutrition on cationanion balance in the two cultivars (Zhu et al., 2001). # 2001 Annals of Botany Company
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Zhu et al.ÐZinc (Zn)-phosphorus (P) Interactions in Two Cultivars of Spring Wheat T A B L E 1. Experimental treatments
Treatment code
Nitrogen source
Phosphorus source
Zn supply
NO-RP-LZ NO-RP-HZ NO-CaP-LZ NO-CaP-HZ
Nitrate Nitrate Nitrate Nitrate
Rock phosphate (RP) Rock phosphate (RP) CaHPO4 (CaP) CaHPO4 (CaP)
Low Zn High Zn Low Zn High Zn
AN-RP-LZ AN-RP-HZ
Ammonium nitrate (AN) Ammonium nitrate (AN)
Rock phosphate (RP) Rock phosphate (RP)
Low Zn High Zn
only only only only
(NO) (NO) (NO) (NO)
T A B L E 2. Phosphorus concentrations in plant tissues, total P uptake (TP) and speci®c P uptake (SPU) of two wheat cultivars grown in sand culture Tissue P concentration mg g ÿ1 d. wt
SPU
mg P per pot
mg P g ÿ1 root d. wt
Treatment
Cultivar
Shoot
NO-RP-LZ
Brookton Krichau Brookton Krichau
0.73 0.89 0.69 0.91 0.05
0.79 0.65 0.71 0.63 0.09
0.86 0.61 0.80 0.63 0.16
1.79 1.77 1.71 1.79 0.15
Brookton Krichau Brookton Krichau
1.96 2.07 1.58 1.93 0.25
1.68 1.40 1.73 1.61 0.15
5.88 4.41 6.66 4.91 0.86
5.94 5.49 4.05 4.37 0.68
Brookton Krichau Brookton Krichau
3.53 3.57 3.37 3.33 0.36
2.97 2.57 2.93 2.22 0.49
12.51 11.89 12.83 12.72 0.63
8.91 8.36 8.45 6.95 0.65
NO-RP-HZ LSD005 AN-RP-LZ AN-RP-HZ LSD005 NO-CaP-LZ NO-CaP-HZ LSD005
M AT E R I A L S A N D M E T H O D S Full experimental details are described in Zhu et al. (2001). A brief summary is provided here. Thoroughly-washed Laer sand was supplemented with two sets of nutrients with dierent nitrogen sources [NH4NO3 or Ca(NO3)2]. Two zinc levels were used: 0.22 and 2.2 mg ZnSO4.7H2O kg ÿ1 sand for low and high Zn supplies respectively. Two P sources were compared: high P supply with 0.5 g CaHPO4 kg ÿ1 sand (CaP) and low P supply with 1 g rock phosphate kg ÿ1 sand (RP) ( particle size 5250 mm; North Carolina, P content around 17 %). Treatment CaP was used as a positive control (sucient P supply) and only nitrate (NO) was used as the nitrogen source. In RP treatments, two nitrogen sources [nitrate only (NO) and ammonium nitrate (AN)] were used to alter the P availability through rhizosphere activities. All treatments and their abbreviated names are listed in Table 1; each treatment was repeated three times. Two cultivars of spring wheat (Triticum aestivum L.) were used: Brookton and Krichau. Brookton has a higher P uptake eciency than Krichau (Zhu et al., unpubl. res.) Four germinated seeds were sown in each pot (6.2 cm diameter and 26 cm depth) ®lled with 1 kg of sand. Each pot was thinned to two plants 3±4 d after emergence. The experiment was conducted in a growth cabinet set at 20 8C
Root
TP
day/15 8C night with a 14 h photoperiod (260 mmol m ÿ2 s ÿ1). Plants were harvested 4 weeks after emergence. After harvest, plant roots were washed thoroughly to remove sand particles, and the plants were divided into shoots and roots. Root and shoot samples were oven dried at 70 8C for 24 h, and dry weights recorded. Tissue samples were then ground. Subsamples were digested with nitric acid (70 %) and analysed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) for P, Zn, Ca, Mg, Na and K. Certi®ed plant material was used in acid digestion and ICP-AES analysis to assure the quality of the data. All data were subjected to analysis of variance (ANOVA) using PC window-based Genstat (Genstat 5 Committee, 1994). R E S U LT S P uptake Concentrations of P in both shoots and roots increased signi®cantly (P 5 0.001) in the series: NO-RP 5 ANRP 5 NO-CaP, irrespective of genotype and Zn supply (Table 2). In treatment NO-CaP (in which external P was not limiting), shoot P concentration did not dier between the two cultivars, whereas root P concentration was slightly
