Abstract

Improved pre-anthesis growth during winter is a basis of yield improvement of Brassica rapa in the short growing season of the Western Australian wheat-belt. Marked genotypic variation was observed in pre-anthesis relative growth rate, time to flowering and dry weight at anthesis between inbred lines in the field during winter. Relative growth rate in the field during winter was positively correlated with growth rate at low temperature in a controlled environment and negatively correlated with potassium leakage from leaf tissue exposed to low temperature. These relationships indicated that genotypes producing more dry weight at anthesis were more tolerant of low temperature normally experienced in the field.

Both relative growth rate at low temperature and potassium leakage from chilled leaf tissue were polygenic traits and were largely controlled by additive gene effects. The narrow sense heritabilities were 0.55 and 0.48 respectively. Selection for reduced potassium leakage in a F₂ population was accompanied by a significant increase in plant dry weight at low temperature. Pollen selection at low temperature had positive effect on progeny growth at low temperature. Progenies derived from fertilization at low temperature were able to accumulate more dry weight at low temperature than progenies derived from fertilization at high temperature.

Introduction

The early maturing characteristic of Brassica rapa is well suited for the dry areas of the WA wheat-belt as it is able to complete life cycle within the short growing season. Cultivars of B. rapa developed in Canada matured within 95 days when grown in the Canadian Prairie (Kasa and Kondra, 1986). However, the yield potential of B. rapa is limited due to a low dry weight accumulation at anthesis (Richards and Thurling, 1978). Increased plant dry weight production at anthesis without delay of flowering would be a basis for yield improvement. As pre-anthesis growth and development occurs at sub-optimal temperature and radiation levels during winter, improvement of pre-anthesis growth would be dependent upon the magnitude of genetic variation in tolerance to low temperature during the period of pre-anthesis growth and development. A previous study with half-sib families revealed substantial genetic variation in pre-anthesis growth and development under low temperature (Thurling and Si, 1991). This study was aimed to estimate the components of genetic variance of character concerned and to evaluate response to selection.

Materials and methods

Inbred-backcross lines of B. rapa were used throughout this study. A highly variable cross-composite population, generated from hybridization between cultivars of sub-species oleifera, dichotoma, trilocularis and chinesis, was used as the pollen source for the creation of the inbred-backcross lines and for pollen selection. Inbred-backcross lines were produced by crossing between Sarson plants from a self-compatible ssp trilocularis and the composite population and followed by two generations of backcrossing to Sarson then advanced by four generations of selfing.

Relative growth rate before flowering in the field during winter and potassium leakage at low temperature was measured as the main indicators of tolerance to low temperature. Relative growth rate was obtained by fitting parabolic functions to the data from 8 harvests over a period of 21 days at three days intervals. The parabolic functions are: dry weight (W) = a + bt + ct², relative growth rate (RGR) = b + 2ct, where t is time (days after sowing). Potassium leakage was calculated from [K₁/(K₁+K₂)] x 100, where K₁ is the amount of potassium leaked from leaf tissue into the incubation solution after chilling at 4°C for 12 hours, K₂ is total potassium left in the tissue after chilling. Potassium concentration was determined with a flame atomic absorption spectrophotometer.

The procedure of triple test cross (Kearsey and Jinks, 1968) was used to study inheritance of relative growth rate and potassium leakage at low temperature. Two inbred lines of BS-4 and BS-27 differing in relative growth rate were the parents of the generation of triple test cross. Pollen selection was carried out by pollinating highly variable pollen grains onto the stigmas of inbred line Srason. Fifteen pollinated plants were transferred to a growth room at 10/2°C and five were transferred to a growth room at 18/13°C. Progenies were evaluated in terms of seedling emergence, potassium leakage and growth at low temperature.

Results and discussion

Marked genotypic variation was observed in pre-anthesis relative growth rate, time to flowering and dry weight at anthesis between 14 inbred lines in the field. A selection of five lines is given in Table 1 to show the genotypic variation in traits measured. Plant dry weight at anthesis was positively correlated with both relative growth rate and time to flowering. However, it would still be possible to early flowering cultivars with increased pre-anthesis growth during winter as there was no correlation between time to flowering and relative growth rate. Relative growth rate in the field during winter was positively correlated with growth rate at low temperature in a controlled environment and negatively correlated with potassium leakage from leaf tissue exposed to low temperature. This finding was consistent with a study with tomato that low ion leakage from chilled leaf tissue was correlated with tolerance to low temperature (Paull, 1981). These relationships indicated that genotypes producing more dry weight at anthesis were more tolerant of low temperature normally experienced in the field.

Table 1. Means for characters at anthesis and pre-anthesis growth of B.rapa selected inbred lines at low temperature

Triple test cross analysis revealed that there was no epistatic effects in governing the genetic modes of relative growth rate and potassium leakage at low temperature. These two traits were largely controlled by additive gene effects. The narrow sense heritabilities were 0.55 and 0.48 respectively (Table 2). The narrow sense heritability was higher than previously reported (Richards and Thurling, 1979). There were two factors for it. Firstly, the parental lines differed greatly in relative growth rate. Secondly, special care was taken to minimise environmental errors.

Table 2. Estimates of genetic and environmental variance components from triple test cross statistics of pre-anthesis growth of B. rapa at low temperature

Selection for reduced potassium leakage in a F₂ population was accompanied by a significant increase in plant dry weight at low temperature. Pollen selection at low temperature had positive effect on progeny growth at low temperature. Progenies derived from fertilization at low temperature were able to accumulate more dry weight at low temperature than progenies from fertilization at high temperature (Table 3). These results are as encouraging as those in a similar study with tomato (Zamir and Gadish, 1987). Given the positive effects of pollen selection by one generation, it would be justified to further assess this technique with large populations over more generations.

Table 3. Effects of pollen selection on time to emergence, potassium leakage and dry weight at low temperature

* Days after emergence

Conclusion

Improvement of pre-anthesis growth of B. rapa at low temperature was achieved by pedigree selection and pollen selection. Selection for reduced potassium leakage from chilled leaf tissue in an F₂ population resulted in a significant increase in plant dry weight at low temperature. Pollen selection at low temperature had positive effect on progeny growth at low temperature. Progenies derived from fertilization at low temperature were able to accumulate more dry weight at low temperature than progenies from fertilization at high temperature.

Acknowledgements

Financial support from AusAID was greatly appreciated.

References

1. Kasa, G. R., and Kondra, Z. P. (1986). Growth analysis of spring-type oilseed rape. Field Crops Research 14, 361-370

2. Kearsey, M. J., and Jinks, J. L. (1968). A general method for detection of additive, dominance and epistatic variation for metric traits: I. Theory. Heredity 23, 403-409

3. Paull, R. E., (1981). Temperature-induced leakage from chilling-sensitive and chilling-resistant plants. Plant Physiology 68, 149-153

4. Richards, R. A., and Thurling, N. (1978). Variation between and within species of rapeseed (Brassica campestris and Brassica napus) in response to drought stress. II. Growth and development under natural drought stresses. Australian Journal of Agricultural Research 29, 479-490

5. Richards, R. A., and Thurling, N. (1979). Genetic analysis of drought stress response in rapeseed (Brassica campestris and Brassica napus). II. Yield improvement and the application of selection indices. Euphytica 28, 169-177

6. Thurling, N. and Si, P. (1991). Low temperature tolerance in Brassica campestris during early development. Proceedings of 8^th International Rapeseed Congress. pp 148-152

7. Zamir, D., and Gadish, I. (1987). Pollen selection for low temperature adaptation in tomato. Theoretical and Applied Genetics 74, 545-548