INFLUENCE OF PLANT-GROWTH AT HIGH CO2 CONCENTRATIONS ON LEAF CONTENT OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE AND INTRACELLULAR-DISTRIBUTION OF SOLUBLE CARBOHYDRATES IN TOBACCO, SNAPDRAGON, AND PARSLEY/

We have examined the possible role of leaf cytosolic hexoses and the e xpression of mannitol metabolism as mechanisms that may affect the rep ression of photosynthetic capacity when plants are grown at 1000 versu s 380 mu L L-1 CO2. In plants grown at high CO2, leaf ribulose-1,5-bis phosphate carboxylase/oxygenase content declined by greater than or eq ual to 20% in tobacco (Nicotiana sylvestris) but was not affected in t he mannitol-producing species snapdragon (Antirrhinum majus) and parsl ey (Petroselinum hortense). In the three species mesophyll glucose and fructose at midday occurred almost entirely in the vacuole (>99%), ir respective of growth CO2 levels. The estimated cytosolic concentration s of glucose and fructose were less than or equal to 100 mu M. In the three species grown at high CO2, total leaf carbohydrates increased 60 to 100%, but mannitol metabolism did not function as an overflow mech anism for the increased accumulation of carbohydrate. In both snapdrag on and parsley grown at ambient or high CO2, mannitol occurred in the chloroplast and cytosol at estimated midday concentrations of 0.1 M or more each. The compartmentation of leaf hexoses and the metabolism of alternate carbohydrates are further considered in relation to photosy nthetic acclimation to high levels of CO2.