Genetic variability of tolerance to freezing in South African wheat cultivars

[摘要] English: In South Africa nearly 47062 ha of wheat has been damaged by frost over the last ten years. This led to an average loss of income close to R 6.7 million annually. The objective of this study was to screen South African wheat cultivars for tolerance to freezing, to study the genetic variability for tolerance to freezing and to study the use of HMW-proteins to screen for tolerance to freezing. An artificial freezing test was used to screen 24 South African wheat cultivars for tolerance to freezing. Tolerance to freezing was obtained by calculating the percentage survival, root length and leaf length at -6 and -12°C. Norstar displayed superior tolerance to freezing at -6 and -12°C. Norstar, Scheepers 69, Belinda, Kavkaz, Molen, Carolus, Jager and PAN 3232 exhibited a high level of tolerance to freezing. The leaves of Caledon, PAN 3232, SST 966, SST 936, Norstar, Tugela DN and PAN 3235 showed a high level of tolerance to freezing. The roots of Kavkaz, Norstar, PAN 3235, Belinda, Jager, Carolus, PAN 3232, Betta and Molen exhibited a high level of tolerance to freezing. A full 6X6 diallel cross was used to determine the genetic variability, combining ability and inheritance of tolerance to freezing. Norstar, Tugela DN, PAN 3349, Karee, SST 66 and Snack were used as parents. Griffing's Model1, Method 1 was used for the analysis. Significant differences in tolerance to freezing were displayed by the F1- hybrids. Crosses such as SnacklNorstar, PAN 3349/Snack and Snack/fugela DN had a low level of tolerance to freezing. Karee/Norstar, SST 66/PAN 3349, Karee/PAN 3349 and SST 66/Norstar were nearly as freezing tolerant as Norstar. The lack of significant differences in the freezing tolerance between reciprocal crosses indicated that freezing tolerance was not affected by cytoplasmic effects. Norstar and Karee had the highest general combining ability. The high GCA:SCA ratio for percentage survival at -12°C and percentage reduction in survival, confirmed the additive gene action of tolerance to freezing. The high heritability estimates for percentage survival (88.18%) and leaf length at -12°C (88.45%) confirmed that tolerance to freezing is a highly heritable character. Selection for freezing tolerance in a breeding programme will be effective. The effect of HMW-proteins to screen for tolerance to freezing was studied using SOS-PAGE. Six wheat cultivars were included, Norstar, Tugela DN, PAN 3349, Karee, SST 66 and Snack. The results confirmed that HMW-proteins in coleoptiles can be used to screen for tolerance to freezing. Accumulation of the 48 kDa protein in Norstar started after the vernalisation requirement was satisfied. The 46 kDa protein is a cold sensitive protein as production stopped during cold acclimation. The 40 kDa band indicated a lack of tolerance to low temperatures. Cold tolerant cultivars were able to accumulate the 25 kDa protein rapidly when they needed protection against low temperatures. Similar reactions to cold hardening were found in the roots of the different cultivars. Snack lost the ability to produce the 68 kDa protein during prolonged periods of low temperatures. Norstar, however, was able to accumulate it during extended cold periods. All the cultivars were able to accumulate the 21, 25 and 30 kDa protein in reaction to cold hardening. The accumulation was activated once vernalisation was completed. The 19 kDa protein acted as an early defence mechanism against low temperatures. Production and accumulation of several proteins can be associated with increased tolerance to freezing. Growth habit and vernalisation requirement did influence the production and accumulation of certain HMW-proteins. A signifficant correlation between the leaf length and the number of HMW-protein bands produced was observed. This indicated that cold hardening did have an effect on the production and accumulation of HMW-proteins in coleoptiles and roots.

[发布机构] University of the Free State