Summary Plant transpiration is controlled by stomata, with S‐ and R‐type anion channels playing key roles in guard cell action. Arabidopsis mutants lacking the ALMT12/QUAC1 R‐type anion channel function in guard cells show only a partial reduction in R‐type channel currents. The molecular nature of these remaining R‐type anion currents is still unclear. To further elucidate this, patch clamp, transcript and gas‐exchange measurements were performed with wild‐type (WT) and different almt mutant plants. The R‐type current fraction in the almt12 mutant exhibited the same voltage dependence, susceptibility to ATP block and lacked a chloride permeability as the WT. Therefore, we asked whether the R‐type anion currents in the ALMT12/QUAC1‐free mutant are caused by additional ALMT isoforms. In WT guard cells, ALMT12, ALMT13 and ALMT14 transcripts were detected, whereas only ALMT13 was found expressed in the almt12 mutant. Substantial R‐type anion currents still remained active in the almt12/13 and almt12/14 double mutants as well as the almt12/13/14 triple mutant. In good agreement, CO2‐triggered stomatal closure required the activity of ALMT12 but not ALMT13 or ALMT14. The results suggest that, with the exception of ALMT12, channel species other than ALMTs carry the guard cell R‐type anion currents.

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