Wheat plants (Triticum durum Desf., cv. Regallo) were grown in the field to study the effects of contrasting [CO₂] conditions (700 versus 370 μmol mol⁻¹) on growth, photosynthetic performance, and C management during the post-anthesis period. The aim was to test whether a restricted capacity of sink organs to utilize photosynthates drives a loss of photosynthetic capacity in elevated CO₂. The ambient ¹³C/¹²C isotopic composition (δ¹³C) of air CO₂ was changed from –10.2‰ in ambient [CO₂] to –23.6‰ under elevated [CO₂] between the 7th and the 14th days after anthesis in order to study C assimilation and partitioning between leaves and ears. Elevated [CO₂] had no significant effect on biomass production and grain filling, and caused an accumulation of C compounds in leaves. This was accompanied by up-regulation of phosphoglycerate mutase and ATP synthase protein content, together with down-regulation of adenosine diphosphate glucose pyrophosphatase protein. Growth in elevated [CO₂] negatively affected Rubisco and Rubisco activase protein content and induced photosynthetic down-regulation. CO₂ enrichment caused a specific decrease in Rubisco content, together with decreases in the amino acid and total N content of leaves. The C labelling revealed that in flag leaves, part of the C fixed during grain filling was stored as starch and structural C compounds whereas the rest of the labelled C (mainly in the form of soluble sugars) was completely respired 48 h after the end of labelling. Although labelled C was not detected in the δ¹³C of ear total organic matter and respired CO₂, soluble sugar δ¹³C revealed that a small amount of labelled C reached the ear. The ¹²CO₂ labelling suggests that during the beginning of post-anthesis the ear did not contribute towards overcoming flag leaf carbohydrate accumulation, and this had a consequent effect on protein expression and photosynthetic acclimation.

Only limited public transcriptomics resources are available for durum wheat and its responses to environmental changes. We developed a quantitative reverse transcription–PCR (qRT–PCR) platform for analysing the expression of primary C and N metabolism genes in durum wheat in leaves (125 genes) and roots (38 genes), based on available bread wheat genes and the identification of orthologs of known genes in other species. We also assessed the expression stability of seven reference genes for qRT–PCR under varying environments. We therefore present a functional qRT–PCR platform for gene expression analysis in durum wheat, and suggest using the ADP-ribosylation factor as a reference gene for qRT–PCR normalization. We investigated the effects of elevated [CO₂] and temperature at two levels of N supply on C and N metabolism by combining gene expression analysis, using our qRT–PCR platform, with biochemical and physiological parameters in durum wheat grown in field chambers. Elevated CO₂ down-regulated the photosynthetic capacity and led to the loss of N compounds, including Rubisco; this effect was exacerbated at low N. Mechanistically, the reduction in photosynthesis and N levels could be associated with a decreased transcription of the genes involved in photosynthesis and N assimilation. High temperatures increased stomatal conductance, and thus did not inhibit photosynthesis, even though Rubisco protein and activity, soluble protein, leaf N, and gene expression for C fixation and N assimilation were down-regulated. Under a future scenario of climate change, the extent to which C fixation capacity and N assimilation are down-regulated will depend upon the N supply.

Elevated [CO₂] (eCO₂) can lead to photosynthetic acclimation and this is often intensified by low nitrogen (N). Despite intensive studies of plant responses to eCO₂, the regulation mechanism of primary metabolism at the whole-plant level in interaction with $NO3−$ supply remains unclear. We examined the metabolic and transcriptional responses triggered by eCO₂ in association with physiological–biochemical traits in flag leaves and roots of durum wheat grown hydroponically in ambient and elevated [CO₂] with low (LN) and high (HN) $NO3−$ supply. Multivariate analysis revealed a strong interaction between eCO₂ and $NO3−$ supply. Photosynthetic acclimation induced by eCO₂ in LN plants was accompanied by an increase in biomass and carbohydrates, and decreases of leaf organic N per unit area, organic acids, inorganic ions, Calvin–Benson cycle intermediates, Rubisco, nitrate reductase activity, amino acids and transcripts for N metabolism, particularly in leaves, whereas $NO3−$ uptake was unaffected. In HN plants, eCO₂ did not decrease photosynthetic capacity or leaf organic N per unit area, but induced transcripts for N metabolism, especially in roots. In conclusion, the photosynthetic acclimation in LN plants was associated with an inhibition of leaf $NO3−$ assimilation, whereas up-regulation of N metabolism in roots could have mitigated the acclimatory effect of eCO₂ in HN plants.