Carbon Dioxide Fixation in the Carbon Economy of Developing Seeds of Lupinus albus (L.).

The effects of CO(2) concentration and illumination on net gas exchange and the pathway of (14)CO(2) fixation in detached seeds from developing fruits of Lupinus albus (L.) have been studied.Increasing the CO(2) concentration in the surrounding atmosphere (from 0.03 to 3.0% [v/v] in air) decreased CO(2) efflux by detached seeds either exposed to the light flux equivalent to that transmitted by the pod wall (500 to 600 micro-Einsteins per square meter per second) in full sunlight or held in darkness. Above 1% CO(2) detached seeds made a net gain of CO(2) in the light (up to 0.4 milligrams of CO(2) fixed per gram fresh weight per hour) but (14)CO(2) injected into the gas space of intact fruits (containing around 1.5% CO(2) naturally) was fixed mainly by the pod and little by the seeds.Throughout development seeds contained ribulose-1,5-bisphosphate carboxylase activity (EC 4.1.1.39), especially in the embryo (up to 99 micromoles of CO(2) fixed per gram fresh weight per hour) and phosphoenolpyruvate carboxylase (EC 4.1.1.31) in both testa (up to 280 micromoles of CO(2) fixed per gram fresh weight per hour) and embryo (up to 355 micromoles of CO(2) fixed per gram fresh weight per hour).In kinetic experiments the most significant early formed product of (14)CO(2) fixation in both light and dark was malate but in the light phosphoglyceric acid and sugar phosphates were also rapidly labeled. (14)CO(2) fixation in the light was linked to the synthesis of sugars and amino acids but in the dark labeled sugars were not formed.