Rubisco activity in guard cells compared with the solute requirement for stomatal opening

• Published in: Plant physiology (Bethesda) 1990-01, Vol.92 (1), p.246-253
• Main Authors: Reckmann, U. (Universitat Gottingen, Gottingen, West Germany), Scheibe, R, Raschke, K
• Format: Article
• Language: English
• Subjects: 140505 > Solar Energy Conversion-- Photochemical, Photobiological, & Thermochemical Conversion-- (1980-); 550201 > Biochemistry-- Tracer Techniques; ACTIVIDAD ENZIMATICA; ACTIVITE ENZYMATIQUE; BASIC BIOLOGICAL SCIENCES; Biological and medical sciences; BIOLOGICAL PATHWAYS; CARBON 14 COMPOUNDS; CARBON COMPOUNDS; CARBON DIOXIDE; CARBON OXIDES; CARBON-CARBON LYASES; CARBOXY-LYASES; CELLULE; CELULAS; CHALCOGENIDES; CHEMICAL REACTIONS; Chemical suspensions; Chloroplasts; ENZYME ACTIVITY; ENZYMES; ESTOMA; FEUILLE; Fundamental and applied biological sciences. Psychology; Guard cells; HOJAS; ISOTOPE APPLICATIONS; LABELLED COMPOUNDS; LEGUMINOSAE; LIASAS; LUMIERE; LUZ; LYASE; LYASES; MAGNOLIOPHYTA; MAGNOLIOPSIDA; Mesophyll; Mesophyll cells; Metabolism; OPENINGS; Osmosis; OXIDES; OXYGEN COMPOUNDS; PHOSPHORYLATION; Photosynthesis, respiration. Anabolism, catabolism; PHYSIOLOGY; PISUM; PISUM SATIVUM; PLANT CELLS; Plant physiology and development; Plants; PLANTS 551001 > Physiological Systems-- Tracer Techniques; Protoplasts; RIBULOSE DIPHOSPHATE CARBOXYLASE; SOLAR ENERGY; Solutes; STOMATA; STOMATE; TRACER TECHNIQUES
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.92.1.246

We investigated whether the reductive pentose phosphate path in guard cells of Pisum sativum had the capacity to contribute significantly to the production of osmotica during stomatal opening in the light. Amounts of ribulose 1,5-bisphophate carboxylase/oxygenase (Rubisco) were determined by the [14C]carboxyarabinitol bisphosphate assay. A guard cell contained about 1.2 and a mesophyll cell about 324 picograms of the enzyme; the ratio was 1:270. The specific activities of Rubisco in guard cells and in mesophyll cells were equal; there was no indication of a specific inhibitor of Rubisco in guard cells. Rubisco activity was 115 femtomol per guard-cell protoplast and hour. This value was different from zero with a probability of 0.99. After exposure of guard-cell protoplasts to 14CO2 for 2 seconds in the light, about one-half of the radioactivity was in phosphorylated compounds and 10% in malate. Guard cells in epidermal strips produced a different labelling pattern; in the light, 10% of the label was in phosphorylated compounds and about 60% in malate. The rate of solute accumulation in intact guard cells was estimated to have been 900 femto-osmol per cell and hour. If Rubisco operated at full capacity in guard cells, and hexoses were produced as osmotica, solutes could be supplied at a rate of 19 femto-osmol per cell and hour, which would constitute 2% of the estimated requirement. The capacity of guard-cell Rubisco to meet the solute requirement for stomatal opening in leaves of Pisum sativum is insignificant