Photosynthesis by inflated pods of a desert shrub, Isomeris arborea.

The photosynthetic capacity and carbon metabolism of the fruits of Isomeris arborea (Capparidaceae), an evergreen shrub endemic to the desert and coastal habitats of Southern California and Baja California, are described. The inflated structure of the pods of I. arborea provides a model system for experimental studies of fruit photosynthesis in native plants since the gas concentration of the internal space can be manipulated and monitored separately from the external pod environment. CO2 released by seed respiration is partially contained in the inner gas space of the pods, resulting in an elevated CO2 environment inside the fruit (500 to 4000 μmol mol-1 depending on the stage of fruit development). A portion of this CO2 is assimilated by the inner layers of the pericarp, but a larger fraction leaks out. The photosynthetic layers of the pericarp use two different sources of CO2: the exocarp fixes exogenous CO2 while the endocarp fixes CO2 released by seed respiration into the pod cavity. Even though the total weight of the fruit increases during development, the combined rates of fixation of externally and internally supplied CO2 remained constant (10-11 μmol CO2 pod-1 h-1). After the pods attain maximum volume, the major change in gas exchange that takes place during fruit growth is a gradual increase in the amount of respiratory CO2 released by the seeds. This shifts the CO2 balance of the fruit from positive, in young fruits, to negative in mature fruits. Pericarp photosynthesis helped support not only the cost of fruit maintenance, but also the cost of fruit growth, particularly during the first stages of fruit development. During later fruiting stages insufficient carbon is fixed to fully supply either respiration or growth.