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

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Abstract
The effects of CO2 concentration and illumination on net gas exchange and the pathway of 14CO2 fixation in detached seeds from developing fruits of L. albus were studied. Increasing the CO2 concentration in the surrounding atmosphere (from 0.03-3.0% [vol/vol] in air) decreased CO2 efflux by detached seeds either exposed to the light flux equivalent to that transmitted by the pod wall (500-600 .mu.Einsteins/m2 per s) in full sunlight or held in darkness. Above 1% CO2 detached seeds made a net gain of CO2 in the light (up to 0.4 mg of CO2 fixed/g fresh wt per h) but 14CO2 injected into the gas space of intact fruits (containing around 1.5% CO2 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 .mu.mol of CO2 fixed/g fresh wt per h) and phosphoenolpyruvate carboxylase (EC 4.1.131) in both testa (up to 280 .mu.mole of CO2 fixed/g fresh wt per h) and embryo (up to 355 .mu.mol of CO2 fixed/g fresh wt per h). In kinetic experiments the most significant early formed product of 14CO2 fixation in both light and dark was malate, but in the light, phosphoglyceric acid sugar phosphates were also rapidly labeled. 14CO2 fixation in the light was linked to the synthesis of sugars and amino acids, but in the dark, labeled sugars were not formed.