ALTERATIONS IN LIGHT-INDUCED STOMATAL OPENING IN A STARCH-DEFICIENT MUTANT OF ARABIDOPSIS-THALIANA L DEFICIENT IN CHLOROPLAST PHOSPHOGLUCOMUTASE ACTIVITY

Stomatal responses to light of Arabidopsis thaliana wild-type plants a nd mutant plants deficient in starch (phosphoglucomutase deficient) we re compared in gas exchange experiments. Stomatal density, size and ul trastructure were identical for the two phenotypes, but no starch was observed in guard cells of the mutant plants whatever the time of day. The overall extent of changes in stomatal conductance during 14 h lig ht-10 h dark cycles was similar for the two phenotypes. However, the s low endogenous stomatal opening occurring in darkness in the wild type was not observed in the mutant plants. Stomata in the mutant plants r esponded much more slowly to blue light (70 mu mol m(-2) s(-1)) though the response to red light (250 mu mol m(-2) s(-1)) was similar to tha t of wild-type plants. In paradermal sections, stomatal responses to r ed light (300 mu mol m(-2) s(-1)) were weak for mild-type plants as we ll as for mutant plants. Stomatal opening was greater under low blue l ight (75 mu mol m(-2) s(-1)) than under red light for the two genotype s. However, in mutant plants, a high chloride concentration (50 mol m( -3)) was necessary to achieve the same stomatal aperture as observed f or the wild-type plants. These results suggest that starch metabolism, via the synthesis of a counter-ion to potassium (probably malate), is required for full stomatal response to blue light but is not involved in the stomatal response to red light.