The Genetics and Physiology of Abiotic Stress Disorder in Swede (Brassica napus var. napobrassica)
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Swedes are extremely common as a root vegetable in Europe, USA, and Canada but are affected by the occasional presence of Brown Heart (BH) disorder affecting the marketable swede root. The incidence of BH has been reported worldwide however it is very difficult to breed resistance due to its sporadic occurrence with no external symptoms to select for. BH has been attributed to boron availability but attempts to link BH appearance definitively with boron deficiency have been difficult. Anecdotal evidence from breeders and growers highlighted the recent co-appearance of BH and frost injury in the field and it was postulated that if an association (physiological or genetic) can be determined between BH appearance and another more easily assessed trait such as frost susceptibility, then a frost tolerance screen may be developed as a useful surrogate method to screen for BH resistance. Frost hardiness assessment of 12 swede genotypes including some F1 hybrids was carried out. Results showed that some genotypes (like Ag31, Me77c and Or13) were more susceptible to frost (EL50 circa -7 ˚C) whilst others (like Gr19 and Ly01) were classified as more tolerant. Breeder trials data from the UK and Germany over a 10 year period showed that 85% of the BH incidence was associated with genotypes that had the frost susceptible lines Ag31, Or13 or Me77c in their parentage. To investigate this association further, frost susceptible and tolerant genotypes, together with a number of their F1 hybrids, were evaluated in a field trial for their response to boron treatments (0.00, 1.35, 1.80 and 2.70 kg B ha-1). At maturity, BH incidence and its severity was predominantly affected by genotype but could be ameliorated by boron application. Ag31 was confirmed to be the most susceptible to BH, and Or13 and Me77c were intermediate in their susceptibility. F1 hybrids between any two susceptible parents were also susceptible to BH. In contrast, genotypes Gr19 and Ly01 were confirmed to be highly resistant to BH and did not show any BH symptoms even at zero boron applied. F1 hybrids between resistant and susceptible lines demonstrated the BH resistant phenotype. Resistance to BH was therefore confirmed as a dominant trait with either a BHBH or BHbh genotype, whilst susceptibility was recessive bhbh. A degree of quantitative variation existed in the severity of the BH suggesting that BH resistance was not a single gene effect. BH severity was significantly negatively correlated (r = - 0.632) with root boron content in susceptible genotypes. The genotypes which were BH resistant in this trial were also more tolerant to frost in screening tests and this association was investigated further at a molecular level. Cold acclimation (CA) for 14 days at 4 ˚C positively affected the response of swede to frost, lowering the EL50 by -1.5˚C, and boron reduced the EL50 by -2.2˚C under non-acclimating conditions and by -1.2˚C under CA. Both boron and CA increased the catalase (CAT) and super oxidase dismutase (SOD) concentrations in swede leaves. Molecular analysis clearly demonstrated the presence of the B.napus cold response gene in swede, BN115, and was shown to be up-regulated due to both CA and boron application but differed between the two genotypes tested. The more frost resistant Gr19 showed a better response than the susceptible Ag31. Boron application reduced EL50 by -2.3˚C for Ag31 and -3.1˚C for Gr19. Given the association between frost tolerance and BH resistance it is suggested that a frost test screen could be used as a useful surrogate method to screen for BH resistance in swede breeding programmes.
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