Photosynthesis-dependent/independent control of stomatal responses to CO2 in mutant barley with surplus electron transport capacity and reduced SLAH3 anion channel transcript.

The mechanisms of stomatal sensitivity to CO2 are yet to be fully understood. The role of photosynthetic and non-photosynthetic factors in stomatal responses to CO2 was investigated in wild-type barley (Hordeum vulgare var. Graphic) and in a mutant (G132) with decreased photochemical and Rubisco capacities. The CO2 and DCMU responses of stomatal conductance (gs), gas exchange, chlorophyll fluorescence and levels of ATP, with a putative transcript for stomatal opening were analysed. G132 had greater gs than the wild-type, despite lower photosynthesis rates and higher intercellular CO2 concentrations (Ci). The mutant had Rubisco-limited photosynthesis at very high CO2 levels, and higher ATP contents than the wild-type. Stomatal sensitivity to CO2 under red light was lower in G132 than in the wild-type, both in photosynthesizing and DCMU-inhibited leaves. Under constant Ci and red light, stomatal sensitivity to DCMU inhibition was higher in G132. The levels of a SLAH3-like slow anion channel transcript, involved in stomatal closure, decreased sharply in G132. The results suggest that stomatal responses to CO2 depend partly on the balance of photosynthetic electron transport to carbon assimilation capacities, but are partially regulated by the CO2 signalling network. High gs can improve the adaptation to climate change in well-watered conditions.