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.