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

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 mu mol mol(-1)), pre-industrial (344 mu mol mol(-1)), current ambient (489 mu mol moll, and super-elevated (1085 mu mol 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 programmes and cultivar selection for Australian wheat under future environmental conditions.

Automated summary

Mesophyll conductance (g(m)) is a crucial factor in limiting photosynthesis, yet its response to long-term growth in variable [CO2] conditions depends on water stress, particularly in crop plants. Plants with higher g m may increase photosynthetic rates (A) without increasing stomatal conductance (g(sc), thereby improving water-use efficiency (WUE).