Assimilate export by leaves of ricinus communis L. growing under normal and elevated carbon dioxide concentrations: the same rate during the day, a different rate at night.

Castor bean (Ricinus communis L.) plants were grown for 5-7 weeks in a controlled environment at 350 &mgr;l l(-1) or 700 &mgr;l l(-1) CO(2). Carbon assimilation, assimilate deposition, dark respiration and assimilate mobilization were measured in leaves 2, 3 and 4 (counted from the base of the plant), and a balance sheet of carbon input and export was elaborated for both CO(2) concentrations. Carbon dioxide assimilation was nearly constant over the illumination period, with only a slight depression occurring at the end of the day in mature source leaves, not in young source leaves. Assimilation was ca. 40% higher at 700 &mgr;l l(-1) than at 350 &mgr;l l(-1) CO(2). The source leaves increased steadily in weight per unit area during the first 3 weeks, more at 700 &mgr;l l(-1) than at 350 &mgr;l l(-1) CO(2). On top of an irreversible weight increase, there was a large gain in dry weight during the day, which was reversed during the night. This reversible weight gain was constant over the life time of the leaf and ca. 80% higher at 700 &mgr;l l(-1) than at 350 &mgr;l l(-1). Most of it was due to carbohydrates. The carbon content (as a percentage) was not altered by the CO(2) treatment. Respiration was 25% higher in high-CO(2) plants when based on leaf area, but the same when based on dry weight. The rate of carbon export via the phloem was the same during the daytime in plants grown at 350 &mgr;l l(-1) and 700 &mgr;l l(-1) CO(2). During the night the low-CO(2) plants had only 50% of the daytime export rate, in contrast to the high-CO(2) plants which maintained the high export rate. It was concluded that the phloem loading system is saturated during the daytime in both CO(2) regimes, whereas during the night the assimilate supply is reduced in plants in the normal CO(2) concentration. Two-thirds of the carbon exported from the leaves was permanently incorporated as plant dry matter in the residual plant parts. This "assimilation efficiency" was the same for both CO(2) regimes. It is speculated that under 350 &mgr;l l(-1) CO(2) the growing Ricinus plant operates at sink limitation during the day and at source limitation during the night.