Illuminating stomatal responses to red light: establishing the role of Ci-dependent versus -independent mechanisms in control of stomatal behaviour.

The stomatal response to red light appears to link stomatal conductance (gs) with photosynthetic rates. Initially, it was suggested that changes in intercellular CO2 concentration (Ci) provide the main cue via a Ci-dependent response. However, evidence for Ci-independent mechanisms suggests an additional, more direct relationship with photosynthesis. While both Ci-dependent and -independent mechanisms clearly function in stomatal red light responses, little is known regarding their relative contribution. The present study aimed to quantify the relative magnitude of Ci-dependent and -independent mechanisms on the stomatal red light response, to characterize their interplay and to assess the putative link between plastoquinone redox state and Ci-independent stomatal responses. Red light response curves measured at a range of Ci values for wild-type Arabidopsis (Col-0) and the CO2 hyposensitive mutant ca1ca4 allowed deconvolution of Ci-dependent and -independent pathways. Surprisingly, we observed that both mechanisms contribute equally to stomatal red light responses, but Ci-independent stomatal opening is suppressed at high Ci. The present data are also consistent with the involvement of the plastoquinone redox state in coordinating the Ci-independent component. Overall, it seems that while Ci-independent mechanisms are distinct from responses to Ci, interplay between these two pathways is important to facilitate effective coordination between gs and photosynthesis.