LOCALIZATION OF PHOTOSYNTHETIC METABOLISM IN THE PARASITIC ANGIOSPERMCUSCUTA-REFLEXA

Cells capable of photosynthesis in the parasitic angiosperm Cuscuta re flexa Roxb. (dodder) are highly localized. Immunolocalization of ribul ose-1,5 bisphosphate carboxylase-oxygenase (Rubisco) and autofluoresce nce of chlorophyll in transverse sections of stems showed that they we re largely restricted to a band of cells adjacent to the vascular bund les, consequently, the concentrations of Rubisco and chlorophyll were low per unit area or fresh weight. When (CO2)-C-14 was supplied to ste m segments of C. reflexa it preferentially accumulated in these cells adjacent to the vasculature. Although the conductance for CO2 movement to the cells containing chlorophyll and Rubisco was very low, both th e light reactions and dark reactions of photosynthesis appeared to be functional. De-epoxidation of the xanthophyll-cycle pigments after exp osure to high light, and the chlorophyll fluorescence parameters, phot ochemical quenching (qP), non-photochemical quenching (NPQ) and the qu antum efficiency of photosystem II (phi PSII) responded normally to ch anges in photon flux density, indicating functional light-driven elect ron transport. The response of CO2 exchange to photon flux density fol lowed a typical hyperbolic curve, and positive rates of CO2 fixation o ccurred when external CO2 was increased to 5%. We propose that CO2 for carbon assimilation is derived from internally respired CO2 and that this layer of photosynthetic cells makes a positive contribution to th e carbon budget of C. reflexa.