The relationship between straw length, root dry weight and zinc efficiency as well as shoot zinc content in hexaploid wheat, Triticum aestivum L.

Rolf Schlegel¹, Ismail Cakmak² and Mariana Atanasova¹

¹Institute of Wheat & Sunflower Res., BG-9520 General Toshevo/Varna (Bulgaria), ² Dep. of Soil Sciences and Plant Nutrition, University of Cukurova, TR-01330 Adana (Turkey)

Key words:  zinc efficiency, zinc content, semi-dwarfs, root dry matter, straw length, wheat

Abstract

In order to elucidate the relationship between plant morphology, i.e. shoot and root characters, and zinc efficiency as well as zinc accumulation in shoots, three types of hexaploid winter wheats with tall, medium-tall and short straw were investigated. Although there was a tendentiously increase root formation (measured as root dry weight) by reduced stem height, no tight correlation could be revealed between root dry weight and Zn efficiency as well as Zn content in shoots. The genotypic variation within three groups of plant height appeared bigger than between the groups. Pleiotropic effects of different genes coding for plant height on other physiological characteristics are assumed. A selection for intensive-type of wheats carrying semi-dwarf genes cannot be an approach for improvement of zinc efficiency and accumulation.

The American varieties 'Eagle' and 'Scout 66' are able to accumulate a high amount of Zn in shoots when there is a sufficient supply. They can be assigned as Zn responsive accumulators.

Introduction

Zinc deficiency is a widespread micronutrient problem in wheat production, particular in calcareous soils of arid and semiarid regions (Reuter et al. 1988, Graham and Welch 1996). In wheat, Zn deficiency occurs on a large area of several countries resulting in severe dropping in grain yield, as found in India (Takkar et al. 1989), Australia (Graham et al. 1992) and Turkey (Cakmak et. al. 1996). Zinc deficiency reduces not only grain and/or dry matter yield but also nutritional quality of grains. Moreover, Zn deficiency enhances sensitivity of cereal crops to different root diseases (Sparrow and Graham 1988, Wilhelm et al. 1988, Grewal et. al. 1996).

Screenings among a wide range of wheats from an international collection revealed already quite a high differentiation of Zn efficiency between genotypes (Cakmak et al. 1998, Schlegel et al. 1998). However, the mechanisms of Zn acquisition and utilisation of cereal plants are not well understood yet. Phytosiderophores might play an important role in Zn uptake under deficient soil conditions (Rengel 1997). But also the root system, i.e. root amount, could contribute to a differential Zn efficiency.

Wheats with different straw length were used for the present study in order to prove that. Since 1935 in Japan ('Norin 10') and later from the 60ies so-called semi-dwarfing genes were intensively introduced in wheat breeding programmes. Such wheats show a comparably short and stiff straw coupled with increased tillering and yield increase despite a range of other modifications. Those intensive wheat types led to out-yields of about 100 % compared to the common material (Borlaug 1983).

On the other hand, some authors demonstrated reduced root system in semi-dwarf wheats associated with reduced yield und stress conditions (Briggle and Vogel 1968, Virmani 1971, Gupta and Virmani 1973, Hurd 1974), while Lupton et al. (1974) observed by radioactive tracing experiments an increase of root system, at least in some specific soil layers. Cholik et al. (1977) did find no correlation between the root system and plant length.

For the own experiment 18 wheat genotypes with different straw length from an international sample were used for studying the relationship between straw length, root development and zinc efficiency.

Material and methods

18 wheat cultivars and strains from a breeder's collection of the Institute of Wheat and Sunflower Research, General Toshevo , Bulgaria (kindly provided by Dr. N. Tsenov) were classified in three groups of plant height: >110 cm, ~90 cm and ~55 cm (cf. Table 1).

Pot experiments were carried out under greenhouse conditions of Adana (Turkey) with 2.2 kg soil/plastic pot with (10 mg Zn/kg soil as Zn SO₄) and without Zn application. All experiments were carried by three replications.  Zinc deficient soil was used from Ezkisehir (Turkey) where Zn deficiency severely occurs in wheat (Cakmak et al. 1996). All pots were randomised every 4-5 days and watered daily to about field capacity using deionised water.

The Zn efficiency of plants was calculated as the ratio of dry matter yield at Zn fertilisation to the yield produced under without Zn fertilisation. Plants were harvested after 40 days of growth. The shoots were dried at +70 °C for determination the dry weight. The sensitivity of the plants to Zn deficiency was evaluated by scoring severity of deficiency symptoms on leaf blades (i.e. necrotic patches on leaves).

Concentrations of Zn in shoots were measured by ICP Atomic Absorption Spectroscopy (ICP-AAS) after ashing samples at +550 °C, and dissolving ash in 3.5 % HCl (Cakmak et al. 1996).

