Localised photosynthetic pigments in the node of a holoparasitic plant: support for shoot growth?

Abstract

Understanding the role of photosynthesis in the holoparasitic genus Cuscuta may give insights into the biology and evolution of these plants. We hypothesised that light irradiance on the node of C. racemosa is important for sustaining the growth of the new shoot through photosynthesis. We blocked light irradiance on the node of detached twigs of C. racemosa for one week and measured twig fresh and dry weight, shoot length and photosynthetic pigment content. Even though fresh and dry weight did not differ between non-irradiated and irradiated twigs, the shoots of non-irradiated nodes were significantly shorter. The non-irradiated plants showed higher amounts of chlorophyll a, b, and lycopene, and a lower concentration of β-carotene. Furthermore, the chlorophyll a/b ratio was lower than that of the control (irradiated) plants. Beyond corroborating our hypothesis, our results suggest that, when light is blocked on the node of C. racemosa, the holoparasite responds in a way similar to that of autotrophic plants under shaded conditions. Our findings suggest a new and previously unacknowledged role for the node of C. racemosa, which would be supporting shoot elongation through photosynthesis.