Mesophyll conductance in two cultivars of wheat grown in glacial to super-elevated CO2 concentrations

Abstract

Mesophyll conductance (g_m) is an important factor limiting photosynthesis. However, g_m response to long-term growth in variable [CO₂] is not well understood, particularly in crop plants. Here, we grew two cultivars of wheat (Halberd and Cranbrook), known to differ in g_m under current environmental conditions, in four [CO₂] treatments: glacial (206 μmol mol ^-1), pre-industrial (344 μmol mol ^-1), current ambient (489 μmol mol ^-1) and super-elevated (1085 μmol mol ^-1), and two water treatments (well-watered and moderate water limitation), to develop an evolutionary and future climate perspective on g_m control of photosynthesis and water use efficiency (WUE). In the two wheat genotypes, g_m increased with rising [CO₂]from glacial to ambient [CO₂], but declined at super-elevated [CO₂]. The responses of g_m to different growth [CO₂]also depend on water stress; however, the specific mechanism of g_m response to [CO₂]remains unclear. Although g_m and g_m/g_sc (mesophyll conductance/stomatal conductance) were strongly associated with the variability of photosynthetic rates (A) and WUE, we found that plants with higher g_m may increase A without increasing g_sc, which increased WUE. These results may be useful to inform plant breeding programs and cultivar selection for Australian wheat under future environmental conditions.