Zhu et al.ÐZinc (Zn)-phosphorus (P) Interactions in Two Cultivars of Spring Wheat T A B L E 3. Proportion of total P allocated to shoots in wheat plants grown in sand culture Low Zn supply Treatment NO-RP AN-RP NO-CaP LSD005
High Zn supply
Brookton
Krichau
Brookton
Krichau
0.56 0.72 0.67
0.63 0.75 0.69
0.59 0.57 0.65
0.65 0.63 0.68
0.025
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SPU of both cultivars signi®cantly in treatment AN-RP, and that of Krichau in treatment NO-CaP. Zn uptake
higher in Brookton than in Krichau under a high Zn supply. In treatments NO-RP and AN-RP, shoot P concentration in Brookton was signi®cantly lower than that in Krichau, except in treatment AN-RP-LZ, and root P concentration in Brookton was signi®cantly higher than in Krichau when Zn supply was low. Zn supply did not aect tissue P concentration signi®cantly, except that high Zn supply was associated with reductions in P concentration in shoots of Brookton in treatment AN-RP and in roots of Krichau in treatment NO-RP. There were highly signi®cant dierences (P 5 0.001) in total P uptake among treatments NO-RP, AN-RP and NOCaP (Table 2). Total P uptake by Brookton was signi®cantly higher than by Krichau in treatments NO-RP and AN-RP, whereas in treatment NO-CaP the cultivars took up the same amount of P. Zn supply did not aect total P uptake by Brookton or Krichau. Low Zn supply caused an increase in the proportion of total P allocated to shoots in treatment AN-RP, but not in treatments NO-RP or NOCaP (Table 3). Generally, the two cultivars had similar values of speci®c P uptake (SPU; phosphoros uptake per unit root dry weight) except in treatment NO-CaP with high Zn supply when Brookton had a higher SPU than Krichau (Table 2). Zn supply did not aect the SPU of either cultivar in treatment NO-RP. However, high Zn supply depressed the
High Zn supply caused a signi®cant increase in total Zn uptake and concentration of Zn in shoots and roots, irrespective of genotype and treatment (Table 4). Zn concentration in shoots fell in the series: NO-RP 4 ANRP 4 NO-CaP. Zn concentration in shoots of Krichau was generally higher than that in shoots of Brookton, especially in RP treatments. Zn concentrations in roots in treatments NO-RP and NO-CaP were signi®cantly higher in Brookton than in Krichau only with high Zn supply; in treatment AN-RP there was no signi®cant dierence in root Zn concentration between the two cultivars. Speci®c Zn uptake (SZU) in treatments NO-RP and ANRP was signi®cantly higher than in treatment NO-CaP (Table 4). There were signi®cant dierences in SZU between Brookton and Krichau in treatments NO-RP and AN-RP but not in treatment NO-CaP. High Zn supply resulted in a signi®cant increase in SZU within each P treatment and genotype (P 5 0.001). The proportion of total Zn allocated to shoots diered signi®cantly between the two cultivars and, in all treatments, Krichau allocated a higher percentage of Zn to shoots than Brookton (data not shown). DISCUSSION This experiment con®rmed that the wheat cultivar Brookton is more ecient in P uptake than Krichau, as found previously (Zhu et al., unpubl. res.), and that the dierence was not aected by Zn supply (Table 2). In RP treatments, changing the nitrogen source from nitrate only to ammonium nitrate caused a substantial increase in growth (Zhu et al., 2001) and total P uptake (Table 2). This was probably due to the increase in H excretion that occurs
T A B L E 4. Total Zn uptake, zinc concentration in plant tissues and speci®c Zn uptake (SZU) of two wheat cultivars grown in sand culture Zn concentrations Total Zn uptake Total Zn in shoots mg per pot
Shoots
mg g ÿ1 d. wt
