EFFECT OF EXCESSIVE ZINC CONTENT IN SOIL ON THE PHOSPHORUS CONTENT IN WHEAT PLANTS

Ewa Stanisławska-Glubiak, Jolanta Korzeniowska
Institute of Soil Science and Plant Cultivation, Department of Soil Tillage and Fertilization in Jelcz-Laskowice, Poland

ABSTRACT

Three microplot trials with spring wheat were run on soil varying for pH level. Increasing Zn doses from 5 to 200 mg·kg ^-1 were tested. Zn applications resulted in a substantial rise of Zn content of wheat shoots. In addition, an interaction of zinc and phosphorus in the plants was found. Contrary to most reports in literature, the interaction was not solely an antagonism. It is only the excessive doses of zinc that restricted phosphorus uptake and lowered the concentration of that element in wheat shoots. Instead, with poor phosphorus supply to the plants moderate zinc doses stimulated phosphorus uptake and raised phosphorus content of plants. Zinc application rate and phosphorus status of plants had a greater impact on Zn-P interaction than did soil pH.

Key words: zinc, excess, phosphorus, interaction, wheat.

INTRODUCTION

Zinc contamination of soils is not a major problem in Poland but none-the-less it may occur locally. The study of the interaction among elements under their excessive supply in the soil is primarily of academic importance. Occasionally, it may be of practical relevance when reclaiming contaminated areas. One of the most important Zn interactions is that involving phosphorus most frequently referred to as antagonism. It is believed that the rise in the concentration of the former will result in the decreased content of the latter [1, 9, 17]. As a rule, investigators describe a decrease in Zn content of plants with increasing phosphorus supply in the soil or with increasing phosphorus fertilizer rates [1, 2, 16]. The studies of the inverse relationship i.e. of the impact of zinc on phosphorus buildup in plants are less numerous, especially when Zn contents of soil reach toxic levels. Occasionally, in cultivated plants lower phosphorus content or decreased phosphorus uptake is found, especially under excessive Zn supply [6, 15]. According to Boawn et al. [5] excess Zn restricts root growth which results in decreased P uptake. Those researchers believe that the cause behind that antagonism may be the precipitation of zinc phosphates in the roots. However it seems that there are also other mechanisms involved. A case in point are the results of a study in which foliar Zn spray of wheat lowered the phosphorus content of grain [12].

It is not always that Zn-P relationship can be referred to as antagonism. At times, increasing Zn rates stimulate phosphorus concentration of plants [3, 12]. Possibly it is the result of a tendency for a given species to maintain the right equilibrium between both elements. Research results suggest that the ratio of both elements must be maintained at an appropriate level. In the study of Li et al. [14] zinc fertilization of barley accompanied by a low phosphorus application caused the yields to increase but slightly whereas a higher phosphorus rate reduced the Zn:P ratio and increased the yields in a distinct manner.

The correct relationship of phosphorus to zinc is different for different species and is continuously modified as the plant grows older. With time, more phosphorus is absorbed and translocated while at the same time less zinc is transported from roots to shoots. The optimum P:Zn ratio for 10 and 20 day-old maize seedlings was found to be 8.5 and 12.4, respectively [1]. At the 6^th leaf stage, when above 200 the ratio is indicative of zinc deficiency [11]. The P:Zn ratio of 100 to 200 in wheat grain is indicative of good or very good zinc supply to the plants [12].

The pattern of zinc vs. phosphorus is also variety dependent [7, 8]. Some varieties within a species are characterized by higher susceptibility to either excess or deficiency of zinc probably because of differences in the ability to take up phosphorus [15, 18]. According to those investigators, the varietal differences for the response to phosphorus in turn are caused by different uptake of zinc by those varieties [14].

The proportions between zinc and phosphorus in the plant differ not only with plant, variety and development stage but are also dependent on physico-chemical properties of the soil. Since it governs nutrient availability soil pH is very important. The objective of this study is to examine the changes of phosphorus concentration of wheat plants under different soil pH as influenced by excessive zinc contents in soil.

EXPERIMENTAL PROCEDURE

The trials were run in the IUNG experiment station of Baborowko near Poznan in concrete-lined micro plots 1 x 1 x 0.6 m in size. The microplots were located in open space, were sunk in the ground and filled with appropriate soil. In dry weather periods the microplots were watered. Three experiments with spring wheat were performed in successive years including two on a medium-heavy soil and one on a very light soil. The soils were characterized with very high or high phosphorus content and medium-high zinc content (Table 1).

