Investigations of water deficit interactions with heat and elevated carbon dioxide in wheat

Future climate is predicted to be characterised by elevated carbon dioxide (eCO2), as well as more incidences of heat and water deficit. eCO2 has been widely reported as enhancing growth, biomass and grain yield. To investigate the interactive effects of abiotic stresses on genotypic performance, an experiment was established in open-top chambers at the University of Rhodes eCO2 facility in Grahamstown, South Africa. The specific objectives of the study were: i) to evaluate the effects of eCO2 on wheat grain yield, yield components and grain quality under heat and terminal water deficit conditions; ii) to identify cultivar sources of tolerance to combined water deficit and heat stress under eCO2; iii) to identify appropriate stress indices that can be used as screening tools for tolerance to combined effects of water deficit and heat stress under eCO2. A total of 19 wheat genotypes were evaluated in three environments varying in CO2, temperature and water deficit during the 2019 winter season. The experiment was laid out in a split-split plot design arranged in blocks inside the chambers. The parameters recorded were: leaf water potential (LWP), biomass content (TB), number of productive tillers (NPT), days to flowering (DTA), days to maturity (DTM), plant height (PH), thousand kernel weight (TKW), number of kernels per spike (KPS), kernel weight per spike (KWS) and total grain weight (TGW). The following stress indices were determined: tolerance index, stress tolerance, yield susceptibility index, mean productivity, geometric mean productivity, stress intensity index and yield index. Elevated atmospheric CO2 ameliorated the negative effects of combined heat and water deficit stress by enhancing LWP, NPT, KPS, TB and TGW. Wheat genotypes responded the same way to CO2 with respect to grain yield. Furthermore, adequate water supply mitigated the adverse effects of heat stress. In addition, the combined effects of eCO2, heat and water deficit are confounding and hypo-additive in nature. The separation of environmental effects revealed that significant genotypic responses on grain yield and biomass were caused by heat and water deficit stress, while eCO2 mitigated their negative effects, promoting growth and reproduction. Both Principal component analysis (PCA) biplot analysis and cultivar superiority measure proved to be reliable statistical tools since they managed to identify 13-5HTSBWYT-H18, Ncema and SST8135 as having both specific adaptations to future climates as well as wide adaption to multiple environments. The genotypes may be used as sources (parents for crosses) for wide adaptation in breeding programmes in the wake of predicted future climate environments. PCA biplot analysis identified mean productivity (MP) and geometric mean productivity (GMP) as the best indices; hence can also be as secondary traits complimenting traditional agronomic and physiological traits in wheat breeding under environments varying in water availability, CO2 and temperature. The interactive effects of eCO2 with heat and water deficit stress did not significantly compromise the grain physical characteristics, flour extraction, protein content, falling number and flour ash. eCO2 ameliorated the negative effects of heat and water deficit by increasing protein content by 4.75 percentThesis (MSc) (Crop Science) -- University of Fort Hare, 202