Localization of photosynthetic metabolism in the parasitic angiosperm Cuscuta reflexa

Abstract

Cells capable of photosynthesis in the parasitic angiosperm Cuscuta reflexa Roxb. (dodder) are highly localized. Immunolocalization of ribulose-1,5 bisphosphate carboxylase-oxygenase (Rubisco) and autofluorescence of chlorophyll in transverse sections of stems showed that they were largely restricted to a band of cells adjacent to the vascular bundles, consequently, the concentrations of Rubisco and chlorophyll were low per unit area or fresh weight. When 14CO2 was supplied to stem 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 the light reactions and dark reactions of photosynthesis appeared to be functional. De-epoxidation of the xanthophyll-cycle pigments after exposure to high light, and the chlorophyll fluorescence parameters, photochemical quenching (qP), non-photochemical quenching (NPQ) and the quantum efficiency of photosystem II (φPSII) responded normally to changes in photon flux density, indicating functional light-driven electron transport. The response of CO2 exchange to photon flux density followed a typical hyperbolic curve, and positive rates of CO2 fixation occurred 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 the carbon budget of C. reflexa.