The genetics of flowering time in Raphanus sativus L. cv. 'Chinese daikon'

Abstract

For the development of an effective plant breeding program, both the presence and identification of genetic variability are essential. The main objective of quantitative genetic studies is to estimate the magnitude of genetic variance, so that predictions about improvements due to a selection program can be made accurately. For the greatest accuracy a knowledge of the relative size of the different genetic variances is required. One of the main objectives of estimating the genetic variance is to estimate the magnitude of the heritability of that character. This enables the breeder to adopt an effective method of selection for the improvement of the crop. If the heritability is high, reliance may be placed mainly on individual plant performance. If it is low, more emphasis should be given to progeny tests and replicated trials in the breeding programs. Phenotypic data are used to infer conclusions about the genotype. Therefore, proper understanding of phenotypic variance is necessary for appropriate interpretations of the data. The phenotype reflects non-genetic as well as genetic influences and these two are not independent. A change in environment does not necessarily cause the same phenotypic response in all genotypes; likewise, a similar genotypic variation may not produce the same phenotypic variation under different environments. This type of interplay is known as genotype-environment interaction. Plant breeders generally agree that such interactions have an important bearing on breeding programs. However, opinions differ as to how to utilize this knowledge for a better breeding program. Some breeders place more emphasis on the "values" of the genotypes, while others consider the "final" character such as yield or quality of prime importance. One major effect of genotype-environment interaction is to reduce the correlation between phenotype and genotype, with the result that inferences become complicated. This is true whether interest is focused on plant improvement procedures or on the mechanism of inheritance. It is probably in the field of developmental physiology that the answers to the basic causes of genotype-environment interaction are likely to be found. The analysis of these interactions, however, lies in the area of quantitative genetics. Better understanding of genotype-environment interactions will definitely prove significant in connection with plant improvement. However, it is quite likely that we may never be able to completely eliminate "unexplained" interactions. 'Chinese half long' is a cultivar of Raphanus sativus grown in Hawaii. It is locally known as Daikon and is the fourth largest vegetable crop by total acreage in the state of Hawaii (Collier ~ ale 1967). One of the main problems faced by the farmers in the production of Daikon is its premature flowering. This net only' reduces the quality of the roots but also affects the yield considerably. Flowering in radish, like other crops, is affected by various environmental factors. The present study was conducted to find whether there exists any genetic variance for flowering time in Daikon and to test the genetic-environment interaction. For this purpose selection in opposite directions was carried out for six generations, followed by replicated field experiments of selected lines at two locations during two seasons. Crossing experiments involving Early and Late parental lines were also conducted in the greenhouse. This study may prove fruitful in developing a better breeding program for Chinese radish in the state of Hawaii.