The need to accelerate the selection of crop genotypes that are both resistant to and productive under abiotic stress is enhanced by global warming and the increase in demand for food by a growing world population. In this thesis, a new method is proposed for evaluation of wheat genotypes in terms of their resilience to stress and their production capacity. The method quantifies the components of the new index related with yield and yield components under abiotic stress. The index method, based on a scoring scale, offers a simple and easy visualization and identification of resilient, productive genotypes, according to their yield and yield components. This new selection method could help breeders and researchers by defining clear and strong criteria to identify genotypes with high resilience and high productivity and to reveal where genotypes express their susceptibility to a stress environment, providing a quantitative classification of contrasts in terms of yield and yield components.
This index method has allowed 1) the identification of contrasting genotypes from a small population (CIMCOG-ROOT, 10 genotypes), and 2) the quantification of their contrasts, in terms of yield and yield components (grain number per spike), both constituting a key opportunity to test whether a stress hormone and/or hormone balance (ABA, ethylene and/or ABA/ethylene) could be used as a physiological trait for breeding for abiotic stress resilience. Due to the complexity of spatial and temporal variation of hormone accumulation (ABA and ethylene) and their different effects in plant development in response to stress environments, it is necessary to investigate how factors other than environment can influence the production of hormones. The factors considered in this work were: day time, water management (irrigation), tissue specificity (leaf and spike) and wheat phenological development (phenological stages).
The present study of hormone (ABA and ethylene) quantification in wheat has shown that the genotypic variation in hormone signalling cannot be identified at every developmental stage of the plant. In fact, only two stages were identified for differences in ethylene emission (late- booting and heading), and two for ABA accumulation (late-booting and half-emergence), both on leaf tissue. However, the ratio ABA/ethylene (ABA/ETH) emerges as a better method to study genotypic variation in response to stress environments, in terms of hormone accumulation. In fact, both tissues, leaf and spike at all stages during the pre-anthesis stage (from booting to heading), have shown significant genotypic variation in terms of ABA/ETH balance.
The resilience index of grain number per spike and this study of hormone (ABA and ethylene) quantification under drought stress (on leaf and spike), have shown that at late-booting stage, a lower leaf hormone ratio ABA/ETH and higher leaf ethylene emission is associated with a higher resilience of spike fertility (grain per spike resilience). However, under controlled conditions, a possible optimum threshold is observed in terms of leaf hormone ratio ABA/ETH. These results have been obtained with statistical significance in eight genotypes under field conditions and four genotypes under controlled environments.
It is suggested in this work that a possible method for early selection of genotypes for high spike fertility resilience under drought stress could be developed by quantifying hormone signalling (leaf ethylene and the leaf ratio ABA/ETH at late-booting stage). However, some improvements in the process of hormone quantification need to be made before recommending this method to breeders.