Roots
SZU mg g ÿ1 root d. wt
Treatment
Cultivar
NO-RP-LZ
Brookton Krichau Brookton Krichau
27.7 30.0 37.5 40.7
15.6 22.2 19.6 29.4
23.8 51.6 28.8 65.1
25.4 22.9 38.1 31.9
58.0 87.4 80.1 114.9
Brookton Krichau Brookton Krichau
64.3 66.5 148.4 130.7
37.5 44.2 52.2 61.3
17.4 27.8 21.6 38.2
27.1 27.8 58.6 61.8
64.9 82.8 90.3 116.4
Brookton Krichau Brookton Krichau
55.5 53.4 105.3 121.4 11.9
29.3 31.3 45.5 60.8 6.8
12.4 13.5 18.3 23.4 4.5
18.6 15.6 39.3 33.0 4.5
39.5 37.5 69.3 66.3 9.5
NO-RP-HZ AN-RP-LZ AN-RP-HZ NO-CaP-LZ NO-CaP-HZ LSD005
944
Zhu et al.ÐZinc (Zn)-phosphorus (P) Interactions in Two Cultivars of Spring Wheat
when ammonium is a signi®cant component of available N (e.g. Raven and Smith, 1976; Gahoonia et al., 1992). Increased solubility of rock phosphate under low pH may also have contributed to increased growth. With a sucient P supply (NO-CaP treatments), the two cultivars displayed the same total P uptake capacity (Table 2). It has been widely reported that Zn de®ciency may be associated with an increase in P uptake and/or tissue P concentration, and it can cause P toxicity in hydroponic culture. Webb and Loneragan (1990) argued that the enhanced P uptake in Zn-de®cient plants was mediated primarily through the concentration of Zn in shoots and roots. Huang et al. (2000) recently demonstrated that Zn de®ciency causes an increase in the expression of P transporter genes in both P-de®cient and P-sucient barley roots. In this study, using sand culture, Zn de®ciency was not evident even under low Zn supplies, and Zn supply had only a marginal eect on tissue P concentration in both wheat cultivars (Table 2). However, an increase in Zn supply did depress speci®c P uptake (SPU) in some cases (Table 2), and the reduction was apparently associated with dierences in total Zn uptake, and root Zn concentration. Dierences in root Zn concentration, and total Zn uptake between low and high Zn supplies in treatments AN-RP and NO-CaP were over 90 %, whereas in treatment NO-RP they were less than 50 % (Table 4). This can be correlated with the signi®cant dierences in SPU associated with Zn supply in treatments AN-RP (both cultivars) and NO-CaP (Krichau), but not in treatment NO-RP. Although the results showed that root Zn concentration was more closely correlated with SPU, the role of shoot Zn concentration could not be excluded. Results (Table 3) showed that low Zn supply slightly increased the proportion of P in shoots in treatment ANRP but not in treatments NO-CaP and NO-RP. This may
be due to the marked reduction in root/shoot ratio under low Zn supply (Zhu et al., 2001). A higher proportion of P allocated to shoots under Zn de®ciency may be the result of an interruption of the negative feedback of P uptake by reducing the translocation (recycling) of P from shoots to roots (basipetal transport) (Marschner and Cakmak, 1986; Webb and Loneragan, 1988). The results provide indirect evidence that low Zn supply may reduce recycling of P when P supply is limited (AN-RP). However, under severe P de®ciency (treatment NO-RP), recycling may be unimportant in determining the distribution of P between shoots and roots as the total P uptake is so low. Large applications of fertilizer P to soils that are low in available Zn can depress tissue Zn concentration or may even induce Zn de®ciency (Robson and Pitman, 1983; Gianquinto et al., 2000). The present results demonstrate that an increase in available soil P was associated with lower Zn concentrations in shoots (Table 4), which is in general agreement with the ®nding of Gianquinto et al. (2000). It has been argued that the P-induced decrease in Zn concentration is caused by a dilution eect of increased shoot growth rather than by reduced