LAUSR

Mesophyll conductance (gm) is an important factor limiting photosynthesis. However, gm 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 gm 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 gm control of photosynthesis and water use efficiency (WUE). In the two wheat genotypes, gm increased with rising [CO2]from glacial to ambient [CO2], but declined at super-elevated [CO2]. The responses of gm to different growth [CO2]also depend on water stress; however, the specific mechanism of gm response to [CO2]remains unclear. Although gm and gm/gsc (mesophyll conductance/stomatal conductance) were strongly associated with the variability of photosynthetic rates (A) and WUE, we found that plants with higher gm may increase A without increasing gsc, which increased WUE. These results may be useful to inform plant breeding programs and cultivar selection for Australian wheat under future environmental conditions.