In order to differentiate soil pH lime or sulfur were applied and thus 3 or 4 different levels of pH were obtained on each soil. Four levels of zinc applied as ZnSO ₄·7H₂O were tested in combination with each soil pH. The rates applied on medium-heavy soil in experiment I were 0, 15, 30 and 100 mg·kg ^-1. In order to obtain more distinct zinc toxicity symptoms in experiment II the Zn rates were elevated to 0, 30, 60 and 200 mg·kg ^-1. On the light soil (experiment III) 0, 5, 15 and 45 mg·kg ^–1 were applied. Each zinc portion was dissolved in 5 l of water and thoroughly mixed with a 20 cm-thick layer of soil. All treatments received the same NPK fertilization as recommended for wheat and suited to given soil conditions. The experiments were laid out as completely randomized designs with four replications. Wheat samples (upper shoot portion 5 cm above the ground) were collected at the beginning of the shooting stage. They were analyzed for phosphorus using the flow spectrophotometry method and for zinc using the AAS method.

RESULTS

In all three experiments the zinc additions applied caused a consistent increase in the zinc concentration of wheat shoots (Table 2). The accompanying changes in phosphorus content were not so unequivocal.

In experiment I on the medium-heavy soil with the pH of 5.7 phosphorus content of non-zinc treated wheat was 0.31% thus falling within the low and optimum zinc contents as reported by Bergmann [4]. The lowest Zn application (15 mg·kg^-1) caused the phosphorus content to increase up to 0.45% (Fig. 1). The second Zn addition in the increasing order (30 mg·kg^-1) resulted in a fall in phosphorus content of plants but only as related to that caused the preceding Zn rate. It is only the rate of 100 mg·kg^-1 Zn that lowered the phosphorus content to a level below the optimum one. Likewise, on the soils with pH 6.6 and 7.0, where the phosphorus supply to the non Zn-treated wheat was relatively low, P content increased initially to decrease with increasing Zn rates verging on the lower limit of the optimum range. It is only on the soil with the highest pH (7.2) that the lowest Zn rate already reduced P content of wheat. In that case the P concentration of the plants from the zero Zn treatment was slightly higher and came within the mid-range of the optimum content.

In experiment II on medium-heavy soil, phosphorus contents of non-Zn treated wheat were much higher than those in experiment I. They came close to the upper limit of the optimum range or even exceeded the optimum (Fig. 2). Supposedly, the differences in the phosphorus status of plants between the experiments were related to weather conditions. Under better phosphorus nutrition the first two zinc rates (30 and 60 mg·kg^-1) did not produce any significant changes in the phosphorus concentration save for the 7.2 soil pH variant where a slight increase in phosphorus content occurred. In that case phosphorus content of non-Zn-treated plants was slightly lower than that in the remaining soil pH variants. The highest Zn rate, 200 mg·kg^-1, caused phosphorus concentration of wheat to decrease substantially, although not below the optimum range. It is noteworthy that the changes in phosphorus concentration of plants in the soil pH variant of 7.5 were much more suave that those in the variants involving lower soil pH.

In experiment III involving a very light soil where much lower zinc rates were applied (up to 45 mg·kg^-1) a similar effect of zinc addition was observed as either increase or decrease in phosphorus content of plants. The relationship of those changes to phosphorus supply to plants was not an unambiguous one. On a very acid soil (pH of 3.9) where phosphorus content came close to the lower limit of the optimum range the first zinc rate caused a slight decline in phosphorus content, the successive rates raising phosphorus contents. On the soil with the pH of 6.3, also characterized with poor phosphorus supply, the reverse was true. Initially, there was a very slight increase in phosphorus content due to the application of the lowest Zn rate, higher applications of Zn resulting in substantial reductions of plant phosphorus concentration. In the soil pH variant of 6.6 with phosphorus supply to plants reduced drastically, phosphorus concentration of plants increased along with increased zinc rates.

In each of the experiments the P to Zn ratio of plants was computed. The values varied extensively from 30 to more than 200. No relationship was found between that ratio and yield. Neither was a clear-cut boundary P to Zn ratio found at which yield decline occurs.

DISCUSSION OF RESULTS

In this study the interaction of phosphorus and zinc was determined, above all, by the amount of zinc addition and by the amount of phosphorus supply to plants. The zinc rates bringing about defined changes in phosphorus concentration of wheat plants were, in turn, dependent on the granulometric composition of the soil which affects zinc sorbing potential.