For the statistical analysis the t-Test and linear correlation coefficients were applied.

Results

As can be taken from Table 1, within and between the three groups of plant height of winter wheats there is a differentiation for the root dry weight (g/plant) under conditions of normal zinc supply. Although there is a clear tendency of increased root dry weight by reduced straw length, the variation within the groups is comparably high in order to receive a statistical significance at P=5% level. Also the correlation between the shoot and root dry weight is not very tight but statistically significant (r=0.48 **, see Fig. 1). It explains that the yield potential of the varieties is not just related to the root and shoot production and/or length of straw.

When the Zn efficiency is analysed under Zn-deficient soil conditions a gross view over the three plant height groups shows a very low differentiation between the tall, medium-tall and short plants for shoots as well as for roots (Table 1 and 2). Nevertheless, there is a slight tendency that reduced straw length and reduced root dry matter is associated with a decrease of Zn efficiency of shoots and roots. The same observation is true for the Zn concentration and content of shoots.

A much bigger variation is found when the individual genotypes within the three groups are considered (Table 3). Among the tall wheats the Russian variety ' Erythrosperm.127' shows 74 % Zn efficiency for shoots and the Canadian variety 'Valor' 83 % for roots, respectively (Table 3). The Zn concentrations and contents under Zn absence and presence are within the range of common wheat material. Only the American varieties 'Eagle' and 'Scout 66' are able to accumulate a high amount of Zn in shoots when there is a sufficient supply. They can be assigned as Zn responsive accumulators.

In the medium-tall group of wheats (Table 3) the Russian strain 'Lutescens 8133' exhibits the highest Zn efficiency for shoots (76 %) and the Russian variety 'Mironovskaya 1' for roots (82 %). Another Russian variety 'Mironovskaya 62' shows as 'Eagle' and 'Scout 66' a high Zn content under Zn supply.

Among the short-straw sample the Bulgarian line 'N2479' shows the highest Zn efficiency for shoots and roots (Table 3), while the Serbian strain 'NS732' accumulates most Zn in shoots under supply. The individual data demonstrate that high and low Zn efficiency can occur in all three samples of straw length with almost the same range. When all single data are compiled a significant correlation (r=+0.69**) exists between the Zn efficiency in shoots and roots (Fig. 2). Despite  this, all other correlation coefficients between the shoot and root dry weight and the Zn efficiency both for shoots and for roots appeared negative (Table 4), i.e. as lower the root and shoot dry matter production is as higher is the Zn efficiency in the shoot and roots. The best correlation coefficient exists between the shoot dry weight and the shoot Zn efficiency (r=-0.72 **).

A final compilation of data of Zn concentrations and contents in wheat shoots together with the shoot and root dry weight under Zn supply as well as shoot and root Zn efficiencies does not show clear correlations between those characters (Table 5), except a tight association between shoot dry weight and shoot content (Fig. 3). The latter means that high shoot dry matter production is basically linked to high Zn contents (r=+0.92 **), no matter whether or not the plants are tall or short, Zn-efficient or not.

Discussion

The results gained during the present experiments support the former studies of Lupton et al. (1974) that reduced straw length in wheat increases the root growth, possibly by translocation of shoot formation potential to root formation. Since the mean values between the three groups of plant height are statistically not yet significant at P=5 % level, it is concluded that genotypic variation within the height groups is big. The genes for plant height may cause similar plant morphology within the groups, however with different pleiotropic interactions influencing physiology. The plant height in the wheats included, obviously is determined by different genes from which at least 22 are described in semi-dwarf wheats (Börner and Mettin 1989). Isogenic lines of wheat could improve the results mentioned above. Such studies are initiated.

Because of slight correlations between straw length, shoot and root dry matter weight and Zn efficiency, it can be assumed that Zn efficiency is not just function of amount of roots and shoots (cf. Table 4). A selection for intensive-type of wheat with semi-dwarf genes cannot improve Zn efficiency per se.  Even more, the throughout negative correlations demonstrate that wheat genotypes with lower shoot and root formation capacities seem to be more predetermined to acquire and/or utilise Zn under Zn-deficient conditions. This has something to do with a plant-internal activation of acquisition mechanisms in genotypes which are not sufficiently furnished with shoots and roots.

The positive correlation between Zn efficiency in roots and shoots is a useful prerequisite for breeding. A selection for improved Zn efficiency of shoots seemed to be accompanied with root Zn efficiency. Additional root screening approaches seem not to be a requirement for practical breeding.

