Breeding and agronomic aspects of bolting in sugar beet

Hybrid varieties of sugar beet either, triploids or diploids are mainly dependent on the reliable females. In this respect, CMS lines or single crosses having high combining ability are considered. For improving a triploid hybrid, genetic enhancement of a teteraploid pollinator expressing maximum heterozygosity in the progenies are of vital importance. Regarding to a diploid hybrid, a good pollinator of E, N, or Z types might be taken into consideration. This project included three long term breeding and one short term agronomic (nutrition requirement) sub-projects that were initiated in 1994. We aimed at developing highly bolting resistant and cercospora tolerant varieties using different breeding programs. The three breeding sub-projects were: 1. Development of O-type and CMS lines for resistance to bolting with the number 100-13-73-005; 2. Development of bolting resistant monogerm or muligerm diploid hybrids numbered 100-13-73-006, and 3. Development of bolting resistant triploid hybrids using laboratory methods numbered 100-13-73-007. The agronomic sub-project was "Study on the nutritional requirements of improved hybrid monogerm of sugar beet" numbered 100-13-78-02. This project was approved by the technical committee of Agricultural Research and Education Organisation (AREO) and performed at Sugar Beet Seed Institute (SBSI). In this project, a simple recurrent selection system using a maintainer sugar beet population segregating for nuclear male sterility was applied in progenies of crosses between two self-fertile monogerms 7112 and 428 with a multigerm 276. F1 hybrids were crossed to an annual and a biannual CMS line. Maintainer genotypes of O-type lines (Nxxzz) were identified and preserved; the non-O types were eliminated. F2 plants were screened for monogermity and multigermity. The CMS and their counterpart O-types were propagated in isolated chambers by several back crossing and simultaneous checking of monogermity and male sterility. In 1998, the best lines were screened for seed characteristics and at the same time single crosses were resulted from crosses between CMS and unrelated O-types. Lines competed beyond the S3 generations were assessed for general combining ability (GCA) in the single crosses. They were also evaluated for resistance to both, bolting and cercospora leaf spot in Dezful and Ghaem Shahr and consequently 10 CMS and their counterpart O-types were reselected for these two characteristics. In 1999, the best F1 CMS combinations were crossed with a numbers of tetraploid and diploid pollinators. After extensive testing of these hybrids, yield components and bolting resistance of those hybrids appeared to be superior to the existing commercial varieties that were released as new hybrids. In respect of improving tetraploid (4n) pollinators, tetraploids of various origins named Jot18, 19669, ET5, Jit13, Hm5513 and HM5514 were cloned and evaluated after multiplication, although cloning, multiplication, and protection of 4n clones were not straightforward. Anyhow, many clones as well as primary parents were selected after evaluation. In addition, a large number of S1 or S2, half-sib and full-sib families were also improved. Tetraploids from Jot18, 19669, Hm5513 and Hm5514 populations were screened and cleaned, and Off-types were eliminated at the initial stages. Bulks of screened half-sibs of 19669 and Jot18 were applied as parents of Rasoul and Dejleh 41 varieties which are normal (N) and E-type varieties, respectively. Both are very valuable monogerm varieties and well known in the sugar beet growing areas. Diploid populations of this project originated from different countries and consisted of E, N, and Z types; they were improved in a separate breeding program that included half-sib family (HSF) selection in diverse diploid populations. From 1994 to 1998, progenies of crosses between improved families and best CMS single crosses were selected in the field for root type and morphological characters, and later reselected for resistance to bolting and Cercospora leaf spot in Dezful, and for sugar content, root yield, sugar yield, non-sugar contents and extractable sugar in various sugar beet growing areas. Among the qualified hybrids, two combinations with the names of Shirin and Gadouk had been released to the sugar beet growers. Fortunately, many S1 and S2 pollinators with resistance to bolting and cercospora leaf spot have been qualified and are being evaluated in another project. By increasing bolting and cercospora resistance in the hybrids, transfer of rhizomania and rhizoctonia resistance genes into these diploids are of priority since new sugar beet varieties must genetically be protected from the two important diseases. Fertiliser requirements were determined in diploid and triploid hybrids of E, N, and Z types at Dezful Station. The best nutrition combinations obtained for Shirin variety were 180 kg N, 75 kg K2O and 150 kg P2O5 per hectare and for Rasoul were 180 kg N, 150 kg P2O5, and 150 kg K2O per hectare. For Shirin variety/, there was no significant differences between 75 and 150 kg K2O treatments. Regarding the nutrition needs, no significant differences were found between a new diploid (261x436) x 206 and a triploid (261x428) x Jot18 hybrid and other commercial varieties. Best combination of micro-elements were found to be 20-30 kg ZnSO4, 15 kg MnSO4, and 15 kg BH3 that had positive effects on sugar yield and root quality. It seems that the experimental soils were rich in micro-element components and varieties showed more or less similar response. Outstanding results of this project reveal that it is possible to access hybrid varieties with high yielding potential and that without introducing Rhizomania and Rhizoctonia resistant genes into new hybrids, supply of commercial varieties to the domestic market are will essentially face a great challenge in near future. It is suggested to use double and triple combinations of different lines and even two resistant lines (each having resistant genes for a specific disease) for improving a uniform variety.