On the medium-heavy soil, up to a certain limit that can be put at 30 mg kg^-1 , Zn fertilization resulted in an increase of phosphorus content of plants poorly supplied with that nutrient. Once that limit of zinc concentration in the soil was exceeded phosphorus content of plants dropped below the value characteristic of non-zinc treated plants. However, in wheat well nourished with phosphorus, zinc rates even as high as 60 mg·kg^-1 did not bring about any significant changes in phosphorus content. Probably in that case the P to N ratio of non Zn-treated plants was too wide on the phosphorus side. By accepting the theory of plants tending to preserve the optimum ratio of both elements there was no need to take up any additional phosphorus. It is only at the highest zinc rate (200 mg·kg^-1) that phosphorus concentration of plants was reduced dramatically against that found in Zn non-treated plants. The reduction of zinc concentration of wheat plants following high zinc rates was probably the result of the disturbed mechanism governing the P to Zn ratio. One of the reasons may be the already mentioned checked root growth due to excessive zinc concentration of the soil solution resulting in restricted phosphorus uptake or precipitation of zinc phosphates in the roots [5]. In a hydroponic study on the hyper-accumulator of zinc Thlaspi caerulescens [19] it was found that up to the zinc level of 20 g·kg^-1 phosphorus content of plant tissues remained at a constant level to decrease abruptly when zinc concentration went above 20 g Zn·kg^-1 .

The authors of this study analyzed the results from other investigations [12] in which a foliar spray of wheat with a small zinc dose (1 kg·ha^-1 ) resulted in an increase in phosphorus content under poor phosphorus nutrition. The increase of phosphorus content in wheat plants poorly supplied with that nutrient may bear out the theory of plants tending to preserve the optimum ratio of both elements.

Moraghan [15], though, failed to find a change in phosphorus content of wheat shoots under soil fertilization with zinc (16 mg·kg^-1). P content remained at the same level as that in non Zn-fertilized wheat but Zn content increased fivefold and there was also a slight increase in yield. In the same study the investigator found a ca. two- to threefold decrease in phosphorus content of bean, soybean, flax and maize plants under Zn fertilization with a simultaneous two- to threefold increase in zinc concentration. The yield increase in those crops was considerable when compared to yield increases in wheat which may be indicative of a lower requirement of wheat for zinc.

As observed in this study, Zn-P relationships on a very light soil were less unambiguous than those on a medium-heavy soil. At low phosphorus supply to plants zinc rates of 5 to 45 mg·kg ^–1 caused either increase or reduction of phosphorus content of plants depending on soil pH. However, the impact of pH was not consistent. Zn-dependent increase in phosphorus concentration was observed both at a pH of 3.9 and 6.6 whereas in the 6.3 pH variant there was a fall in the phosphorus content of plants.

There are few studies from which inferences can be made regarding the effect of soil pH on zinc-phosphorus interaction. Erratic patterns of changes in zinc concentration of grasses fertilized with phosphorus are explained by the researchers as brought about by soil acidity [10]. In a study of the response of wheat to excessive zinc doses (500 mg·kg^-1) zinc toxicity was alleviated due to high phosphorus fertilization but only on a very acidic soil [13]. Presumably, the reason was a very good availability of zinc under low soil pH and, at the same time, poor availability to plants of phosphorus compounds which restricted the possibility to regulate the ratio of P to Zn. Available forms of phosphorus are most abundant at pH_KCl 6-7 whereas the solubility of zinc compounds decreases with increasing pH_KCl and is low within the range of 6 to 7 [9].

It could have been expected from this study that on a soil with a pH >7.0 the reported relationships between zinc and phosphorus would not occur because of poor availability of zinc to plants. However, on the medium-heavy soil at pH variants of >7.0 the same tendencies were found as those for lower pH variants, all changes in phosphorus concentrations showing less abrupt patterns. It shows that zinc was taken up by wheat from the alkaline soil as well albeit less intensively. Even though solubility of zinc forms decreases proportionally to the increase in soil pH the formation of complex anions and organic-mineral associations may sustain its high mobility in alkaline soils, too [9].

CONCLUSIONS

1. The Zn-P interaction in zinc-fertilized wheat plants was more affected by zinc dose and phosphorus status of plants than by soil pH.

2. Zn-P interaction was not always an antagonistic one. Elevated zinc content of soil and plant enhanced phosphorus uptake by the plant under poor phosphorus nutrition.

3. Excessive zinc content of soil restricted phosphorus uptake by and lowered phosphorus concentration of plants.

4. Plants growing under elevated zinc content of soil should be well supplied with phosphorus which sometimes necessitates increased phosphorus fertilization.

REFERENCES

1. Ali T., Srivastava PC., Singh TA.; 1990. The effect of zinc and phosphorus fertilization on zinc and phosphorus nutrition of maize during early growth. Pol. J. Soil Sci.; 1: 79-87.