The Zn contents in the shoots were also not clearly related to the Zn efficiency or plant height. There is even a negative tendency. More Zn-efficient genotypes seem to have a lower potential for Zn accumulation in the shoot. An independent genetic control of Zn efficiency and Zn deposition in shoots can be assumed, as found in several other studies (Graham et al. 1983, Cakmak et al. 1998, Schlegel et al. 1997, 1998).

References

Borlaug N E (1983) Contribution of conventional plant breeding to food production. Science 219, 689-693.

Börner A and Mettin D (1989) Genetische Grundlagen der Halmverkürzung (Dwarfismus) beim Weizen and Möglichkeiten der züchterischen Nutzung. Kulturpflanze 37, 29-55.

Briggle L W and Vogel O A (1968) Breeding short stature, disease resistant wheats in the United States. Euphytica Suppl.1, 107-130.

Cakmak I, A Yilmaz, M Kalayci, H Ekiz, B Torun, B Erenoglu and H-J Braun, (1996) Zinc deficiency as a critical nutritional problem in wheatproduction in Central Anatolia. Plant and Soil 180, 165-172.

Cakmak  I, Torun B, Erenoglu B, Ozturk L, Marschner H, Kalayci M and Yilmaz A (1998) Morphological  and physiological  differences in the response of cereals to zinc deficiency. In: H J Braun et al. (eds.), Wheat: Prospects for Global Improvement: 427-435.

Cholik F A, Welsh J R and Cole C V (1977) Rooting patterns of semi-dwarf and tall winter wheat cultivars under dryland field conditions. Crop Sci. 17, 637-639.

Graham R and R M Welch (1996) Breeding for staple-food crops with high micronutrient density: Working Papers on Agricultural Strategies for Micronutrients, No.3. Intern. Food Policy Institute, Washington D.C., 1-148.

Graham, R D, Davies, W J, Sparrow, D H R and J S Ascher (1983) Tolerance of barley and other cereals to manganese-deficient calcareous soils of South Australia. In M R Saric and B C Loughman (eds.). Genetic Aspects of Plant Nutrition. M Nijhoff, Netherlands.

Graham R D, Ascher J S and Hynes S C (1992) Selecting zinc-efficient cereal genotypes for soils of low zinc status. Plant Soil 146, 241-250.

Grewal H S, Graham R D and Rengel Z (1996) Genotypic variation in zinc deficiency and resistance to crown rot disease (Fusarium graminearum Schw. Group 1) in wheat. Plant and Soil 186, 219-226.

Gupta A P and Virmani S M (1973) Note on the rooting pattern of three-gene-dwarf wheats. Indian J. Agric. Sci. 43, 971-973.

Hurd  E A (1974) Phenotype and drought tolerance in wheat.  Agric. Meteorol. 14, 39-55.

Lupton F G H, Oliver R H and Ellis F B (1974) Root and shoot growth in semi-dwarf and taller winter wheats. Ann. Appl. Biol. 77, 129-144.

Rengel Z (1997) Root exudation and microflora populations in rhizosphere of crop genotypes differing in tolerance to micronutrient deficiency.  In: T. Ando (ed.), Plant Nutrition - for sustainable food production and environment (Kluwer Acad. Publ., Japan), 243-248.

Reuter D J, Cartwright B, Judson G J, McFarlane J D, Maschmedt D J, Robinson J B (1988) Trace elements in South Australian agriculture. Dep. of Agric. South Australia Techn. Rep. 139, 1-34.

Schlegel R, Cakmak I , Torun B, Eker S and Koleli N (1997) The effect of rye genetic information on zinc, copper, manganese and iron concentration of  wheat shoots in zinc deficient soil. Cer. Res. Comm. 25, 177-184.

Schlegel R, Cakmak I, Torun B, Eker S, Tolay I, Ekiz H, Kalayci M and Braun, H J (1998) Screening for zinc efficiency among wheat relatives and their utilisation for alien gene transfer. In: H J Braun et al. (eds.), Wheat: Prospects for Global Improvement: 347-352.

Sparrow D H R and Graham R D (1988) Susceptibility of zinc-deficient wheat plants to colonization by Fusarim graminearum Schw. Group I. plant and Soil 112, 261-266.

Takkar P N, Chibba I M and Mehta S K (1989) Twenty years of coordinated research of micronutrients in soil and plants. Indian Inst. Soil Sci., Bhopal. IISS Bull.

Virmani S M (1971) Rooting patterns in dwarf wheats. Indian J. Agron. 16, 33-36.

Willhelm N S, Graham R D and Rovira A D (1988) Application of different sources of manganese sulfate decreases take-all (Gaeumannomyces graminis var. tritici) of wheat grown in a manganese deficient soil. Austr. J. Agric. Res. 39, 1-10.