Abstract
As a consequence of global climate change, heat stress together with other abiotic stresses will remain an important determinant of future food security. Wheat (Triticum aestivum L.) is the third most important crop of the world feeding one third of the world population. Being a crop of temperate climate, wheat is sensitive to heat stress. We need to understand how our crops will perform in these changing climatic conditions and how we can develop varieties, which are more tolerant.
The PhD study focussed on understanding heat tolerance in wheat with a combined approach of plant physiology and quantitative genetics in particular, plant phenotyping based quantitative trait loci (QTL) discovery for a physiological trait under heat stress. Chlorophyll a fluorescence trait, Fv/Fm was used as a phenotyping tool, as it reflects the effect of heat stress on maximum photochemical efficiency of photosystem II (PSII), which is a fundamental process in photosynthesis.
The first study was conducted to identify cultivars differing in Fv/Fm as a measure of heat tolerance during reproductive phase. The proportion of the total variation in cultivar Fv/Fm that was due to the genotypic difference was termed genetic determination (GD). An initial mass screening of 1,274 wheat cultivars (diverse origin) showed a GD of 8.5%. A stronger heat treatment was given in the second screening with 138 selected cultivars resulting in larger differentiation of cultivars (GD 15.4%). The GD further increased to 27.9% in the third screening with 41 selected cultivars. The Fv/Fm was influenced by heat stress and the difference between the cultivars appeared only during the heat stress. Further analysis of other chlorophyll fluorescence parameters showed similar or higher GD, but they did not reveal the genetic difference among cultivars due to heat stress as the GD of most of them remained similar in control and stress.
The second study investigated if it was possible to use detached leaves to screen for heat tolerance instead of intact plants. The previously selected 41 cultivars, known to differ in v/Fm, were used. The main difference between the detached leaves and intact plant was the vast difference in time scale of reaction and the severity of heat stress. All the chlorophyll fluorescence parameters remained almost unaffected in control, indicating that the detachment itself did not cause the difference. The correlation of the cultivar response in intact plant versus detached leaf was low. Overall, the result suggests that selection of cultivars by detached leaves may operate for different genetic factors than in intact plants. In the third study, the previously selected high and low groups of cultivars (from first study) were used to validate what does a difference in Fv/Fm under heat stress means in term of overall photosynthesis and plant performance under heat stress. The cultivars were ealuated at a relatively milder and diurnally varying heat stress mimicking the naturally occurring heat waves. Despite the narrow differences in Fv/Fm between the two groups a significant correlation of cultivar Fv/Fm was found with the cultivar response at 40 °C for 3 days (with first study). This shows that the selection of cultivars at a stronger heat stress was valid also at a milder stress.
Overall, the high Fv/Fm cultivars maintained the functionality of PSII and thereby photosynthesis under heat stress by retaining chlorophyll to stay green during the heat tress and by lowering their leaf temperature through better evaporative cooling via high ranspiration and stomatal conductance. Despite the difference in stomatal conductance, the high and low groups maintained the same level of intracellular CO2 but differed in the net photosynthesis, suggesting differences in biochemical functions or repair mechanisms in PSII components under heat stress. An observed positive correlation between cultivar differences in Fv/Fm and dry matter accumulation during heat stress is a step forward to document that phenotypic differences measured by phenomic approaches can be translated into overall plant performance under heat stress.
The fourth study focussed on associating phenotypic variations identified by Fv/Fm to genetic differences by QTL mapping. A total of three significant QTLs explaining about 12-16% of phenotypic variation for Fv/Fm was found in the three mapping populations (F2) generated by crossing three male parents with highest Fv/Fm with a common female parent with lowest Fv/Fm (selected in the first study). The short arm region in chromosome 3B and 1D appears to be important genomic regions associated with Fv/Fm during heat stress. Remarkably, the QTLs responded in a heat inducible way, which supports our previous results that differences in cultivar Fv/Fm were heat stress driven.
The PhD thesis work contributes to the better understanding of physiological and genetic basis of heat tolerance in wheat. With a combined approach of physiological phenotyping and genetic analysis it was possible to link small phenotypic differences for a key physiological process in the photosynthesis to QTLs. Future works may reveal how important these identified QTLs are in terms of improving heat tolerance in wheat. Similar approach of plant phenotyping based gene discovery can be used to understand other physiological traits in plants in response to different abiotic stresses, which may lead to identification of genes for physiological traits that may confer better adaptation to changing climatic conditions. Eventually, combining all the identified “good genes” may aid in developing stress tolerant cultivars to overcome environmental constraints and thereby, meet the increasing demand of future food security.
