Are Mistletoes Shade Plants? CO2Assimilation and Chlorophyll Fluorescence of Temperate Mistletoes and their Hosts

Mistletoes usually have slower rates of photosynthesis than their hosts. This study examines CO 2 assimilation, chlorophyll fluorescence and the chlorophyll content of temperate host–parasite pairs (nine hosts parasitized by Ileostylus micranthus and Carpodetus serratus parasitized by Tupeia antarctica ). The hosts of I. micranthus had higher mean annual CO 2 assimilation (3.59 ± 0.41 μmol m −2 s −1 ) than I. micranthus (2.42 ± 0.20 μmol m −2 s −1 ), and C. serratus (2.41 ± 0.43 μmol m −2 s −1 ) showed higher CO 2 assimilation than T. antarctica (0.67 ± 0.64 μmol m −2 s −1 ). Hosts saturated at significantly higher electron transport rates (ETR) and light levels than mistletoes. The positive relationship between CO 2 assimilation and electron transport suggests that the lower CO 2 assimilation rates in mistletoes are a consequence of lower electron transport rates. When photosynthetic rates, ETR and chlorophyll a / b ratios were adjusted for photosynthetically active radiation, hosts did not have significantly higher CO 2 assimilation (3.21 ± 0.37 μmol m −2 s −1 ) than mistletoes (2.54 ± 0.41 μmol m −2 s −1 ), but still had significantly higher ETR and chlorophyll a / b ratios. The electron transport rates, saturating light and chlorophyll a / b ratios of sun leaves from mistletoes were similar to host shade leaves. These responses indicate that in comparison with their hosts, mistletoe leaves have the photosynthetic characteristics of the leaves of shade plants.