A qualitative and quantitative evaluation of freezing stress in wheat

English: In South Africa loss of income as a result of frost damage to wheat has had far reaching consequences for the wheat industry in recent times. This is the result of early maturing wheat as well as the occurrence of late frost in early spring when wheat is most susceptible to frost damage, therefore intensifying the risk of frost stress. The objectives of this study were to: a) evaluate the quantitative and qualitative characteristics of different growth types (winter, intermediate and spring wheat) for tolerance to frost (freezing) during the tillering, flag leaf, flowering and hard dough growth stages at different sub-zero temperatures; b) evaluate different growth types for frost tolerance at different flag leaf growth stages; c) evaluate different growth types for frost tolerance at different flowering stages; and d) compile a guide with illustrations of frost injury symptoms that could be used by wheat growers and other participants in the wheat industry. a) Artificial freezing was used to evaluate the reaction response of different growth types at different temperatures (0 to –12°C with 3°C increments) and at different growth stages (tillering, flag leaf, flowering and hard dough). A quantitative evaluation showed that all cultivars were to some degree sensitive to frost damage. Cultivars 1 to 3 showed the highest degree of sensitivity at the flag leaf stage, while cultivar 4 proved to be more sensitive at the flowering stage. Though the growth stages differed in terms of sensitivity to frost stress, it was evident that temperatures below –6°C led to a reduction in growth and development, and subsequently a reduction in the parameters measured. Finally, the different genotypes had a profound influence on the reaction of wheat to frost injury, with the winter type being more tolerant than the spring type. b) A qualitative evaluation of different growth types, in terms of protein content and stirring number, was conducted at different temperatures and different growth stages. Results obtained at the different growth stages, before grain filling commenced, showed no differences. Only the hard dough stage seemed to be negatively influenced by the cumulative effect of a decrease in temperature. Generally a decrease in temperature led to a decrease in grain quality at different growth stages. c) Artificial freezing was also used to evaluate the reaction of two growth types at different temperatures (–5° to –9°C with 2°C increments) and at different flag leaf stages (early flag leaf, flag leaf and emerging of awns). The quantitative evaluation showed cultivar 1 (winter type) to be more tolerant to frost injury than cultivar 2 (intermediate type). However, both cultivars were highly sensitive to frost injury at the early flag leaf than at the flag leaf and emergence of the awns stages. Furthermore, the primary spikes were shown to sustain the highest degree of frost injury during this trial. d) During this trial the reaction response of two growth types at different temperatures (–5° to –9°C with 2°C increments) and at different flowering stages (0, 50 and 100% flowering), was evaluated. The quantitative evaluation showed cultivar 1 (winter type) to be more frost tolerant than cultivar 2 (intermediate type). At the different flowering stages both cultivars proved to be highly sensitive to frost injury at temperatures lower than –5°C. No significant differences were obtained for cultivar 1 at the different flowering stages, but cultivar 2 was more sensitive at 0% flowering than at other growth stages. e) A guide was finally compiled to assist the producer, agronomist, insurance companies and other role players in the wheat industry. This guide consists of short discussions supported by photographs to illustrate frost damage to South African wheat.