The PhD study focussed on understanding heat tolerance in wheat with a combined approach of plant physiology and quantitative genetics in particular, plant phenotyping based quantitative trait loci (QTL) discovery for a physiological trait under heat stress. Chlorophyll a fluorescence trait, Fv/Fm was used as a phenotyping tool, as it reflects the effect of heat stress on maximum photochemical efficiency of photosystem II (PSII), which is a fundamental process in photosynthesis.
The first study was conducted to identify cultivars differing in Fv/Fm as a measure of heat tolerance during reproductive phase. The proportion of the total variation in cultivar Fv/Fm that was due to the genotypic difference was termed genetic determination (GD). An initial mass screening of 1,274 wheat cultivars (diverse origin) showed a GD of 8.5%. A stronger heat treatment was given in the second screening with 138 selected cultivars resulting in larger differentiation of cultivars (GD 15.4%). The GD further increased to 27.9% in the third screening with 41 selected cultivars. The Fv/Fm was influenced by heat stress and the difference between the cultivars appeared only during the heat stress. Further analysis of other chlorophyll fluorescence parameters showed similar or higher GD, but they did not reveal the genetic difference among cultivars due to heat stress as the GD of most of them remained similar in control and stress.
The second study investigated if it was possible to use detached leaves to screen for heat tolerance instead of intact plants. The previously selected 41 cultivars, known to differ in v/Fm, were used. The main difference between the detached leaves and intact plant was the vast difference in time scale of reaction and the severity of heat stress. All the chlorophyll fluorescence parameters remained almost unaffected in control, indicating that the detachment itself did not cause the difference. The correlation of the cultivar response in intact plant versus detached leaf was low. Overall, the result suggests that selection of cultivars by detached leaves may operate for different genetic factors than in intact plants. In the third study, the previously selected high and low groups of cultivars (from first study) were used to validate what does a difference in Fv/Fm under heat stress means in term of overall photosynthesis and plant performance under heat stress. The cultivars were ealuated at a relatively milder and diurnally varying heat stress mimicking the naturally occurring heat waves. Despite the narrow differences in Fv/Fm between the two groups a significant correlation of cultivar Fv/Fm was found with the cultivar response at 40 °C for 3 days (with first study). This shows that the selection of cultivars at a stronger heat stress was valid also at a milder stress.
Overall, the high Fv/Fm cultivars maintained the functionality of PSII and thereby photosynthesis under heat stress by retaining chlorophyll to stay green during the heat tress and by lowering their leaf temperature through better evaporative cooling via high ranspiration and stomatal conductance. Despite the difference in stomatal conductance, the high and low groups maintained the same level of intracellular CO2 but differed in the net photosynthesis, suggesting differences in biochemical functions or repair mechanisms in PSII components under heat stress. An observed positive correlation between cultivar differences in Fv/Fm and dry matter accumulation during heat stress is a step forward to document that phenotypic differences measured by phenomic approaches can be translated into overall plant performance under heat stress.
The fourth study focussed on associating phenotypic variations identified by Fv/Fm to genetic differences by QTL mapping. A total of three significant QTLs explaining about 12-16% of phenotypic variation for Fv/Fm was found in the three mapping populations (F2) generated by crossing three male parents with highest Fv/Fm with a common female parent with lowest Fv/Fm (selected in the first study). The short arm region in chromosome 3B and 1D appears to be important genomic regions associated with Fv/Fm during heat stress. Remarkably, the QTLs responded in a heat inducible way, which supports our previous results that differences in cultivar Fv/Fm were heat stress driven.
The PhD thesis work contributes to the better understanding of physiological and genetic basis of heat tolerance in wheat. With a combined approach of physiological phenotyping and genetic analysis it was possible to link small phenotypic differences for a key physiological process in the photosynthesis to QTLs. Future works may reveal how important these identified QTLs are in terms of improving heat tolerance in wheat. Similar approach of plant phenotyping based gene discovery can be used to understand other physiological traits in plants in response to different abiotic stresses, which may lead to identification of genes for physiological traits that may confer better adaptation to changing climatic conditions. Eventually, combining all the identified “good genes” may aid in developing stress tolerant cultivars to overcome environmental constraints and thereby, meet the increasing demand of future food security.
| Originalsprog | Engelsk |
|---|
| Forlag | Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen |
|---|---|
| Antal sider | 182 |
| Status | Udgivet - 2013 |