Carbon/nitrogen relations in C3 cereal crops within a climate change context: implications on grain yield and quality

In a context of the expanding world population and global climate change, food security is becoming a challenge for worldwide society. To meet the increasing global agricultural demands, crop yields enhancement has been attempted since the green revolution and cereals production, in particular wheat, has increased since then by releasing high yielding new cultivars. However, improvement in crop yields has slowed since the 1990s and the recent gains in global crop production fall short of the expected demands mainly due to global warming. At a global scale, the relatively decrease in wheat production is principally because of the adverse effects of abiotic stresses that are increasing in intensity and frequency under climate change scenario. Low water availability and extreme temperatures will negatively affect the growth and productivity of major crop species including durum wheat. In the Mediterranean area, the process of grain filling is coinciding with dry and hot environmental conditions affecting final yield quantitatively and qualitatively as well. Moreover, studies conducted recently remarked that grain mineral composition is shifted and total protein content in grains is reduced when durum wheat grows in the presence of high CO2 concentration ([CO2]). It is clear that commercialised wheat genotypes are becoming more vulnerable to global climate change which is affecting not only grain yield but also quality. Thus, the understanding of physiological mechanisms that enable plants to adapt to drought stress and increasing atmospheric [CO2] could help in screening and selection of genotypes with suitable grain yield and quality, and using these traits in breeding programs. On the other hand, the increase in nitrogen fertilizers application in wheat crops is consequently stimulating plant growth and increasing grain yield, nitrogen and protein concentration in kernels ensuring, thereby, good bread/pasta making quality and mitigating the negative effect of changing climate on grain production. Nevertheless, the excessive nitrogen supply can lead to environment pollution and may probably accentuate climate warming by increasing nitrous oxide (N2O) emission. For this reason, optimizing nitrogen use efficiency (NUE) is a tool to increase crop yields while preserving the environment. Within this context, the main objective of this work is the use of new wheat selection criteria to identify, in an integrative manner, genotypes and crop management practices conferring high nitrogen use efficiency to reach higher yield and better grain quality under increasing [CO2] and low water availability. For this purpose, in the first chapter (I), a meta-analysis study was carried out to provide an overview of the effects and interactions of multiple climate factors, specifically high [CO2], drought, and elevated temperature on the productivity and grain quality of C3 cereals. Findings presented in this chapter showed that despite of the positive effect of elevate [CO2] on grain yield, this trait seems to be mitigated by heat and drought stress. Grain quality was also impacted by changing climate, characterized by an increase in carbohydrates and decrease in protein and minerals. In the second chapter (II), we assessed the grain quality trait of wheat archived samples since 1850 collected from many countries to evaluate the nutritional quality changes in grain under changing climate. This study confirmed the results foundin the previous chapter and showed an imbalance in carbohydrate/protein content marked after the 60s, adding to an impoverishment in minerals. Yield results from Broadbalk wheat experiment in Rothamsted (UK) showed an improvement of wheat yield since the green revolution attributed mainly to the introducing of semi-dwarf high yielding genotypes. In chapter (III), to investigate the impact of nitrogen fertilization on yield and grain quality, an experiment was performed where 20 durum wheat genotypes were fertilized since anthesis with two N fertilization levels under greenhouse conditions. Within these genotypes, only 6 lines were selected with high and low nitrogen use efficiency to characterize agronomic and quality traits. As expected, nitrogen supply increased grain yield while no effect was detected in thousand-grain weight. Grain soluble sugars, gluten fractions, mineral composition, and polyphenol concentrations were also improved by N application. The comparison among genotypes revealed that high yielding genotypes had higher grain carbohydrate concentrations while higher concentrations in grain minerals, gluten fractions, and polyphenols were recorded in low yielding cultivars. Finally, in chapter (IV), 4 durum wheat genotypes and 6 tritordeum lines with higher and lower NUE were exposed to high [CO2] and drought stress in greenhouses, in order to characterize post-anthesis nutrient remobilization from leaves and ears sustaining grain filling, together with agronomic characterization under such conditions. It seems that the increase of atmospheric [CO2] could attenuate the negative effect of drought on grain yield. Carbon and nitrogen metabolism in leaves and ears were altered under high CO2 enrichment and larger effect was observed when it was combined with drought, and the relative contribution of each organ to grain filling was strongly affected by growing conditions.