Zn uptake by roots (Singh et al., 1988; Gianquinto et al., 2000). However, in the present case, reduced Zn concentrations cannot be fully explained by a dilution eect. For example, Brookton had similar shoot and root biomass in treatments AN-RP and NO-CaP (Zhu et al., 2001), but tissue Zn concentration in Brookton was signi®cantly lower in the NO-CaP treatment than in AN-RP (Table 4). This is further shown by the signi®cant increase in speci®c Zn uptake with low soil P availability over high P availability (Table 4), and the inverse relationship between shoot P and Zn concentrations (Fig. 1). The results also indicate that P uptake eciency may aect Zn uptake. Shoot Zn concentration in Krichau
Shoot Zn concentration (m g g-1 d.wt)
45
40 R = 0.44, P < 0.01
35
30
25
20
15
3
3.5
4
4.5
5
5.5
6
Total P uptake (mg per plant) F I G . 1. The relationship between total P uptake and shoot Zn concentration in plants of double haploid lines from the population derived from a cross between Brookton and Krichau, grown in a sand culture system (authors' unpubl. res.).
Zhu et al.ÐZinc (Zn)-phosphorus (P) Interactions in Two Cultivars of Spring Wheat (with low P uptake eciency) was higher than in Brookton (with high P uptake eciency), especially under P stress (Table 4). The higher Zn concentration in Krichau may be due in part to a `concentration eect' (i.e. less biomass production) resulting from low P uptake eciency. However, changes in biomass between treatments were less marked than changes in shoot Zn concentration (Table 3 in Zhu et al., 2001 and Table 4), and total shoot Zn accumulation in Krichau was higher than that in Brookton (Table 4). The dierences in shoot Zn concentration between the two cultivars may result from at least two processes. Firstly, high P uptake eciency may depress root Zn uptake. This is demonstrated in Table 4, showing that under P stress Krichau always had a higher SZU than Brookton. Secondly, high P uptake eciency may also involve a high rate of P transport from root to shoot via the xylem, and this may hinder Zn translocation from root to shoot, as the proportion of shoot Zn was consistently lower in Brookton than in Krichau (data not shown). Results from a separate experiment involving selected doubled haploid lines derived from a cross between Brookton and Krichau have shown that shoot Zn concentration decreased with an increase in total P uptake by dierent lines (Fig. 1 Zhu et al., unpubl. res.). In conclusion, the results of this experiment suggest that breeding crop cultivars for high P uptake eciency may inadvertently depress Zn uptake and tissue Zn concentration, and that Zn eciency should be considered when selecting cultivars with high P uptake eciency. The possible reduction in Zn concentration in cultivars with high P uptake eciency when grown in P-de®cient soils may in turn result in grains with low Zn content, particularly in calcareous soils in which Zn de®ciency often co-occurs with P de®ciency. However, Zn eciency is controlled by other factors that are independent of P nutrition, and selecting cultivars with both high P and Zn eciencies is possible. The results also suggested that Zn supply may not aect the expression of plant P uptake eciency. AC K N OW L E D GE M E N T S We wish to thank the Corporative Research Centre for Molecular Plant Breeding for ®nancial support. Technical assistance from Andrew Barritt is greatly appreciated. We also thank Ms Teresa Fowles for ICP analyses. L I T E R AT U R E C I T E D Cakmak I, Marschner H. 1986. Mechanism of phosphorus-induced zinc de®ciency in cotton. I. Zinc de®ciency-enhanced uptake rate of phosphorus. Physiologia Plantarum 68: 483±490.
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