2. Bednarek W., Lipiński W.; 1996. Zaopatrzenie jęczmienia jarego w mangan i cynk w warunkach zróżnicowanego nawożenia fosforem , magnezem i wapnowania. [Providing the spring barley with manganese and zinc in conditions of differentiated phosphorus and magnesium fertilization and liming]. Zesz. Probl. Post. Nauk Rol.; 434(1): 30-35; [in Polish].

3. Benedycka Z., Krauze A.; 1995. Zastosowanie fosforu z mikroelementami (Zn, Mo, B) w dolistnym dokarmianiu bobiku. [Application of phosphorus with micronutrients (Zn, Mo, B) for foliar fertilization of faba bean]. Acta Acad. Agricult. Tech. Olst. Agricult.; 61(496): 31-37; [in Polish].

4. Bergmann W.; 1986. Farbatlas Ernahrungsstorungen bei Kulturpflanzen. Visuelle und analytische Diagnose. VEB Gustav Fischer Verlag, Jena; [in German].

5. Boawn LC., Rasmussen PE.; 1971. Crop response to excessive zinc fertilization of alkaline soil. Agron. J.; 63: 874-876.

6. Chaney RL., 1993. Zinc phytotoxicity. In: Zinc in Soils and Plants. Ed. A D Robson. Kluwer Academic Publishers, Dordrecht.: 135-150.

7. Erdal I., Yilmaz A., Taban S., Eker S., Torun B., Cakmak I.; 2002. Phytic acid and phosphorus concentrations in seeds of wheat cultivars grown whit and without zinc fertilization. J. Plant Nutr.; 25(1): 113-127.

8. Grewal HS., Williams R.; 1999. Alfalfa genotypes differ in their ability to tolerance zinc deficiency. Plant and Soil.; 214(1-2): 39-48.

9. Kabata-Pendias A., Pendias H.; 1999. Biogeochemia pierwiastków sladowych. [Biogeochemistry of trace elements]. Wyd. Nauk. PWN, Warszawa; [in Polish].

10. Kabata-Pendias A., Gajda J., Gałczyńska B.; 1966. Wpływ nawozów potasowych i fosforowych na zawartosc pierwiastków sladowych w trawach. [Effect of potassium and phosphorus fertilizers on concentration of trace elements in grass]. Pam. Puł.; 22: 231-243; [in Polish].

11. Kadar I., Gulyas F., Gaspar L., Zilahy P.; 2000. Mineral nutrition of maize (Zea mays L.) on chernozem soil I. Novenytermeles; 49(4): 371-388.

12. Krauze A.; 1996. Wpływ nawożenia cynkiem na plony i jakosc ziarna pszenicy w zależnosci od dawki i sposobu nawożenia fosforem. [The effect of zinc fertilization on the yield and grain quality of winter wheat in relation to the rate and method of phosphorus fertilization]. Zesz. Probl. Post. Nauk Rol.; 434(1): 185-191; [in Polish].

13. Kwiecień M., Filipek T.; 2002. Reakcja kukurydzy na toksyczne ilosci kadmu i cynku w zależnosci od pH gleby oraz nawożenia fosforem. [Response of corn to toxic cadmium and zinc in relation to soil pH and phosphorus fertilization]. Zesz. Probl. Post. Nauk Rol.; 482: 317-322; [in Polish].

14. Li HY., Zhu YG., Smith SE., Smith FA.; 2003. Phosphorus-zinc interactions in two barley cultivars differing in phosphorus and zinc efficiencies. J. Plant Nutr.; 26 (5): 1085-1099.

15. Moraghan JT.; 1984. Differential responses of five species to phosphorus and zinc fertilizers. Commun. Soil Sci. Plant Anal.; 15(4): 437-447.

16. Spiak Z., Wall L.; 2000. Wpływ nawożenia mineralnego na zawartosc cynku w glebach zanieczyszczonych przez Hutę Miedzi “Głogów”. [Effect of mineral fertilization on zinc content in soils contaminated by the copper smelter]. Zesz. Probl. Post. Nauk Rol.; 471(2): 1135-1143; [in Polish].

17. Tisdale SL., Nelson WL.; 1975. Soil fertility and fertilizers, 3rd Ed. Macmillan Publishing Co., New York.

18. Weber R., Żurawski H., Hryńczuk B.; 1997. Reakcje odmian pszenicy jarej na zagrożenie zwišzane z nadmiarem cynku w zależnosci od pH gleby. [Response of spring wheat cultivars to zinc excess hazard in relation to soil pH]. Biul. IHAR; 204: 197-203; [in Polish].

19. Zhao FJ., Shen ZG., McGrath SP.; 1998. Solubility of zinc and interactions between zinc and phosphorus in the hyperaccumulator Thlaspi caerulescens. Plant Cell and Environment; 21(1): 108-114.

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