Abstract

Oilseed rape (Brassica napus L.) is adapted to fertile and pH neutral soils. However Poland is the country with a great percentage of acid soils - 60% of agricultural land. Aluminium (Al) toxicity is a serious growth limiting factor when the soil pH is lower than 5.5.

Two sets of spring oilseed rape genotypes, one consisting of 27 CMS ogu lines, and the other consisting of their maintainers - 27 inbred lines, were tested for Al tolerance using the nutrient solution test. The resistance of root apical meristem to damages caused by the aluminium ions under controlled conditions (pH, temperature), measured by the ability to continue root elongation after Al-shock, was used as an indication of Al tolerance of tested genotypes. Each line showed a linear decline in percentage of Al tolerant plants to increasing Al concentration - 5, 10 and 20 mg L^-1. Considerable genetic variability of tolerance to Al stress among two sets of tested lines was revealed. It has been statistically proved that the CMS ogu lines had better Al tolerance than their maintainers. It seems that CMS ogu system introduced to oilseed rape from male sterile Raphanus sativus may be useful for the development of hybrid spring oilseed rape production on soils with acidity problem.

Introduction

Soil acidity limits plant growth in many parts of the world (Adams 1981). In Poland approximately 60% of arable soils are acid and in a lot these soils, aluminium (Al) toxicity is the primary growth-limiting factor for plants (Boguszewski 1980). Toxic concentrations of Al are generally found in soils having pH values about 5.5 (Foy 1988). A severe inhibition of root growth is the main direct effect of Al on plants. This reduction in root growth decreases water and nutrient uptake and leads to poor yield. Moreover, excess of exchangeable or soluble Al is especially undesirable in subsoil because it reduces rooting depth and branching and it predisposes plants to drought injury (Alam 1981).

Oilseed rape is adapted to pH neutral soils but growers in Poland very frequent have to sow oilseed rape in different soil conditions, even with Al-toxicity problem.

First spring composite hybrid Margo shows the increase of yield performance over traditional cultivars, but it remains to be proven that hybrid oilseed rape with the foreign cytoplasm offers any growth and vigour advantage in acid soils. Comparison of the Al tolerance of CMS ogu lines and theirs maintainers was the aim of this study.

Materials and Method

Plant material

Two sets of spring oilseed rape genotypes, one consisting of twenty seven CMS ogu lines (BC₄), and the other consisting of their maintainers - 27 self-pollinated lines (S₄), were tested for Al tolerance. All material was bred at Experimental Station Malyszyn.

Laboratory test

The laboratory test in water culture was carried out using the modified method elaborated by Aniol (1981) for estimation of Al tolerance of cereals. Germinated seeds were put on the nutrient solution which contained 0.4 mM CaCl₂, 0.65 mM KNO₃, 0.25 mM MgCl₂, 0.01 mM (NH₄)₂SO₄ and 0.04 mM NH₄NO₃, with pH 4.0 and in the temperature of 25⁰C.

Four-day seedlings were transferred to the similar nutrient solution with added aluminium. The concentrations used were: 5, 10 and 20 mg L^-1Al. After exposure to aluminium, seedlings were removed, thoroughly washed for 2-3 min in runny tap water and placed for 48 hours in identical nutrient solution without aluminium. Then, the roots of seedlings were stained for 10 min with 0.1% solution of Eriochrome cyjanine R. After staining the excess of staining solution was removed by washing under tap water. If the aluminium treatment did not destroy the root apical meristem, part of the root which grew after Al treatment was white and contrasted with violet stained root part exposed to aluminium. The root regrowth of 100 seedlings for each studied object was evaluated. The experiment was replicated twice.

Results and discussion

Considerable genotype variability appeared among two sets of investigated lines of spring oilseed rape (Table 1) since there were significant differences for each of three Al treatments. Results indicated that each investigated line showed a linear decline of seedling percentage with root regrowth to increasing Al concentration. Thus it can be concluded that all investigated lines were not only heterozygous with respect to Al tolerance, but that each level of Al tolerance was determined by different genes. For practical purposes further studies on inheritance of Al tolerance of oilseed rape are needed. This will provide a better understanding of genetic determination of aluminium tolerance and its utilisation in breeding.

Tolerance to Al stress is apparently relative rather than absolute since differing root regrowth may occur depending on the level of Al stress (Aniol, 1991). It may be true that different genotypes could have different performance of tolerance in different Al concentration. This phenomenon has been reported earlier in cereals (Aniol, 1991). Thus, the concentration of aluminium used in a nutrient solution test should be chosen according to a screening purpose.

Generally, the CMS lines could be ranked in order of Al-tolerance much the same as could be ranked their self-pollinated maintainers. However, some selected lines of both sets were significantly different from their analogues. These may be associated with using of BC₄ CMS lines, which do not have full genetic information of theirs maintainers, and more backcrossing (i.e. BC₆) are needed to transfer of all traits.

The comparison between the average level of aluminium tolerance of CMS ogu lines and average level of Al tolerance of their maintainers showed that occurrence of tolerant seedling was more frequent among the former than among the latter (Fig. 1). CMS ogu lines had significantly (P<0.05) higher mean percentage of seedlings with root regrowth than the maintainers in lower external Al concentrations (i.e. 5 and 10 mg L^-1). These results would suggest that incorporation of Brassica napus genome into male sterile cytoplasm of Raphanus sativus has the significant positive effect on Al-tolerance of hybrids.

Table 1. Aluminium tolerance of CMS ogu lines (BC₄) and their self-pollinated maintainers expressed as percentage of seedlings with root regrowth after Al stress

Fig. 1. Reaction of CMS ogu lines and maintainers to aluminium at different external Al concentration (mg L^-1) in nutrient solution.

Conclusion

The root regrowth characteristic appears to be a useful indicator for screening of Al tolerant germplasm and evaluating Al tolerance response. However, verification and establishment of the association of Al tolerance in nutrient solution with plant productivity on soils with acidity problems entails further investigation of a number of hybrids and classical oilseed rape varieties in different environmental conditions.

Acknowlegements

The authors wish to thank Professor Andrzej Aniol for useful consultations.

REFERENCES

1. Adams F. 1981. Nutritional imbalances and constraints to plant growth in acid soils. Journal of Plant Nutrition, 4, pp.81-87.

2. Alam S.M. 1981. Influence of aluminium on plant growth and mineral nutrition of barley. Communication in Soil Science and Plant Analyses, 12, pp. 121-138.

3. Aniol A. 1981. Metody okreslania tolerancyjnosci zboz na toksyczne dzialanie jonow glinu. Biuletyn IHAR, 143, pp. 3-14.

4. Aniol A. 1991. Genetics of acid tolerant plant. In: R.J. Wright, V.C. Baligar & R.P. Murrmann (Eds). Plant - Soil Interactions at Low pH, pp. 1007-1017. Kluwer Academic Publishers, Dodrecht, The Netherlands.

5. Boguszewski W. 1980. Wapnowanie gleb. PWRiL.Foy C.D. 1988. Plant adaptation to acid, aluminium – toxic soils. Communications in Soil Science and Plant Analyses, 19(7-12), pp. 959-987.