DIFFERENTIAL STIMULATION OF PEP-CARBOXYLATION IN GUARD-CELLS AND MESOPHYLL-CELLS BY AMMONIUM OR FUSICOCCIN

Dark CO2-fixation in guard cells of Vicia faba was much more sensitive to ammonium than in mesophyll cells. Addition of ammonium (5.0 mol m-3; pH(o) 7.6) caused up to a 7-fold increase in dark CO2-fixation rates in guard cell protoplasts (GCP), whereas in leaf slices, mesophyll cells, and mesophyll protoplasts the increase was only about 1.4-fold. In both cell or tissue types, total CO2-fixation rates were higher in the light (2-12-fold higher in GCP and 28-fold in mesophyll); these rates were only slightly changed by ammonium treatment. However, separation of C-14-labelled products after fixation of CO2 in the light by GCP revealed a large ammonium-induced shift in carbon flow from starch and sugars to typical products of C4-metabolism (mainly malate and aspartate). In contrast, in mesophyll cells amino acid and malate labelling was only moderately increased by ammonium at the expense of sucrose. The data suggest that in vivo ammonium might facilitate stomatal opening and/or delay stomatal closing through an increased production of organic acids.