The evaluation and characterisation of South African wheat cultivars for temperature stress tolerance

English: Extreme temperatures are detrimental to plant growth and development and thus affect the productivity of various crops around the world. In South Africa, nearly 544 000 hectares of wheat were insured against frost damage over the last 15 years. Wheat in most of the western parts of Free State, experiences high temperature stress in later stages of growth. In this study, 15 cultivars were evaluated to test their response to high and low temperature stress. Ten of the cultivars were tested to determine the influence of high and low temperature stress on yield and yield components. There was a high and positive correlation between total grain yield and other yield components. Some cultivars performed well and gave good yields under both high and low temperature stress, indicating stress tolerance. Other cultivars showed temperature stress sensitivity, as they yielded poorly. Low temperature stress proved to the most devastating as some cultivars failed to produce any seed and some had poor and shrunken seeds. Accumulation of metabolites during stress has been correlated with the level of stress tolerance. A study to evaluate the effect of high and low temperature stress on proteins and carbohydrate (glucose and sucrose) content in seedlings was done. There was a significant difference in the increase of protein content between high and low temperature stress. Protein content increased significantly more under low temperature stress than high temperature stress. All cultivars except PAN 3349 had increased protein content under low temperature stress but a few cultivars exhibited high temperature sensitivity as they failed to increase their protein content. Changes were also observed on carbohydrate level. There was a notable increase in glucose and sucrose content in certain cultivars. It was also noted that sucrose content increased significantly more than the glucose content and that not all the cultivars that exhibited increased sucrose content necessarily had a glucose content increase as well. Different screening methods were evaluated for their efficiency as measure of stress tolerance. Results from the cell membrane stability test from this study were inconclusive, which raises questions on the reliability and efficiency of this method as a measure of high temperature tolerance. Another test that was conducted is TTC, which gave good and reliable results It showed that there are definite high temperature tolerance differences in South African cultivars. Some cultivars proved to be high temperature tolerant as they had high TTC reduction under stress temperatures, whereas some cultivars showed sensitivity with low TTC reduction. It was also found that some cultivars are able to avoid stress, whereby they tolerate stress for only short periods. A crown survival study was also done and leaf-length and percentage survival were studied as measures of freezing tolerance. Results indicated a high level of freezing tolerance in cultivars, as they remained viable and were able to grow new leaves after exposure to stress. The cultivars in this study can be grouped as tolerant, intermediate and sensitive. On the bases of this study the tests were reliable and can be utilised as a measure for freezing tolerance. The use of pro line content as indication of tolerance to stress has been established and in use for a long time. The effect of high and low temperature stress was studied with the aim of evaluating proline content as a measure of temperature stress. Results showed increased levels of proline after temperature treatments in comparison to a control treatment in some cultivars. Some cultivars exhibited an increase of proline content after both high and low temperature treatments, whereas some had increased proline content for either low or high temperature stress. Proline content can be used as a screening tool for tolerance, but it must be used in conjunction with other screening methods like TTC.